The Concise Guide to PHARMACOLOGY 2015/16: Enzymes

1The authors state that there are no conflicts of interest to declare.

2Enzymes are protein catalysts facilitating the conversion of substrates into products. The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC‐IUBMB) classifies enzymes into families, using a four number code, on the basis of the reactions they catalyse. There are six main families:

3EC 1.‐.‐.‐ Oxidoreductases;

4EC 2.‐.‐.‐ Transferases;

5EC 3.‐.‐.‐ Hydrolases;

6EC 4.‐.‐.‐ Lyases;

7EC 5.‐.‐.‐ Isomerases;

8EC 6.‐.‐.‐ Ligases.

9Although there are many more enzymes than receptors in biology, and many drugs that target prokaryotic enzymes are effective medicines, overall the number of enzyme drug targets is relatively small [367, 401], which is not to say that they are of modest importance.

10The majority of drugs which act on enzymes act as inhibitors; one exception is metformin, which appears to stimulate activity of AMP‐activated protein kinase, albeit through an imprecisely‐defined mechanism. Kinetic assays allow discrimination of competitive, non‐competitive, and un‐competitive inhibitors. The majority of inhibitors are competitive (acting at the enzyme's ligand recognition site), non‐competitive (acting at a distinct site; potentially interfering with co‐factor or co‐enzyme binding) or of mixed type. One rare example of an uncompetitive inhibitor is lithium ions, which are effective inhibitors at inositol monophosphatase only in the presence of high substrate concentrations. Some inhibitors are irreversible, including a group known as suicide substrates, which bind to the ligand recognition site and then couple covalently to the enzyme. It is beyond the scope of the Guide to give mechanistic information about the inhibitors described, although generally this information is available from the indicated literature.

11Many enzymes require additional entities for functional activity. Some of these are used in the catalytic steps, while others promote a particular conformational change. Co‐factors are tightly bound to the enzyme and include metal ions and heme groups. Co‐enzymes are typically small molecules which accept or donate functional groups to assist in the enzymatic reaction. Examples include ATP, NAD, NADP and S‐adenosylmethionine, as well as a number of vitamins, such as riboflavin (vitamin B1) and thiamine (vitamin B2). Where co‐factors/co‐enzymes have been identified, the Guide indicates their involvement.

12This is a complete listing of enzyme families included in the online IUPHAR/BPS Guide to PHARMACOLOGY database. Summary information is provided for a subset of enzyme families (those with page numbers) in the tables below. Family members judged to be of significant pharmacological interest have been included, with further enzymes listed in the database.

136028 Protein Kinases (EC 2.7.x.x)

14‐ AGC: Containing PKA, PKC, PKG families

15‐ DMPK family

16‐ GEK subfamily

17‐ Other DMPK family kinases

186028 Rho kinase

19‐ G protein‐coupled receptor kinases

20‐ BARK/GRK2 subfamily

21‐ GRK1/3 subfamily

22‐ MAST family

23‐ NDR family

24‐ PDK1 family

25‐ Protein kinase A

26‐ Akt (Protein kinase B)

276029 Protein kinase C (PKC)

286029 Alpha subfamily

296029 Delta subfamily

306030 Eta subfamily

31‐ Iota subfamily

32‐ Protein kinase G (PKG)

33‐ Protein kinase N (PKN) family

34‐ RSK family

35‐ MSK subfamily

36‐ p70 subfamily

37‐ RSK subfamily

38‐ RSKR subfamily

39‐ RSKL family

40‐ SGK family

41‐ YANK family

42‐ Atypical

43‐ ABC1 family

44‐ ABC1‐A subfamily

45‐ ABC1‐B subfamily

46‐ Alpha kinase family

47‐ ChaK subfamily

48‐ eEF2K subfamily

49‐ Other alpha kinase family kinases

50‐ BCR family

51‐ Bromodomain kinase (BRDK) family

52‐ G11 family

53‐ Phosphatidyl inositol 3' kinase‐related kinases (PIKK) family

54‐ ATR subfamily

556030 FRAP subfamily

56‐ SMG1 subfamily

57‐ TRRAP subfamily

58‐ Other PIKK family kinases

59‐ RIO family

60‐ RIO1 subfamily

61‐ RIO2 subfamily

62‐ RIO3 subfamily

63‐ PDHK family

64‐ Pyruvate dehydrogenase kinase (PDHK) family

65‐ TAF1 family

66‐ TIF1 family

67‐ CAMK: Calcium/calmodulin‐dependent protein kinases

68‐ CAMK‐like (CAMKL) family

69‐ AMPK subfamily

70‐ BRSK subfamily

71‐ CHK1 subfamily

72‐ HUNK subfamily

73‐ LKB subfamily

74‐ MARK subfamily

75‐ MELK subfamily

76‐ NIM1 subfamily

77‐ NuaK subfamily

78‐ PASK subfamily

79‐ QIK subfamily

80‐ SNRK subfamily

81‐ CAMK‐unique family

82‐ CASK family

83‐ DCAMKL family

84‐ Death‐associated kinase (DAPK) family

85‐ MAPK‐Activated Protein Kinase (MAPKAPK) family

86‐ MAPKAPK subfamily

87‐ MKN subfamily

88‐ Myosin Light Chain Kinase (MLCK) family

89‐ Phosphorylase kinase (PHK) family

90‐ PIM family

91‐ Protein kinase D (PKD) family

92‐ PSK family

93‐ RAD53 family

94‐ Trbl family

95‐ Trio family

96‐ CK1: Casein kinase 1

97‐ Casein kinase 1 (CK1) family

98‐ Tau tubulin kinase (TTBK) family

99‐ Vaccina related kinase (VRK) family

100‐ CMGC: Containing CDK, MAPK, GSK3, CLK families

101‐ CLK family

102‐ Cyclin‐dependent kinase (CDK) family

103‐ CCRK subfamily

104‐ CDK1 subfamily

1056031 CDK4 subfamily

106‐ CDK5 subfamily

107‐ CDK7 subfamily

108‐ CDK8 subfamily

109‐ CDK9 subfamily

110‐ CDK10 subfamily

111‐ CRK7 subfamily

112‐ PITSLRE subfamily

113‐ TAIRE subfamily

114‐ Cyclin‐dependent kinase‐like (CDKL) family

115‐ Dual‐specificity tyrosine‐(Y)‐phosphorylation regulated kinase (DYRK) family

116‐ Dyrk1 subfamily

117‐ Dyrk2 subfamily

118‐ HIPK subfamily

119‐ PRP4 subfamily

120‐ Glycogen synthase kinase (GSK) family

1216031 GSK subfamily

122‐ Mitogen‐activated protein kinases (MAP kinases)

123‐ ERK subfamily

124‐ Erk7 subfamily

125‐ JNK subfamily

126‐ p38 subfamily

127‐ nmo subfamily

128‐ RCK family

129‐ SRPK family

130‐ Other protein kinases

131‐ CAMKK family

132‐ Meta subfamily

133‐ Aurora kinase (Aur) family

134‐ Bub family

135‐ Bud32 family

136‐ Casein kinase 2 (CK2) family

137‐ CDC7 family

138‐ Haspin family

139‐ IKK family

140‐ IRE family

141‐ MOS family

142‐ NAK family

143‐ NIMA (never in mitosis gene a)‐ related kinase (NEK) family

144‐ NKF1 family

145‐ NKF2 family

146‐ NKF4 family

147‐ NKF5 family

148‐ NRBP family

149‐ Numb‐associated kinase (NAK) family

150‐ Other‐unique family

1516032 Polo‐like kinase (PLK) family

152‐ PEK family

153‐ GCN2 subfamily

154‐ PEK subfamily

155‐ Other PEK family kinases

156‐ SgK493 family

157‐ Slob family

158‐ TBCK family

159‐ TOPK family

160‐ Tousled‐like kinase (TLK) family

161‐ TTK family

162‐ Unc‐51‐like kinase (ULK) family

163‐ VPS15 family

164‐ WEE family

165‐ Wnk family

166‐ Miscellaneous protein kinases

167‐ actin‐binding proteins ADF family

168‐ Twinfilin subfamily

169‐ SCY1 family

170‐ Hexokinases

171‐ STE: Homologs of yeast Sterile 7, Sterile 11, Sterile 20 kinases

1726032 STE7 family

173‐ STE11 family

174‐ STE20 family

175‐ FRAY subfamily

176‐ MSN subfamily

177‐ NinaC subfamily

178‐ PAKA subfamily

179‐ PAKB subfamily

180‐ STLK subfamily

181‐ TAO subfamily

182‐ STE20 family

183‐ STE‐unique family

184‐ TK: Tyrosine kinase

1856033 Abl family

1866033 Ack family

187‐ Csk family

188‐ Fak family

189‐ Fer family

1906034 Janus kinase (JakA) family

1916034 Src family

192‐ Syk family

1936035 Tec family

194‐ TKL: Tyrosine kinase‐like

195‐ Interleukin‐1 receptor‐associated kinase (IRAK) family

196‐ Leucine‐rich repeat kinase (LRRK) family

197‐ LIM domain kinase (LISK) family

198‐ LIMK subfamily

199‐ TESK subfamily

200‐ Mixed Lineage Kinase (MLK) family

201‐ HH498 subfamily

202‐ ILK subfamily

203‐ LZK subfamily

204‐ MLK subfamily

205‐ TAK1 subfamily

2066035 RAF family

207‐ Receptor interacting protein kinase (RIPK) family

208‐ TKL‐unique family

2096036 Peptidases and proteinases

210‐ AA: Aspartic (A) Peptidases

2116036 A1: Pepsin

212‐ AD: Aspartic (A) Peptidases

2136037 A22: Presenilin

214‐ CA: Cysteine (C) Peptidases

215‐ C1: Papain

216‐ C2: Calpain

217‐ C12: Ubiquitin C‐terminal hydrolase

218‐ C19: Ubiquitin‐specific protease

219‐ C54: Aut2 peptidase

220‐ CD: Cysteine (C) Peptidases

221‐ C13: Legumain

2226037 C14: Caspase

223‐ CE: Cysteine (C) Peptidases

224‐ C48: Ulp1 endopeptidase

225‐ M‐: Metallo (M) Peptidases

226‐ M79: Prenyl protease 2

227‐ MA: Metallo (M) Peptidases

2286037 M1: Aminopeptidase N

2296038 M2: Angiotensin‐converting (ACE and ACE2)

2306038 M10: Matrix metallopeptidase

2316039 M12: Astacin/Adamalysin

232‐ M13: Neprilysin

233‐ M49: Dipeptidyl‐peptidase III

234‐ MC: Metallo (M) Peptidases

235‐ M14: Carboxypeptidase A

236‐ ME: Metallo (M) Peptidases

237‐ M16: Pitrilysin

238‐ MF: Metallo (M) Peptidases

239‐ M17: Leucyl aminopeptidase

240‐ MG: Metallo (M) Peptidases

241‐ M24: Methionyl aminopeptidase

242‐ MH: Metallo (M) Peptidases

243‐ M18: Aminopeptidase I

244‐ M20: Carnosine dipeptidase

2456039 M28: Aminopeptidase Y

246‐ MJ: Metallo (M) Peptidases

2476040 M19: Membrane dipeptidase

248‐ MP: Metallo (M) Peptidases

249‐ M67: PSMD14 peptidase

250‐ PA: Serine (S) Peptidases

2516040 S1: Chymotrypsin

252‐ PB: Threonine (T) Peptidases

2536041 T1: Proteasome

254‐ T2: Glycosylasparaginase precursor

255‐ PC: Cysteine (C) Peptidases

256‐ C26: Gamma‐glutamyl hydrolase

257‐ SB: Serine (S) Peptidases

2586042 S8: Subtilisin

259‐ SC: Serine (S) Peptidases

2606042 S9: Prolyl oligopeptidase

261‐ S10: Carboxypeptidase Y

262‐ S28: Lysosomal Pro‐Xaa carboxypeptidase

263‐ S33: prolyl aminopeptidase

2646042 Acetylcholine turnover

2656044 Adenosine turnover

2666045 Amino acid hydroxylases

2676046 L‐Arginine turnover

2686047 Arginase

2696047 Arginine:glycine amidinotransferase

2706047 Dimethylarginine dimethylaminohydrolases

2716048 Nitric oxide synthases

2726048 Carboxylases and decarboxylases

2736049 Carboxylases

2746050 Decarboxylases

2756052 Catecholamine turnover

2766055 Ceramide turnover

2776055 Serine palmitoyltransferase

278‐ 3‐ketodihydrosphingosine reductase

2796056 Ceramide synthase

2806057 Sphingolipid Δ 14‐desaturase

2816058 Sphingomyelin synthase

2826058 Sphingomyelin phosphodiesterase

2836059 Neutral sphingomyelinase coupling factors

2846059 Ceramide glucosyltransferase

2856060 Acid ceramidase

2866060 Neutral ceramidases

2876061 Alkaline ceramidases

2886061 Ceramide kinase

2896062 Chromatin modifying enzymes

290‐ Enzymatic bromodomain‐containing proteins

291‐ Bromodomain kinase (BRDK) family

292‐ TAF1 family

293‐ TIF1 family

294‐ 1.14.11.‐ Histone demethylases

295‐ 2.1.1.43 Histone methyltransferases (HMTs)

296‐ 2.3.1.48 Histone acetyltransferases (HATs)

297‐ 3.6.1.3 ATPases

2986062 2.1.1.‐ Protein arginine N‐methyltransferases

2996062 3.5.1.‐ Histone deacetylases (HDACs)

3006063 Cyclic nucleotide turnover

3016063 Adenylyl cyclases

3026064 Soluble guanylyl cyclase

3036065 Exchange protein activated by cyclic AMP (Epac)

3046066 Phosphodiesterases, 3',5'‐cyclic nucleotide

3056069 Cytochrome P450

3066069 CYP1 family

3076070 CYP2 family

3086070 CYP3 family

3096071 CYP4 family

3106072 CYP5, CYP7 and CYP8 families

3116072 CYP11, CYP17, CYP19, CYP20 and CYP21 families

3126073 CYP24, CYP26 and CYP27 families

3136074 CYP39, CYP46 and CYP51 families

3146076 Eicosanoid turnover

3156075 Endocannabinoid turnover

3166077 Cyclooxygenase

3176077 Prostaglandin synthases

3186079 Lipoxygenases

3196080 Leukotriene and lipoxin metabolism

3206081 GABA turnover

3216082 Glycerophospholipid turnover

322‐ Lipid modifying kinases

3236083 1‐phosphatidylinositol 4‐kinase family

3246083 Phosphatidylinositol‐4‐phosphate 3‐kinase family

3256084 Phosphatidylinositol 3‐kinase family

3266084 Phosphatidylinositol‐4,5‐bisphosphate 3‐kinase family

3276085 1‐phosphatidylinositol‐3‐phosphate 5‐kinase family

3286085 Type I PIP kinases (1‐phosphatidylinositol‐4‐phosphate 5‐kinase family)

3296086 Type II PIP kinases (1‐phosphatidylinositol‐5‐phosphate 4‐kinase family)

3306100 Sphingosine kinase

3316087 Phosphoinositide‐specific phospholipase C

3326088 Phospholipase A2

3336089 Phosphatidylcholine‐specific phospholipase D

3346090 Lipid phosphate phosphatases

3356082 Phosphatidylinositol kinases

3366091 Haem oxygenase

3376092 Hydrogen sulphide synthesis

3386093 Hydrolases

3396093 Inositol phosphate turnover

3406094 Inositol 1,4,5‐trisphosphate 3‐kinases

3416094 Inositol polyphosphate phosphatases

3426094 Inositol monophosphatase

3436095 Lanosterol biosynthesis pathway

3446097 Nucleoside synthesis and metabolism

3456099 Sphingosine 1‐phosphate turnover

3466100 Sphingosine kinase

3476100 Sphingosine 1‐phosphate phosphatase

3486101 Sphingosine 1‐phosphate lyase

3496101 Thyroid hormone turnover

350‐ 1.4.3.13 Lysyl oxidases

351‐ 1.13.11.‐ Dioxygenases

3526103 1.14.11.29 2‐oxoglutarate oxygenases transferases

353‐ 2.3.‐.‐ Acyltransferases

3546103 2.4.2.30 poly(ADP‐ribose)polymerases

3556104 2.5.1.58 Protein farnesyltransferase

356‐ 3.1.‐.‐ Ester bond enzymes

357‐ 3.1.1.‐ Carboxylic Ester Hydrolases

358‐ 3.2.1.‐ Glycosidases

359‐ 3.4.21.46 Complement factor D

3606062 3.5.1.‐ Histone deacetylases (HDACs)

3616104 3.5.3.15 Peptidyl arginine deiminases (PADI)

362‐ 3.6.5.2 Small monomeric GTPases

3636104 RAS subfamily

3646105 4.2.1.1 Carbonate dehydratases

365‐ 5.‐.‐.‐ Isomerases

366‐ 5.2.‐.‐ Cis‐trans‐isomerases

3676105 5.99.1.2 DNA Topoisomerases

368Protein kinases (E.C. 2.7.11.‐) use the co‐substrate ATP to phosphorylate serine and/or threonine residues on target proteins. Analysis of the human genome suggests the presence of 518 protein kinases in man, with over 100 protein kinase‐like pseudogenes [313]. It is beyond the scope of the Concise Guide to list all these protein kinase activities; the full listing may be seen at www.GuidetoPHARMACOLOGY.org.

369Most inhibitors of these enzymes have been assessed in cell‐free investigations and so may appear to ‘lose’ potency and selectivity in intact cell assays. In particular, ambient ATP concentrations may be influential in responses to inhibitors, since the majority are directed at the ATP binding site [103].

370Eglen R et al. (2011) Drug discovery and the human kinome: recent trends. Pharmacol. Ther. 130: 144‐56 [PMID:21256157]

371Graves LM et al. (2013) The dynamic nature of the kinome. Biochem. J. 450: 1‐8 [PMID:23343193]

372Liu Q et al. (2013) Developing irreversible inhibitors of the protein kinase cysteinome. Chem. Biol. 20: 146‐59 [PMID:23438744]

373Martin KJ et al. (2012) Selective kinase inhibitors as tools for neuroscience research. Neuropharmacology 63: 1227‐37 [PMID:22846224]

374Tarrant MK et al. (2009) The chemical biology of protein phosphorylation. Annu. Rev. Biochem. 78: 797‐825 [PMID:19489734]

375Wu‐Zhang AX et al. (2013) Protein kinase C pharmacology: refining the toolbox. Biochem. J. 452: 195‐209 [PMID:23662807]

376Rho kinase (also known as P160ROCK, Rho‐activated kinase) is activated by members of the Rho small G protein family, which are activated by GTP exchange factors, such as ARHGEF1(Q92888, p115‐RhoGEF), which in turn may be activated by Gα 12/13 subunits [269].

377Protein kinase C is the target for the tumour‐promoting phorbol esters, such as tetradecanoyl‐β‐phorbol acetate (TPA, also known as phorbol 12‐myristate 13‐acetate).

378Classical protein kinase C isoforms: PKCα, PKCβ, PKCγ. Members of the classical protein kinase C family are activated by Ca2+ and diacylglycerol, and may be inhibited by GF109203X, calphostin C, GÖ 6983, chelerythrine and Ro31‐8220.

379Novel protein kinase C isoforms: PKCδ, PKCε, PKCη, PKCθ and PKCμ. Members of the classical protein kinase C family are activated by diacylglycerol and may be inhibited by calphostin C, GÖ 6983 and chelerythrine.

380Atypical protein kinase C isoforms: PKCι, PKCζ.

381Peptidases and proteinases hydrolyse peptide bonds, and can be simply divided on the basis of whether terminal peptide bonds are cleaved (exopeptidases and exoproteinases) at the amino terminus (aminopeptidases) or carboxy terminus (carboxypeptidases). Non‐terminal peptide bonds are cleaved by endopeptidases and endoproteinases, which are divided into serine endopeptidases (EC 3.4.21.‐), cysteine endopeptidases (EC 3.4.22.‐), aspartate endopeptidases (EC 3.4.23.‐), metalloendopeptidases (EC 3.4.24.‐) and threonine endopeptidases (EC 3.4.25.‐).

382It is beyond the scope of the Guide to list all peptidase and proteinase activities; this summary focuses on selected enzymes of significant pharmacological interest.

383Presenilin (PS)‐1 or ‐2 act as the catalytic component/essential co‐factor of the γ‐secretase complex responsible for the final carboxy‐terminal cleavage of amyloid precursor protein (APP) [249] in the generation of amyloid beta (Aβ) [6, 471]. Given that the accumulation and aggregation of Aβ in the brain is pivotal in the development of Alzheimer's disease (AD), inhibition of PS activity is one mechanism being investigated as a therapeutic option for AD [177]. Several small molecule inhibitors of PS‐1 have been investigated, with some reaching early clinical trials, but none have been formally approved. Dewji et al (2015) have reported that small peptide fragments of human PS‐1 can significantly inhibit Aβ production (total Aβ, Aβ40 and Aβ42) both in vitro and when infused in to the brains of APP transgenic mice [111]. The most active small peptides in this report were P4 [PMID: 25923432] and P8 [PMID: 25923432], from the amino‐terminal domain of PS‐1.

384Caspases, (E.C. 3.4.22.‐) which derive their name from Cysteine ASPartate‐specific proteASES, include at least two families; initiator caspases (caspases 2, 8, 9 and 10), which are able to hydrolyse and activate a second family of effector caspases (caspases 3, 6 and 7), which themselves are able to hydrolyse further cellular proteins to bring about programmed cell death. Caspases are heterotetrameric, being made up of two pairs of subunits, generated by a single gene product, which is proteolysed to form the mature protein. Members of the mammalian inhibitors of apoptosis proteins (IAP) are able to bind the procaspases, thereby preventing maturation to active proteinases.

385CARD16 (Caspase recruitment domain‐containing protein 16, caspase‐1 inhibitor COP, CARD only domain‐containing protein 1, pseudo interleukin‐1β converting enzyme, pseudo‐ICE, ENSG00000204397) shares sequence similarity with some of the caspases.

386Aminopeptidases catalyze the cleavage of amino acids from the amino (N) terminus of protein or peptide substrates, and are involved in many essential cellular functions. Members of this enzyme family may be monomeric or multi‐subunit complexes, and many are zinc metalloenzymes [480].

387Matrix metalloproteinases (MMP) are calcium‐ and zinc‐dependent proteinases regulating the extracellular matrix and are often divided (e.g. [501]) on functional and structural bases into gelatinases, collagenases, stromyelinases and matrilysins, as well as membrane type‐MMP (MT‐MMP).

388A number of small molecule ‘broad spectrum’ inhibitors of MMP have been described, including marimastat and batimastat.

389Tissue inhibitors of metalloproteinase (TIMP) proteins are endogenous inhibitors acting to chelate MMP proteins: TIMP1(TIMP1, P01033), TIMP2(TIMP2, P16035), TIMP3(TIMP3, P35625), TIMP4(TIMP4, Q99727)

390ADAM (A Disintegrin And Metalloproteinase domain containing proteins) metalloproteinases cleave cell‐surface or transmembrane proteins to generate soluble and membrane‐limited products.

391ADAMTS (with thrombospondin motifs) metalloproteinases cleave cell‐surface or transmembrane proteins to generate soluble and membrane‐limited products.

392Additional ADAM family members include AC123767.2 (cDNA FLJ58962, moderately similar to mouse ADAM3, ENSG00000231168), AL160191.3 (ADAM21‐like protein, ENSG00000235812), AC136428.3‐2 (ENSG00000185520) and ADAMDEC1 (decysin 1, ENSG00000134028).

393Other ADAMTS family members include AC104758.12‐5 (FLJ00317 protein Fragment ENSG00000231463), AC139425.3‐1 (ENSG00000225577), and AC126339.6‐1 (ENSG00000225734).

394folate hydrolase is also known as NAALADase as it is responsible for the hydrolysis of N‐acetaspartylglutamate to form N‐acetylaspartate and L‐glutamate. In the gut, the enzyme assists in the assimilation of folate by hydrolysing dietary poly‐gamma‐glutamylfolate. The enzyme is highly expressed in the prostate, and its expression is up‐regulated in cancerous tissue. A tagged version of the antibody capromab has been used for imaging purposes.

395The T1 macropain beta subunits form the catalytic proteinase core of the 20S proteasome complex [86]. This catalytic core enables the degradation of peptides with Arg, Phe, Tyr, Leu, and Glu adjacent to the cleavage site. The β5 subunit is the principal target of the approved drug proteasome inhibitor bortezomib.

396One member of this family has garnered intense interest as a clinical drug target. As liver PCSK9 acts to maintain cholesterol homeostasis, it has become a target of intense interest for clinical drug development. Inhibition of PCSK9 can lower low‐density cholesterol (LDL‐C) by clearing LDLR‐bound LDL particles, thereby lowering circulating cholesterol levels. It is hypothesised that this action may improve outcomes in patients with atherosclerotic cardiovascular disease [297, 416, 462]. Therapeutics which inhibit PCSK9 are viewed as potentially lucrative replacements for statins, upon statin patent expiry. Several monoclonal antibodies including alirocumab, evolocumab, bococizumab, RG‐7652 and LY3015014 are under development. One RNAi therapeutic, code named ALN‐PCS02, is also in development [99, 139, 146].

397Acetylcholine is familiar as a neurotransmitter in the central nervous system and in the periphery. In the somatic nervous system, it activates nicotinic acetylcholine receptors at the skeletal neuromuscular junction. It is also employed in the autonomic nervous system, in both parasympathetic and sympathetic branches; in the former, at the smooth muscle neuromuscular junction, activating muscarinic acetylcholine receptors. In the latter, acetylcholine is involved as a neurotransmitter at the ganglion, activating nicotinic acetylcholine receptors. Acetylcholine is synthesised in neurones through the action of choline O‐acetyltransferase and metabolised after release through the extracellular action of acetylcholinesterase and cholinesterase. Choline is accumulated from the extracellular medium by selective transporters (see SLC5A7 and the SLC44 family). Acetylcholine is accumulated in synaptic vesicles through the action of the vesicular acetylcholine transporter SLC18A3.

398A number of organophosphorus compounds inhibit acetylcholinesterase and cholinesterase irreversibly, including pesticides such as chlorpyrifos‐oxon, and nerve agents such as tabun, soman and sarin. AChE is unusual in its exceptionally high turnover rate which has been calculated at 740 000/min/molecule [505].

399Abreu‐Villaça Y et al. (2011) Developmental aspects of the cholinergic system. Behav. Brain Res. 221: 367‐78 [PMID:20060019]

400Bellier JP et al. (2011) Peripheral type of choline acetyltransferase: biological and evolutionary implications for novel mechanisms in cholinergic system. J. Chem. Neuroanat. 42: 225‐35 [PMID:21382474]

401Engel AG et al. (2015) Congenital myasthenic syndromes: pathogenesis, diagnosis, and treatment. Lancet Neurol 14: 420‐34 [PMID:25792100]

402Rosini M et al. (2014) Multi‐target design strategies in the context of Alzheimer's disease: acetylcholinesterase inhibition and NMDA receptor antagonism as the driving forces. Neurochem. Res. 39: 1914‐23 [PMID:24493627]

403Zimmermann M. (2013) Neuronal AChE splice variants and their non‐hydrolytic functions: redefining a target of AChE inhibitors? Br. J. Pharmacol. 170: 953‐67 [PMID:23991627]

404A multifunctional, ubiquitous molecule, adenosine acts at cell‐surface G protein‐coupled receptors, as well as numerous enzymes, including protein kinases and adenylyl cyclase. Extracellular adenosine is thought to be produced either by export or by metabolism, predominantly through ecto‐5'‐nucleotidase activity (also producing inorganic phosphate). It is inactivated either by extracellular metabolism viaadenosine deaminase (also producing ammonia) or, following uptake by nucleoside transporters, viaadenosine deaminase or adenosine kinase (requiring ATP as co‐substrate). Intracellular adenosine may be produced by cytosolic 5'‐nucleotidases or through S‐adenosylhomocysteine hydrolase (also producing L‐homocysteine).

405An extracellular adenosine deaminase activity, termed ADA2 or adenosine deaminase growth factor (ADGF, CECR1, Q9NZK5) has been identified [94, 309], which is insensitive to EHNA[546]. Other forms of adenosine deaminase act on ribonucleic acids and may be divided into two families: ADAT1(Q9BUB4) deaminates transfer RNA; ADAR(EC 3.5.4.37, also known as 136 kDa double‐stranded RNA‐binding protein, P136, K88DSRBP, Interferon‐inducible protein 4); ADARB1 (EC 3.5.‐.‐,, also known as dsRNA adenosine deaminase) and ADARB2 (EC 3.5.‐.‐, also known as dsRNA adenosine deaminase B2, RNA‐dependent adenosine deaminase 3) act on double‐stranded RNA. Particular polymorphisms of the ADA gene result in loss‐of‐function and severe combined immunodeficiency syndrome. Adenosine deaminase is able to complex with dipeptidyl peptidase IV (EC 3.4.14.5, DPP4, also known as T‐cell activation antigen CD26, TP103, adenosine deaminase complexing protein 2) to form a cell‐surface activity [248].

406Antonioli L et al. (2013) CD39 and CD73 in immunity and inflammation. Trends Mol Med 19: 355‐67 [PMID:23601906]

407Boison D. (2013) Adenosine kinase: exploitation for therapeutic gain. Pharmacol. Rev. 65: 906‐43 [PMID:23592612]

408Cortés A et al. (2015) Moonlighting adenosine deaminase: a target protein for drug development. Med Res Rev 35: 85‐125 [PMID:24933472]

409Tomaselli S et al. (2014) The RNA editing enzymes ADARs: mechanism of action and human disease. Cell Tissue Res. 356: 527‐32 [PMID:24770896]

410Zimmermann H et al. (2012) Cellular function and molecular structure of ecto‐nucleotidases. Purinergic Signal. 8: 437‐502 [PMID:22555564]

411The amino acid hydroxylases (monooxygenases), EC.1.14.16.‐, are iron‐containing enzymes which utilise molecular oxygen and sapropterin as co‐substrate and co‐factor, respectively. In humans, as well as in other mammals, there are two distinct L‐Tryptophan hydroxylase 2 genes. In humans, these genes are located on chromosomes 11 and 12 and encode two different homologous enzymes, TPH1 and TPH2.

412Amireault P et al. (2013) Life without peripheral serotonin: insights from tryptophan hydroxylase 1 knockout mice reveal the existence of paracrine/autocrine serotonergic networks. ACS Chem Neurosci 4: 64‐71 [PMID:23336045]

413Daubner SC et al. (2011) Tyrosine hydroxylase and regulation of dopamine synthesis. Arch. Biochem. Biophys. 508: 1‐12 [PMID:21176768]

414Flydal MI et al. (2013) Phenylalanine hydroxylase: function, structure, and regulation. IUBMB Life 65: 341‐9 [PMID:23457044]

415Roberts KM et al. (2013) Mechanisms of tryptophan and tyrosine hydroxylase. IUBMB Life 65: 350‐7 [PMID:23441081]

416Tekin I et al. (2014) Complex molecular regulation of tyrosine hydroxylase. J Neural Transm 121: 1451‐81 [PMID:24866693]

417L‐arginine is a basic amino acid with a guanidino sidechain. As an amino acid, metabolism of L‐arginine to form L‐ornithine, catalysed by arginase, forms the last step of the urea production cycle. L‐Ornithine may be utilised as a precursor of polyamines (see Carboxylases and Decarboxylases) or recycled via L‐argininosuccinic acid to L‐arginine. L‐Arginine may itself be decarboxylated to form agmatine, although the prominence of this pathway in human tissues is uncertain. L‐Arginine may be used as a precursor for guanidoacetic acid formation in the creatine synthesis pathway under the influence of arginine:glycine amidinotransferase with L‐ornithine as a byproduct. Nitric oxide synthase uses L‐arginine to generate nitric oxide, with L‐citrulline also as a byproduct.

418L‐Arginine in proteins may be subject to post‐translational modification through methylation, catalysed by protein arginine methyltransferases. Subsequent proteolysis can liberate asymmetric NG,NG‐dimethyl‐L‐arginine(ADMA), which is an endogenous inhibitor of nitric oxide synthase activities. ADMA is hydrolysed by dimethylarginine dimethylhydrolase activities to generate L‐citrulline and dimethylamine.

419Moncada S et al. (1997) International Union of Pharmacology Nomenclature in Nitric Oxide Research. Pharmacol. Rev. 49: 137‐42 [PMID:9228663]

420Tang L et al. (2014) Targeting NOS as a therapeutic approach for heart failure. Pharmacol. Ther. 142: 306‐15 [PMID:24380841]

421Tratsiakovich Y et al. (2013) Arginase as a target for treatment of myocardial ischemia‐reperfusion injury. Eur. J. Pharmacol. 720: 121‐3 [PMID:24183975]

422Whiteley CG. (2014) Arginine metabolising enzymes as targets against Alzheimers' disease. Neurochem. Int. 67: 23‐31 [PMID:24508404]

423Yang Y et al. (2013) Protein arginine methyltransferases and cancer. Nat. Rev. Cancer 13: 37‐50 [PMID:23235912]

424Arginase (EC 3.5.3.1) are manganese‐containing isoforms, which appear to show differential distribution, where the ARG1 isoform predominates in the liver and erythrocytes, while ARG2 is associated more with the kidney.

425Nω‐hydroxyarginine, an intermediate in NOS metabolism of L‐arginine acts as a weak inhibitor and may function as a physiological regulator of arginase activity. Although isoform‐selective inhibitors of arginase are not available, examples of inhibitors selective for arginase compared to NOS are Nω‐hydroxy‐nor‐L‐arginine[483], S‐(2‐boronoethyl)‐L‐cysteine[90, 256] and 2(S)‐amino‐6‐boronohexanoic acid [22, 90].

426Dimethylarginine dimethylaminohydrolases (DDAH, EC 3.5.3.18) are cytoplasmic enzymes which hydrolyse NG,NG‐dimethyl‐L‐arginine to form dimethylamine and L‐citrulline.

427Nitric oxide synthases (NOS, E.C. 1.14.13.39) utilise L‐arginine(not D‐arginine) and molecular oxygen to generate nitric oxide and L‐citrulline. The nomenclature suggested by NC‐IUPHARof NOS I, II and III [340] has not gained wide acceptance. eNOS and nNOS isoforms are activated at concentrations of calcium greater than 100 nM, while iNOS shows higher affinity for Ca2+/calmodulin(CALM1 CALM2 CALM3, P62158) and thus appears to be constitutively active. All the three isoforms are homodimers and require sapropterin, flavin adenine dinucleotide, flavin mononucleotide and NADPH for catalytic activity. L‐NAME is an inhibitor of all three isoforms, with an IC50 value in the micromolar range.

428The reductase domain of NOS catalyses the reduction of cytochrome c and other redox‐active dyes [322]. NADPH:O2 oxidoreductase catalyses the formation of superoxide anion/H2O2 in the absence of L‐arginine and sapropterin.

429The carboxylases allow the production of new carbon‐carbon bonds by introducing HCO3 − or CO2 into target molecules. Two groups of carboxylase activities, some of which are bidirectional, can be defined on the basis of the cofactor requirement, making use of biotin (EC 6.4.1.‐) or vitamin K hydroquinone (EC 4.1.1.‐).

430Dicarboxylic acids including citric acid are able to activate ACC1/ACC2 activity allosterically. PCC is able to function in forward and reverse modes as a ligase (carboxylase) or lyase (decarboxylase) activity, respectively. Loss‐of‐function mutations in GGCX are associated with clotting disorders.

431The decarboxylases generate CO2 and the indicated products from acidic substrates, requiring pyridoxal phosphate or pyruvic acid as a co‐factor.

432Bale S et al. (2010) Structural biology of S‐adenosylmethionine decarboxylase. Amino Acids 38: 451‐60 [PMID:19997761]

433Jitrapakdee S et al. (2008) Structure, mechanism and regulation of pyruvate carboxylase. Biochem. J. 413: 369‐87 [PMID:18613815]

434Lietzan AD et al. (2014) Functionally diverse biotin‐dependent enzymes with oxaloacetate decarboxylase activity. Arch. Biochem. Biophys. 544: 75‐86 [PMID:24184447]

435Moya‐García AA et al. (2009) Structural features of mammalian histidine decarboxylase reveal the basis for specific inhibition. Br. J. Pharmacol. 157: 4‐13 [PMID:19413567]

436Tong L. (2013) Structure and function of biotin‐dependent carboxylases. Cell. Mol. Life Sci. 70: 863‐91 [PMID:22869039]

437Vance JE et al. (2013) Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells. Biochim. Biophys. Acta 1831: 543‐54 [PMID:22960354]

438Catecholamines are defined by the presence of two adjacent hydroxyls on a benzene ring with a sidechain containing an amine. The predominant catacholamines in mammalian biology are the neurotransmitter/hormones dopamine, (‐)‐noradrenaline (norepinephrine) and (‐)‐adrenaline(epinephrine). These hormone/transmitters are synthesized by sequential metabolism from L‐phenylalanine via L‐tyrosine. Hydroxylation of L‐tyrosine generates levodopa, which is decarboxylated to form dopamine. Hydroxylation of the ethylamine sidechain generates (‐)‐noradrenaline (norepinephrine), which can be methylated to form (‐)‐adrenaline(epinephrine). In particular neuronal and adrenal chromaffin cells, the catecholamines dopamine, (‐)‐noradrenaline and (‐)‐adrenaline are accumulated into vesicles under the influence of the vesicular monoamine transporters (VMAT1/SLC18A1 and VMAT2/SLC18A2). After release into the synapse or the bloodstream, catecholamines are accumulated through the action cell‐surface transporters, primarily the dopamine (DAT/SLC6A3) and norepinephrine transporter (NET/SLC6A2). The primary routes of metabolism of these catecholamines are oxidation via monoamine oxidase activities of methylation via catechol O‐methyltransferase.

439Al‐Nuaimi SK et al. (2012) Monoamine oxidase inhibitors and neuroprotection: a review. Am J Ther 19: 436‐48 [PMID:22960850]

440Fitzpatrick PF. (2012) Allosteric regulation of phenylalanine hydroxylase. Arch. Biochem. Biophys. 519: 194‐201 [PMID:22005392]

441Flydal MI et al. (2013) Phenylalanine hydroxylase: function, structure, and regulation. IUBMB Life 65: 341‐9 [PMID:23457044]

442Ma Z et al. (2013) Structure‐based drug design of catechol‐O‐methyltransferase inhibitors for CNS disorders. Br J Clin Pharmacol [PMID:23713800]

443Shih JC et al. (2011) Transcriptional regulation and multiple functions of MAO genes. J Neural Transm 118: 979‐86 [PMID:21359973]

444Ceramides are a family of sphingophospholipids synthesized in the endoplasmic reticulum, which mediate cell stress responses, including apoptosis, autophagy and senescence, Serine palmitoyltransferase generates 3‐ketosphinganine, which is reduced to sphinganine(dihydrosphingosine). N‐Acylation allows the formation of dihydroceramides, which are subsequently reduced to form ceramides. Once synthesized, ceramides are trafficked from the ER to the Golgi bound to the ceramide transfer protein, CERT (COL4A3BP, Q9Y5P4). Ceramide can be metabolized via multiple routes, ensuring tight regulation of its cellular levels. Addition of phosphocholine generates sphingomyelin while carbohydrate is added to form glucosyl‐ or galactosylceramides. Ceramidase re‐forms sphingosine or sphinganine from ceramide or dihydroceramide. Phosphorylation of ceramide generates ceramide phosphate. The determination of accurate kinetic parameters for many of the enzymes in the sphingolipid metabolic pathway is complicated by the lipophilic nature of the substrates.

445The functional enzyme is a heterodimer of SPT1 (LCB1) with either SPT2 (LCB2) or SPT3 (LCB2B); the small subunits of SPT (ssSPTa or ssSPTb) bind to the heterodimer to enhance enzymatic activity. The complexes of SPT1/SPT2/ssSPTa and SPT1/SPT2/ssSPTb were most active with palmitoylCoA as substrate, with the latter complex also showing some activity with stearoylCoA [190]. Complexes involving SPT3 appeared more broad in substrate selectivity, with incorporation of myristoylCoA prominent for SPT1/SPT3/ssSPTa complexes, while SP1/SPT3/ssSPTb complexes had similar activity with C16, C18 and C20 acylCoAs [190].

446This family of enzymes, also known as sphingosine N‐acyltransferase, is located in the ER facing the cytosol with an as‐yet undefined topology and stoichiometry. Ceramide synthase in vitro is sensitive to inhibition by the fungal derived toxin, fumonisin B1.

447DEGS1 and DEGS2 are 4TM proteins.

448DEGS1 activity is inhibited by a number of natural products, including curcumin and Δ9‐tetrahydrocannabinol[130].

449Following translocation from the ER to the Golgi under the influence of the ceramide transfer protein, sphingomyelin synthases allow the formation of sphingomyelin by the transfer of phosphocholine from the phospholipid phosphatidylcholine.

450Sphingomyelin synthase‐related protein 1 is structurally related but lacks sphingomyelin synthase activity.

451Also known as sphingomyelinase.

452Protein FAN [2] and polycomb protein EED [383] allow coupling between TNF receptors and neutral sphingomyelinase phosphodiesterases.

453The six human ceramidases may be divided on the basis of pH optimae into acid, neutral and alkaline ceramidases, which also differ in their subcellular location.

454The six human ceramidases may be divided on the basis of pH optimae into acid, neutral and alkaline ceramidases, which also differ in their subcellular location.

455ASAH2B appears to be an enzymatically inactive protein, which may result from gene duplication and truncation.

456The six human ceramidases may be divided on the basis of pH optimae into acid, neutral and alkaline ceramidases, which also differ in their subcellular location.

457A ceramide kinase‐like protein has been identified in the human genome (CERKL, Q49MI3).

458Castro BM et al. (2014) Ceramide: a simple sphingolipid with unique biophysical properties. Prog. Lipid Res. 54: 53‐67 [PMID:24513486]

459Halmer R et al. (2014) Sphingolipids: important players in multiple sclerosis. Cell. Physiol. Biochem. 34: 111‐8 [PMID:24977485]

460Ito M et al. (2014) New insight into the structure, reaction mechanism, and biological functions of neutral ceramidase. Biochim. Biophys. Acta 1841: 682‐91 [PMID:24064302]

461Khavandgar Z et al. (2015) Sphingolipid metabolism and its role in the skeletal tissues. Cell. Mol. Life Sci. 72: 959‐69 [PMID:25424644]

462Lowther J et al. (2012) Structural, mechanistic and regulatory studies of serine palmitoyltransferase. Biochem. Soc. Trans. 40: 547‐54 [PMID:22616865]

463Saied EM et al. (2014) Small molecule inhibitors of ceramidases. Cell. Physiol. Biochem. 34: 197‐212 [PMID:24977492]

464Chromatin modifying enzymes, and other chromatin‐modifying proteins, fall into three broad categories: writers, readers and erasers. The function of these proteins is to dynamically maintain cell identity and regulate processes such as differentiation, development, proliferation and genome integrity via recognition of specific 'marks' (covalent post‐translational modifications) on histone proteins and DNA [267]. In normal cells, tissues and organs, precise co‐ordination of these proteins ensures expression of only those genes required to specify phenotype or which are required at specific times, for specific functions. Chromatin modifications allow DNA modifications not coded by the DNA sequence to be passed on through the genome and underlies heritable phenomena such as X chromosome inactivation, aging, heterochromatin formation, reprogramming, and gene silencing (epigenetic control).

465To date at least eight distinct types of modifications are found on histones. These include small covalent modifications such as acetylation, methylation, and phosphorylation, the attachment of larger modifiers such as ubiquitination or sumoylation, and ADP ribosylation, proline isomerization and deimination. Chromatin modifications and the functions they regulate in cells are reviewed by Kouzarides (2007) [267].

466Writer proteins include the histone methyltransferases, histone acetyltransferases, some kinases and ubiquitin ligases.

467Readers include proteins which contain methyl‐lysine‐recognition motifs such as bromodomains, chromodomains, tudor domains, PHD zinc fingers, PWWP domains and MBT domains.

468Erasers include the histone demethylases and histone deacetylases (HDACs and sirtuins).

469Dysregulated epigenetic control can be associated with human diseases such as cancer [129], where a wide variety of cellular and protein abberations are known to perturb chromatin structure, gene transcription and ultimately cellular pathways [24, 439]. Due to the reversible nature of epigenetic modifications, chromatin regulators are very tractable targets for drug discovery and the development of novel therapeutics. Indeed, small molecule inhibitors of writers (e.g. azacitidine and decitabine target the DNA methyltransferases DNMT1 and DNMT3 for the treatment of myelodysplastic syndromes [165, 520]) and erasers (e.g. the HDAC inhibitors vorinostat, romidepsin and belinostat for the treatment of T‐cell lymphomas [144, 254]) are already being used in the clinic. The search for the next generation of compounds with improved specificity against chromatin‐associated proteins is an area of intense basic and clinical research [56]. Current progress in this field is reviewed by Simó‐Riudalbas and Esteller (2015) [440].

470Protein arginine N‐methyltransferases (PRMT, EC 2.1.1.‐) encompass histone arginine N‐methyltransferases (PRMT4, PRMT7, EC 2.1.1.125) and myelin basic protein N‐methyltransferases (PRMT7, EC 2.1.1.126). They are dimeric or tetrameric enzymes which use S‐adenosyl methionine as a methyl donor, generating S‐adenosylhomocysteine as a by‐product. They generate both mono‐methylated and di‐methylated products; these may be symmetric (SDMA) or asymmetric (NG,NG‐dimethyl‐L‐arginine) versions, where both guanidine nitrogens are monomethylated or one of the two is dimethylated, respectively.

471Histone deacetylases act as erasers of epigenetic acetylation marks on lysine residues in histones. Removal of the acetyl groups facilitates tighter packing of chromatin (heterochromatin formation) leading to transcriptional repression.

472The histone deacetylase family has been classified in to five subfamilies based on phylogenetic comparison with yeast homologues:

473Class I contains HDACs 1, 2, 3 and 8

474Class IIa contains HDACs 4, 5, 7 and 9

475Class IIb contains HDACs 6 and 10

476Class III contains the sirtuins (SIRT1‐7)

477Class IV contains only HDAC11.

478Classes I, II and IV use Zn+ as a co‐factor, whereas catalysis by Class III enzymes requires NAD+ as a co‐factor, and members of this subfamily have ADP‐ribosylase activity in addition to protein deacetylase function [420].

479HDACs have more general protein deacetylase activity, being able to deacetylate lysine residues in non‐histone proteins [82] such as microtubules [221], the hsp90 chaperone [268] and the tumour suppressor p53 [302].

480Dysregulated HDAC activity has been identified in cancer cells and tumour tissues [288, 410], making HDACs attractive molecular targets in the search for novel mechanisms to treat cancer [522]. Several small molecule HDAC inhibitors are already approved for clinical use: romidepsin, belinostat, vorinostat, panobinostat, belinostat, valproic acid and chidamide. HDACs and HDAC inhibitors currently in development as potential anti‐cancer therapeutics are reviewed by Simó‐Riudalbas and Esteller (2015) [440].

481Cyclic nucleotides are second messengers generated by cyclase enzymes from precursor triphosphates and hydrolysed by phosphodiesterases. The cellular actions of these cyclic nucleotides are mediated through activation of protein kinases (cAMP‐ and cGMP‐dependent protein kinases), ion channels (cyclic nucleotide‐gated, CNG, and hyperpolarization and cyclic nucleotide‐gated, HCN) and guanine nucleotide exchange factors (GEFs, Epac).

482Adenylyl cyclase, E.C. 4.6.1.1, converts ATP to cyclic AMP and pyrophosphate. Mammalian membrane‐bound adenylyl cyclases are typically made up of two clusters of six TM domains separating two intracellular, overlapping catalytic domains that are the target for the nonselective activators forskolin, NKH477(except AC9, [392]) and Gα s(the stimulatory G protein α subunit). Adenosine and its derivatives (e.g. 2',5'‐dideoxyadenosine), acting through the P‐site, appear to be physiological inhibitors of adenylyl cyclase activity [485]. Three families of adenylyl cyclase are distinguishable: calmodulin(CALM1 CALM2 CALM3, P62158)‐stimulated (AC1, AC3 and AC8), Ca2+‐inhibitable (AC5, AC6 and AC9) and Ca2+‐insensitive (AC2, AC4 and AC7) forms.

483Nitric oxide has been proposed to inhibit AC5 and AC6 selectively [210], although it is unclear whether this phenomenon is of physiological significance. A soluble adenylyl cyclase has been described (ADCY10, Q96PN6 [48]), unaffected by either Gα or Gβ γ subunits, which has been suggested to be a cytoplasmic bicarbonate (pH‐insensitive) sensor [75]. It can be inhibited selectively by KH7 (pIC50 5.0‐5.5) [208].

484Soluble guanylyl cyclase (GTP diphosphate‐lyase (cyclising)), E.C. 4.6.1.2, is a heterodimer comprising α and β chains, both of which have two subtypes in man (predominantly α1β1; [544]). A haem group is associated with the β chain and is the target for the endogenous ligand nitric oxide (NO∙), and, potentially, carbon monoxide [150]. The enzyme converts guanosine‐5'‐triphosphate to the intracellular second messenger 3',5'‐guanosine monophosphate (cyclic GMP).

485ODQ also shows activity at other haem‐containing proteins [134], while YC1 may also inhibit cGMP‐hydrolysing phosphodiesterases [149, 159].

486Epacs are members of a family of guanine nucleotide exchange factors (ENSFM00250000000899), which also includes RapGEF5(GFR, KIAA0277, MR‐GEF, Q92565) and RapGEFL1 (Link‐GEFII, Q9UHV5). They are activated endogenously by cyclic AMP and with some pharmacological selectivity by 8‐pCPT‐2'‐O‐Me‐cAMP[126]. Once activated, Epacs induce an enhanced activity of the monomeric G proteins, Rap1 and Rap2 by facilitating binding of guanosine‐5'‐triphosphate in place of guanosine 5'‐diphosphate, leading to activation of phospholipase C [423].

4873',5'‐Cyclic nucleotide phosphodiesterases (PDEs, 3',5'‐cyclic‐nucleotide 5'‐nucleotidohydrolase), E.C. 3.1.4.17, catalyse the hydrolysis of a 3',5'‐cyclic nucleotide (usually cyclic AMP or cyclic GMP). Isobutylmethylxanthine is a nonselective inhibitor with an IC50 value in the millimolar range for all isoforms except PDE 8A, 8B and 9A. A 2',3'‐cyclic nucleotide 3'‐phosphodiesterase (E.C. 3.1.4.37 CNPase) activity is associated with myelin formation in the development of the CNS.

488PDE1A, 1B and 1C appear to act as soluble homodimers, while PDE2A is a membrane‐bound homodimer. PDE3A and PDE3B are membrane‐bound.

489PDE4 isoforms are essentially cyclic AMP specific. The potency of YM976 at other members of the PDE4 family has not been reported. PDE4B‐D long forms are inhibited by extracellular signal‐regulated kinase (ERK)‐mediated phosphorylation [212, 213]. PDE4A‐D splice variants can be membrane‐bound or cytosolic [217]. PDE4 isoforms may be labelled with [3H]rolipram.

490PDE6 is a membrane‐bound tetramer composed of two catalytic chains (PDE6A or PDE6C and PDE6B), an inhibitory chain (PDE6G or PDE6H) and the PDE6D chain. The enzyme is essentially cyclic GMP specific and is activated by the α‐subunit of transducin (Gα t) and inhibited by sildenafil, zaprinast and dipyridamole with potencies lower than those observed for PDE5A. Defects in PDE6B are a cause of retinitis pigmentosa and congenital stationary night blindness.

491Cooper DM et al. (2014) Adenylate cyclase‐centred microdomains. Biochem. J. 462: 199‐213 [PMID:25102028]

492Derbyshire ER et al. (2012) Structure and regulation of soluble guanylate cyclase. Annu. Rev. Biochem. 81: 533‐59 [PMID:22404633]

493Francis SH et al. (2011) Mammalian cyclic nucleotide phosphodiesterases: molecular mechanisms and physiological functions. Physiol. Rev. 91: 651‐90 [PMID:21527734]

494Nicol X et al. (2014) Routes to cAMP: shaping neuronal connectivity with distinct adenylate cyclases. Eur. J. Neurosci. 39: 1742‐51 [PMID:24628976]

495Schmidt M et al. (2013) Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions. Pharmacol. Rev. 65: 670‐709 [PMID:23447132]

496Steegborn C. (2014) Structure, mechanism, and regulation of soluble adenylyl cyclases ‐ similarities and differences to transmembrane adenylyl cyclases. Biochim. Biophys. Acta 1842: 2535‐47 [PMID:25193033]

497The cytochrome P450 enzyme family (CYP450), E.C. 1.14.‐.‐, were originally defined by their strong absorbance at 450 nm due to the reduced carbon monoxide‐complexed haem component of the cytochromes. They are an extensive family of haem‐containing monooxygenases with a huge range of both endogenous and exogenous substrates. Listed below are the human enzymes; their relationship with rodent CYP450 enzyme activities is obscure in that the species orthologue may not mediate metabolism of the same substrates. Although the majority of CYP450 enzyme activities are concentrated in the liver, the extrahepatic enzyme activities also contribute to patho/physiological processes. Genetic variation of CYP450 isoforms is widespread and likely underlies a significant proportion of the individual variation to drug administration. Further family members are included on the online database at www.GuidetoPHARMACOLOGY.org

498Guengerich FP et al. (2011) Orphans in the human cytochrome P450 superfamily: approaches to discovering functions and relevance in pharmacology. Pharmacol. Rev. 63: 684‐99 [PMID:21737533]

499Lorbek G et al. (2012) Cytochrome P450s in the synthesis of cholesterol and bile acids–from mouse models to human diseases. FEBS J. 279: 1516‐33 [PMID:22111624]

500Orr ST et al. (2012) Mechanism‐based inactivation (MBI) of cytochrome P450 enzymes: structure‐activity relationships and discovery strategies to mitigate drug‐drug interaction risks. J. Med. Chem. 55: 4896‐933 [PMID:22409598]

501Peñas‐Lledó EM et al. (2014) CYP2D6 variation, behaviour and psychopathology: implications for pharmacogenomics‐guided clinical trials. Br J Clin Pharmacol 77: 673‐83 [PMID:24033670]

502Ross AC et al. (2011) Cytochrome P450s in the regulation of cellular retinoic acid metabolism. Annu. Rev. Nutr. 31: 65‐87 [PMID:21529158]

503Shahabi P et al. (2014) Human cytochrome P450 epoxygenases: variability in expression and role in inflammation‐related disorders. Pharmacol. Ther. 144: 134‐61 [PMID:24882266]

504Werk AN et al. (2014) Functional gene variants of CYP3A4. Clin. Pharmacol. Ther. 96: 340‐8 [PMID:24926778]

505The principle endocannabinoids are 2‐arachidonoylglycerol(2AG) and anandamide(N‐arachidonoylethanolamine, AEA), thought to be generated on demand rather than stored, although this may not always be the case [10]. Mechanisms for release and re‐uptake of endocannabinoids (and related entities) are unclear, although candidates for intracellular transport have been suggested. For the generation of 2‐arachidonoylglycerol, the key enzyme involved is diacylglycerol lipase (DGL), whilst several routes for anandamide synthesis have been described, the best characterized of which involves N‐acylphosphatidylethanolamine‐phospholipase D (NAPE‐PLD, [438]). Inactivation of these endocannabinoids appears to occur predominantly through monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH) for 2‐arachidonoylglycerol and anandamide, respectively. Note that these enzymes also contribute to the turnover of many endogenous ligands inactive at CB1 and CB2 cannabinoid receptors, such as N‐oleoylethanolamide, N‐palmitoylethanolamine and 2‐oleoyl glycerol. In vitro experiments indicate that the endocannabinoids are also substrates for oxidative metabolism via cyclooxygenase, lipoxygenase and cytochrome P450 enzyme activities [9, 145, 450].

506Many of the compounds described as inhibitors are irreversible and so potency estimates will vary with incubation time. FAAH2 is not found in rodents [518] and few of the inhibitors described have been assessed at this enzyme activity. 2‐arachidonoylglycerol has been reported to be hydrolysed by multiple enzyme activities from neural preparations, including ABHD6(Q9BV23) [41], ABHD12 (8N2K0) [41], neuropathy target esterase (PNPLA6, Q8IY17[316]) and carboxylesterase 1 (CES1, P23141[533]). Although these have been incompletely defined, WWL70 has been described to inhibit ABHD6 selectively with a pIC50 value of 7.2 [284]. Other selective inhibitors of NAAA (with respect to FAAH) have been described, but these are not yet commercially available.

507Blankman JL et al. (2013) Chemical probes of endocannabinoid metabolism. Pharmacol. Rev. 65: 849‐71 [PMID:23512546]

508Fowler CJ. (2013) Transport of endocannabinoids across the plasma membrane and within the cell. FEBS J. 280: 1895‐904 [PMID:23441874]

509Hermanson DJ et al. (2014) Substrate‐selective COX‐2 inhibition as a novel strategy for therapeutic endocannabinoid augmentation. Trends Pharmacol. Sci. 35: 358‐67 [PMID:24845457]

510Savinainen JR et al. (2012) The serine hydrolases MAGL, ABHD6 and ABHD12 as guardians of 2‐arachidonoylglycerol signalling through cannabinoid receptors. Acta Physiol (Oxf) 204: 267‐76 [PMID:21418147]

511Ueda N et al. (2013) Metabolism of endocannabinoids and related N‐acylethanolamines: canonical and alternative pathways. FEBS J. 280: 1874‐94 [PMID:23425575]

512Wellner N et al. (2013) N‐acylation of phosphatidylethanolamine and its biological functions in mammals. Biochim. Biophys. Acta 1831: 652‐62 [PMID:23000428]

513Eicosanoids are 20‐carbon fatty acids, where the usual focus is the polyunsaturated analogue arachidonic acid and its metabolites. Arachidonic acid is thought primarily to derive from phospholipase A2 action on membrane phosphatidylcholine, and may be re‐cycled to form phospholipid through conjugation with coenzyme A and subsequently glycerol derivatives. Oxidative metabolism of arachidonic acid is conducted through three major enzymatic routes: cyclooxygenases; lipoxygenases and cytochrome P450‐like epoxygenases, particularly CYP2J2. Isoprostanes are structural analogues of the prostanoids (hence the nomenclature D‐, E‐, F‐isoprostanes and isothromboxanes), which are produced in the presence of elevated free radicals in a non‐enzymatic manner, leading to suggestions for their use as biomarkers of oxidative stress. Molecular targets for their action have yet to be defined.

514Prostaglandin (PG) G/H synthase, most commonly referred to as cyclooxygenase (COX, (5Z,8Z,11Z,14Z)‐icosa‐5,8,11,14‐tetraenoate,hydrogen‐donor: oxygen oxidoreductase) activity, catalyses the formation of PGG2 from arachidonic acid. Hydroperoxidase activity inherent in the enzyme catalyses the formation of PGH2 from PGG2. COX‐1 and ‐2 can be nonselectively inhibited by ibuprofen, ketoprofen, naproxen, indomethacin and paracetamol (acetaminophen). PGH2 may then be metabolised to prostaglandins and thromboxanes by various prostaglandin synthases in an apparently tissue‐dependent manner.

515Subsequent to the formation of PGH2, the cytochrome P450 activities thromboxane synthase (CYP5A1, TBXAS1, P24557, EC 5.3.99.5) and prostacyclin synthase (CYP8A1, PTGIS, Q16647, EC 5.3.99.4) generate thromboxane A2 and prostacyclin (PGI2), respectively. Additionally, multiple enzyme activities are able to generate prostaglandin E2(PGE2), prostaglandin D2(PGD2) and prostaglandin F2α(PGF2α). PGD2 can be metabolised to 9α,11β‐prostacyclin F2α through the multifunctional enzyme activity of AKR1C3. PGE2 can be metabolised to 9α,11β‐prostaglandin F2α through the 9‐ketoreductase activity of CBR1. Conversion of the 15‐hydroxyecosanoids, including prostaglandins, lipoxins and leukotrienes to their keto derivatives by the NAD‐dependent enzyme HPGD leads to a reduction in their biological activity.

516YS121 has been reported to inhibit mPGES1 and 5‐LOX with a pIC50value of 5.5 [263].

517The lipoxygenases (LOXs) are a structurally related family of non‐heme iron dioxygenases that function in the production, and in some cases metabolism, of fatty acid hydroperoxides. For arachidonic acid as substrate, these products are hydroperoxyeicosatetraenoic acids (HPETEs). In humans there are five lipoxygenases, the 5S‐(arachidonate: oxygen 5‐oxidoreductase), 12R‐(arachidonate 12‐lipoxygenase, 12R‐type), 12S‐(arachidonate: oxygen 12‐oxidoreductase), and two distinct 15S‐(arachidonate: oxygen 15‐oxidoreductase) LOXs that oxygenate arachidonic acid in different positions along the carbon chain and form the corresponding 5S‐, 12S‐, 12R‐, or 15S‐hydroperoxides, respectively.

518An 8‐LOX (EC 1.13.11.40, arachidonate:oxygen 8‐oxidoreductase) may be the mouse orthologue of 15‐LOX‐2 [158]. Some general LOX inhibitors are nordihydroguiaretic acid and esculetin. Zileuton and caffeic acid are used as 5‐lipoxygenase inhibitors, while baicalein and CDC are 12‐lipoxygenase inhibitors. The specificity of these inhibitors has not been rigorously assessed with all LOX forms: baicalein, along with other flavonoids, such as fisetin and luteolin, also inhibits 15‐LOX‐1 [414].

519Leukotriene A4(LTA4), produced by 5‐LOX activity, and lipoxins may be subject to further oxidative metabolism; ω‐hydroxylation is mediated by CYP4F2 and CYP4F3, while β‐oxidation in mitochondria and peroxisomes proceeds in a manner dependent on coenzyme A conjugation. Conjugation of LTA4 at the 6 position with reduced glutathione to generate LTC4 occurs under the influence of leukotriene C4 synthase, with the subsequent formation of LTD4 and LTE4, all three of which are agonists at CysLT receptors. LTD4 formation is catalysed by γ‐glutamyltransferase, and subsequently dipeptidase 2 removes the terminal glycine from LTD4 to generate LTE4. Leukotriene A4 hydrolase converts the 5,6‐epoxide LTA4 to the 5‐hydroxylated LTB4, an agonist for BLT receptors. LTA4 is also acted upon by 12S‐LOX to produce the trihydroxyeicosatetraenoic acids lipoxins LXA4 and LXB4. Treatment with a LTA4 hydrolase inhibitor in a murine model of allergic airway inflammation increased LXA4 levels, in addition to reducing LTB4, in lung lavage fluid [400].

520LTA4 hydrolase is also involved in biosynthesis of resolvin Es. Aspirin has been reported to increase endogenous formation of 18S‐hydroxyeicosapentaenoate (18S‐HEPE) compared with 18R‐HEPE, a resolvin precursor. Both enantiomers may be metabolised by human recombinant 5‐LOX; recombinant LTA4hydrolase converted chiral 5S(6)‐epoxide‐containing intermediates to resolvin E1 and 18S‐resolvin E1 [358].

521LTA4H is a member of a family of arginyl aminopeptidases (ENSFM00250000001675), which also includes aminopeptidase B (RNPEP, 9H4A4) and aminopeptidase B‐like 1 (RNPEPL1, Q9HAU8). Dipeptidase 1 and 2 are members of a family of membrane dipeptidases, which also includes (DPEP3, Q9H4B8) for which LTD4 appears not to be a substrate.

522Durand T et al. (2011) Isoprostanes and phytoprostanes: Bioactive lipids. Biochimie 93: 52‐60 [PMID:20594988]

523Floyd CN et al. (2014) Mechanisms of aspirin resistance. Pharmacol. Ther. 141: 69‐78 [PMID:23993980]

524Haeggström JZ et al. (2011) Lipoxygenase and leukotriene pathways: biochemistry, biology, and roles in disease. Chem. Rev. 111: 5866‐98 [PMID:21936577]

525Korotkova M et al. (2014) Persisting eicosanoid pathways in rheumatic diseases. Nat Rev Rheumatol 10: 229‐41 [PMID:24514915]

526Krieg P et al. (2014) The role of lipoxygenases in epidermis. Biochim. Biophys. Acta 1841: 390‐400 [PMID:23954555]

527Rodríguez M et al. (2014) Polarization of the innate immune response by prostaglandin E2: a puzzle of receptors and signals. Mol. Pharmacol. 85: 187‐97 [PMID:24170779]

528Smith WL et al. (2011) Enzymes of the cyclooxygenase pathways of prostanoid biosynthesis. Chem. Rev. 111: 5821‐65 [PMID:21942677]

529Tai HH et al. (2011) Regulation of 15‐hydroxyprostaglandin dehydrogenase (15‐PGDH) by non‐steroidal anti‐inflammatory drugs (NSAIDs). Prostaglandins Other Lipid Mediat. 96: 37‐40 [PMID:21763448]

530The inhibitory neurotransmitter γ‐aminobutyrate (GABA, 4‐aminobutyrate) is generated in neurones by glutamic acid decarboxylase. GAD1 and GAD2 are differentially expressed during development, where GAD2 is thought to subserve a trophic role in early life and is distributed throughout the cytoplasm. GAD1 is expressed in later life and is more associated with nerve terminals [128] where GABA is principally accumulated in vesicles through the action of the vesicular inhibitory amino acid transporter SLC32A1. The role of γ‐aminobutyraldehyde dehydrogenase (ALDH9A1) in neurotransmitter GABA synthesis is less clear. Following release from neurons, GABA may interact with either GABAA or GABAB receptors and may be accumulated in neurones and glia through the action of members of the SLC6 family of transporters. Successive metabolism through GABA transaminase and succinate semialdehyde dehydrogenase generates succinic acid, which may be further metabolized in the mitochondria in the tricarboxylic acid cycle.

531Errichiello L et al. (2011) Temporal lobe epilepsy and anti glutamic acid decarboxylase autoimmunity. Neurol. Sci. 32: 547‐50 [PMID:21468678]

532Kim KJ et al. (2011) Succinic semialdehyde dehydrogenase: biochemical‐molecular‐clinical disease mechanisms, redox regulation, and functional significance. Antioxid. Redox Signal. 15: 691‐718 [PMID:20973619]

533McQuail JA et al. (2015) Molecular aspects of age‐related cognitive decline: the role of GABA signaling. Trends Mol Med 21: 450‐60 [PMID:26070271]

534Pan ZZ. (2012) Transcriptional control of Gad2. Transcription 3: 68‐72 [PMID:22414751]

535Verrotti A et al. (2012) Seizures and type 1 diabetes mellitus: current state of knowledge. Eur. J. Endocrinol. 167: 749‐58 [PMID:22956556]

536Phospholipids are the basic barrier components of membranes in eukaryotic cells divided into glycerophospholipids (phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol and its phosphorylated derivatives) and sphingolipids (ceramide phosphorylcholine and ceramide phosphorylethanolamine).

537Phosphatidylinositol may be phosphorylated at either 3‐ or 4‐ positions on the inositol ring by PI 3‐kinases or PI 4‐kinases, respectively.

538Phosphatidylinositol 3‐kinasesPhosphatidylinositol 3‐kinases (PI3K, provisional nomenclature) catalyse the introduction of a phosphate into the 3‐position of phosphatidylinositol (PI), phosphatidylinositol 4‐phosphate (PIP) or phosphatidylinositol 4,5‐bisphosphate (PIP2). There is evidence that PI3K can also phosphorylate serine/threonine residues on proteins. In addition to the classes described below, further serine/threonine protein kinases, including ATM (Q13315) and mTOR(P42345), have been described to phosphorylate phosphatidylinositol and have been termed PI3K‐related kinases. Structurally, PI3K have common motifs of at least one C2, calcium‐binding domain and helical domains, alongside structurally‐conserved catalytic domains. Wortmannin and LY294002 are widely‐used inhibitors of PI3K activities. Wortmannin is irreversible and shows modest selectivity between Class I and Class II PI3K, while LY294002 is reversible and selective for Class I compared to Class II PI3K.

539Class I PI3Ks (EC 2.7.1.153) phosphorylate phosphatidylinositol 4,5‐bisphosphate to generate phosphatidylinositol 3,4,5‐trisphosphate and are heterodimeric, matching catalytic and regulatory subunits. Class IA PI3Ks include p110α, p110β and p110δ catalytic subunits, with predominantly p85 and p55 regulatory subunits. The single catalytic subunit that forms Class IB PI3K is p110γ. Class IA PI3Ks are more associated with receptor tyrosine kinase pathways, while the Class IB PI3K is linked more with GPCR signalling.

540Class II PI3Ks (EC 2.7.1.154) phosphorylate phosphatidylinositol to generate phosphatidylinositol 3‐phosphate (and possibly phosphatidylinositol 4‐phosphate to generate phosphatidylinositol 3,4‐bisphosphate). Three monomeric members exist, PI3K‐C2α, β and γ, and include Ras‐binding, Phox homology and two C2domains.

541The only class III PI3K isoform (EC 2.7.1.137) is a heterodimer formed of a catalytic subunit (VPS34) and regulatory subunit (VPS15).

542Phosphatidylinositol 4‐kinasesPhosphatidylinositol 4‐kinases (EC 2.7.1.67) generate phosphatidylinositol 4‐phosphate and may be divided into higher molecular weight type III and lower molecular weight type II forms.

543Type I PIP kinases are required for the production of the second messenger phosphatidylinositol 4,5‐bisphosphate (PtdIns(4,5)P2) by phosphorylating PtdIns(4)P [397]. This enzyme family is also known as type I PIP(5)Ks.

544Type II PIP kinases are essential for the production of the second messenger phosphatidylinositol 4,5‐bisphosphate (PtdIns(4,5)P2) by phosphorylating PtdIns(5)P [397]. This enzyme family is also known as type II PIP(5)Ks.

545Neubauer HA et al. (2013) Roles, regulation and inhibitors of sphingosine kinase 2. FEBS J. 280: 5317‐36 [PMID:23638983]

546Truman JP et al. (2014) Evolving concepts in cancer therapy through targeting sphingolipid metabolism. Biochim. Biophys. Acta 1841: 1174‐88 [PMID:24384461]

547Phosphoinositide‐specific phospholipase C (PLC, EC 3.1.4.11), catalyses the hydrolysis of PIP2 to IP3 and 1,2‐diacylglycerol, each of which have major second messenger functions. Two domains, X and Y, essential for catalytic activity, are conserved in the different forms of PLC. Isoforms of PLC‐β are activated primarily by G protein‐coupled receptors through members of the Gq/11 family of G proteins. The receptor‐mediated activation of PLC‐γ involves their phosphorylation by receptor tyrosine kinases (RTK) in response to activation of a variety of growth factor receptors and immune system receptors. PLC‐ε1 may represent a point of convergence of signalling via both G protein‐coupled and catalytic receptors. Ca2+ ions are required for catalytic activity of PLC isoforms and have been suggested to be the major physiological form of regulation of PLC‐δ activity. PLC has been suggested to be activated non‐selectively by the small molecule m3M3FBS[21], although this mechanism of action has been questioned [271]. The aminosteroid U73122 has been described as an inhibitor of phosphoinositide‐specific PLC [447], although its selectivity among the isoforms is untested and it has been reported to occupy the H1 histamine receptor [222].

548A series of PLC‐like proteins (PLCL1, Q15111; PLCL2, Q9UPR0 and PLCH1, Q4KWH8) form a family with PLCδ and PLCζ1 isoforms, but appear to lack catalytic activity.

549PLC‐δ2 has been cloned from bovine sources [328].

550Phospholipase A2(PLA2, EC 3.1.1.4) cleaves the sn‐2 fatty acid of phospholipids, primarily phosphatidylcholine, to generate lysophosphatidylcholine and arachidonic acid. Most commonly‐used inhibitors (e.g. bromoenol lactone, arachidonyl trifluoromethyl ketone or methyl arachidonyl fluorophosphonate) are either non‐selective within the family of phospholipase A2 enzymes or have activity against other eicosanoid‐metabolising enzymes.

551Secreted or extracellular forms: sPLA2‐1B, sPLA2‐2A, sPLA2‐2D, sPLA2‐2E, sPLA2‐2F, sPLA2‐3, sPLA2‐10 and sPLA2‐12A

552Cytosolic, calcium‐dependent forms: cPLA2‐4A, cPLA2‐4B, cPLA2‐4C, cPLA2‐4D, cPLA2‐4E and cPLA2‐4F

553Other forms: PLA2‐G5, iPLA2‐G6, PLA2‐G7 and PAFAH2 (platelet‐activating factor acetylhydrolase 2)

554The sequence of PLA2‐2C suggests a lack of catalytic activity, while PLA2‐12B (GXIIB, GXIII sPLA2‐like) appears to be catalytically inactive [413]. A further fragment has been identified with sequence similarities to Group II PLA2 members. Otoconin 90 (OC90) shows sequence homology to PLA2‐G10.

555A binding protein for secretory phospholipase A2 has been identified which shows modest selectivity for sPLA2‐1B over sPLA2‐2A, and also binds snake toxin phospholipase A2[12]. The binding protein appears to have clearance function for circulating secretory phospholipase A2, as well as signalling functions, and is a candidate antigen for idiopathic membraneous nephropathy [27].

556PLA2‐G7 and PAFAH2 also express platelet‐activating factor acetylhydrolase activity (EC 3.1.1.47).

557Phosphatidylcholine‐specific phospholipase D (PLD, EC 3.1.1.4) catalyses the formation of phosphatidic acid from phosphatidylcholine. In addition, the enzyme can make use of alcohols, such as butanol in a transphosphatidylation reaction [399].

558A lysophospholipase D activity (ENPP2, Q13822, also known as ectonucleotide pyrophosphatase/phosphodiesterase 2, phosphodiesterase I, nucleotide pyrophosphatase 2, autotaxin) has been described, which not only catalyses the production of lysophosphatidic acid (LPA) from lysophosphatidylcholine, but also cleaves ATP (see Goding et al., 2003 [175]). Additionally, an N‐acylethanolamine‐specific phospholipase D (NAPEPLD, Q6IQ20) has been characterized, which appears to have a role in the generation of endocannabinoids/endovanilloids, including anandamide[362]. This enzyme activity appears to be enhanced by polyamines in the physiological range [292] and fails to transphosphatidylate with alcohols [381].

559Three further, less well‐characterised isoforms are PLD3 (PLD3, Q8IV08, other names Choline phosphatase 3, HindIII K4L homolog, Hu‐K4), PLD4 (PLD4, Q96BZ4, other names Choline phosphatase 4, Phosphatidylcholine‐hydrolyzing phospholipase, D4C14orf175 UNQ2488/PRO5775) and PLD5 (PLD5, Q8N7P1). PLD3 has been reported to be involved in myogenesis [365]. PLD4 is described not to have phospholipase D catalytic activity [539], but has been associated with inflammatory disorders [360, 468, 484]. Sequence analysis suggests that PLD5 is catalytically inactive.

560Lipid phosphate phosphatases, divided into phosphatidic acid phosphatases or lipins catalyse the dephosphorylation of phosphatidic acid (and other phosphorylated lipid derivatives) to generate inorganic phosphate and diacylglycerol. PTEN, a phosphatase and tensin homolog (BZS, MHAM, MMAC1, PTEN1, TEP1) is a phosphatidylinositol 3,4,5‐trisphosphate 3‐phosphatase which acts as a tumour suppressor by reducing cellular levels of PI 3,4,5‐P, thereby toning down activity of PDK1 and PKB. Loss‐of‐function mutations are frequently identified as somatic mutations in cancers.

561Astudillo AM et al. (2012) Dynamics of arachidonic acid mobilization by inflammatory cells. Biochim. Biophys. Acta 1821: 249‐56 [PMID:22155285]

562Berridge MJ. (2009) Inositol trisphosphate and calcium signalling mechanisms. Biochim. Biophys. Acta 1793: 933‐40 [PMID:19010359]

563Brown HA et al. (2011) Introduction to lipid biochemistry, metabolism, and signaling. Chem. Rev. 111: 5817‐20 [PMID:21951202]

564Bunney TD et al. (2011) PLC regulation: emerging pictures for molecular mechanisms. Trends Biochem. Sci. 36: 88‐96 [PMID:20870410]

565Clayton EL et al. (2013) Mammalian phosphatidylinositol 4‐kinases as modulators of membrane trafficking and lipid signaling networks. Prog. Lipid Res. 52: 294‐304 [PMID:23608234]

566Dan P et al. (2012) Phospholipase A_2 activities in skin physiology and pathology. Eur. J. Pharmacol. 691: 1‐8 [PMID:22819703]

567Harden TK et al. (2011) Mechanism of activation and inactivation of Gq/phospholipase C‐β signaling nodes. Chem. Rev. 111: 6120‐9 [PMID:21988240]

568Hermansson M et al. (2011) Mechanisms of glycerophospholipid homeostasis in mammalian cells. Prog. Lipid Res. 50: 240‐57 [PMID:21382416]

569Hui DY. (2012) Phospholipase A(2) enzymes in metabolic and cardiovascular diseases. Curr. Opin. Lipidol. 23: 235‐40 [PMID:22327613]

570Kok BP et al. (2012) Unlike two peas in a pod: lipid phosphate phosphatases and phosphatidate phosphatases. Chem. Rev. 112: 5121‐46 [PMID:22742522]

571Liu Y et al. (2010) Phosphoinositide phosphatases in cell biology and disease. Prog. Lipid Res. 49: 201‐17 [PMID:20043944]

572Lyon AM et al. (2013) Structural insights into phospholipase C‐β function. Mol. Pharmacol. 84: 488‐500 [PMID:23880553]

573Quach ND et al. (2014) Secretory phospholipase A2 enzymes as pharmacological targets for treatment of disease. Biochem. Pharmacol. 90: 338‐48 [PMID:24907600]

574Selvy PE et al. (2011) Phospholipase D: enzymology, functionality, and chemical modulation. Chem. Rev. 111: 6064‐119 [PMID:21936578]

575Song MS et al. (2012) The functions and regulation of the PTEN tumour suppressor. Nat. Rev. Mol. Cell Biol. 13: 283‐96 [PMID:22473468]

576Stephens L et al. (2013) More paths to PI3Kγ. PLoS Biol. 11: e1001594 [PMID:23853549]

577Sánchez‐Fernández G et al. (2014) Gαq signalling: the new and the old. Cell. Signal. 26: 833‐48 [PMID:24440667]

578Woscholski R. (2014) Chemical intervention tools to probe phosphoinositide‐dependent signalling. Biochem. Soc. Trans. 42: 1343‐8 [PMID:25233413]

579Yagami T et al. (2014) The role of secretory phospholipase A_2 in the central nervous system and neurological diseases. Mol. Neurobiol. 49: 863‐76 [PMID:24113843]

580Haem oxygenase (heme,hydrogen‐donor:oxygen oxidoreductase (α‐methene‐oxidizing, hydroxylating)), E.C. 1.14.99.3, converts heme into biliverdin and carbon monoxide, utilizing NADPH as cofactor.

581The existence of a third non‐catalytic version of haem oxygenase, HO3, has been proposed, although this has been suggested to be a pseudogene [202]. The chemical tin protoporphyrin IX acts as a haem oxygenase inhibitor in rat liver with an IC50 value of 11 nM [120].

582Abraham NG et al. (2008) Pharmacological and clinical aspects of heme oxygenase. Pharmacol. Rev. 60: 79‐127 [PMID:18323402]

583George EM et al. (2014) The heme oxygenases: important regulators of pregnancy and preeclampsia. Am. J. Physiol. Regul. Integr. Comp. Physiol. 307: R769‐77 [PMID:24898840]

584Gozzelino R et al. (2010) Mechanisms of cell protection by heme oxygenase‐1. Annu. Rev. Pharmacol. Toxicol. 50: 323‐54 [PMID:20055707]

585Poulos TL. (2014) Heme enzyme structure and function. Chem. Rev. 114: 3919‐62 [PMID:24400737]

586Rochette L et al. (2013) Carbon monoxide: mechanisms of action and potential clinical implications. Pharmacol. Ther. 137: 133‐52 [PMID:23026155]

587Wegiel B et al. (2013) The social network of carbon monoxide in medicine. Trends Mol Med 19: 3‐11 [PMID:23140858]

588Hydrogen sulfide is a putative gasotransmitter, with similarities to nitric oxide and carbon monoxide. Although the enzymes indicated have multiple enzymatic activities, the focus here is the generation of hydrogen sulphide and the enzymatic characteristics are described accordingly. Cystathionine β‐synthase and cystathionine γ‐lyase are pyridoxal phosphate‐dependent enzymes, while 3‐mercaptopyruvate sulfurtransferase functions as a pyridoxal phosphate‐independent pathway.

589Bełtowski J. (2015) Hydrogen sulfide in pharmacology and medicine–An update. Pharmacol Rep 67: 647‐58 [PMID:25933982]

590Li L et al. (2011) Hydrogen sulfide and cell signaling. Annu. Rev. Pharmacol. Toxicol. 51: 169‐87 [PMID:21210746]

591Nagy P et al. (2014) Chemical aspects of hydrogen sulfide measurements in physiological samples. Biochim. Biophys. Acta 1840: 876‐91 [PMID:23769856]

592Wallace JL et al. (2015) Hydrogen sulfide‐based therapeutics: exploiting a unique but ubiquitous gasotransmitter. Nat Rev Drug Discov 14: 329‐45 [PMID:25849904]

593Wang R et al. (2015) The role of H2S bioavailability in endothelial dysfunction. Trends Pharmacol. Sci. [PMID:26071118]

594listed in this section are hydrolases not accumulated in other parts of the Concise Guide, such as monoacylglycerol lipase and acetylcholinesterase. Pancreatic lipase is the predominant mechanism of fat digestion in the alimentary system; its inhibition is associated with decreased fat absorption. CES1 is present at lower levels in the gut than CES2 (P23141), but predominates in the liver, where it is responsible for the hydrolysis of many aliphatic, aromatic and steroid esters. Hormone‐sensitive lipase is also a relatively non‐selective esterase associated with steroid ester hydrolysis and triglyceride metabolism, particularly in adipose tissue. Endothelial lipase is secreted from endothelial cells and regulates circulating cholesterol in high density lipoproteins.

595Markey, G.M. (2011) Carboxylesterase 1 (Ces1): from monocyte marker to major player. J Clin Pathol 64: 107‐109 [PMID:21177752]

596The sugar alcohol D‐myo‐inositol is a component of the phosphatidylinositol signalling cycle, where the principal second messenger is inositol 1,4,5‐trisphosphate, IP3, which acts at intracellular ligand‐gated ion channels, IP3 receptors to elevate intracellular calcium. IP3 is recycled to inositol by phosphatases or phosphorylated to form other active inositol polyphosphates. Inositol produced from dephosphorylation of IP3 is recycled into membrane phospholipid under the influence of phosphatidyinositol synthase activity (CDP‐diacylglycerol‐inositol 3‐phosphatidyltransferase [EC 2.7.8.11]).

597Inositol 1,4,5‐trisphosphate 3‐kinases (E.C. 2.7.1.127, ENSFM00250000001260) catalyse the generation of inositol 1,3,4,5‐tetrakisphosphate (IP4) from IP3. IP3 kinase activity is enhanced in the presence of calcium/calmodulin(CALM1 CALM2 CALM3, P62158) [91].

598Members of this family exhibit phosphatase activity towards IP3, as well as towards other inositol derivatives, including the phospholipids PIP2 and PIP3. With IP3 as substrate, 1‐phosphatase (EC 3.1.3.57) generates 4,5,‐IP2, 4‐phosphatases (EC 3.1.3.66, ENSFM00250000001432) generate 1,5,‐IP2 and 5‐phosphatases (E.C. 3.1.3.36 or 3.1.3.56) generate 1,4,‐IP2.

599In vitro analysis suggested IP3 and IP4 were poor substrates for SKIP, synaptojanin 1 and synaptojanin 2, but suggested that PIP2 and PIP3 were more efficiently hydrolysed [422].

600Inositol monophosphatase (E.C. 3.1.3.25, IMPase, myo‐inositol‐1(or 4)‐phosphate phosphohydrolase) is a magnesium‐dependent homodimer which hydrolyses myo‐inositol monophosphate to generate myo‐inositol and phosphate. Glycerol may be a physiological phosphate acceptor. Li+ is a nonselective un‐competitive inhibitor more potent at IMPase 1 (pKi ca. 3.5, [324]; pIC50 3.2, [359]) than IMPase 2 (pIC50 1.8‐2.1, [359]). IMPase activity may be inhibited competitively by L690330(pKi 5.5, [324]), although the enzyme selectivity is not yet established.

601Polymorphisms in either of the genes encoding these enzymes have been linked with bipolar disorder [443, 444, 540]. Disruption of the gene encoding IMPase 1, but not IMPase 2, appears to mimic the effects of Li+ in mice [97, 98].

602Barker CJ et al. (2013) New horizons in cellular regulation by inositol polyphosphates: insights from the pancreatic β‐cell. Pharmacol. Rev. 65: 641‐69 [PMID:23429059]

603Billcliff PG et al. (2014) Inositol lipid phosphatases in membrane trafficking and human disease. Biochem. J. 461: 159‐75 [PMID:24966051]

604Chiu CT et al. (2010) Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders. Pharmacol. Ther. 128: 281‐304 [PMID:20705090]

605Pirruccello M et al. (2012) Inositol 5‐phosphatases: insights from the Lowe syndrome protein OCRL. Trends Biochem. Sci. 37: 134‐43 [PMID:22381590]

606Schell MJ. (2010) Inositol trisphosphate 3‐kinases: focus on immune and neuronal signaling. Cell. Mol. Life Sci. 67: 1755‐78 [PMID:20066467]

607Lanosterol is a precursor for cholesterol, which is synthesized primarily in the liver in a pathway often described as the mevalonate or HMG‐CoA reductase pathway. The first two steps (formation of acetoacetyl CoA and the mitochondrial generation of (S)‐3‐hydroxy‐3‐methylglutaryl‐CoA) are also associated with oxidation of fatty acids.

608Miziorko HM. (2011) Enzymes of the mevalonate pathway of isoprenoid biosynthesis. Arch. Biochem. Biophys. 505: 131‐43 [PMID:20932952]

609Rozman D et al. (2010) Perspectives of the non‐statin hypolipidemic agents. Pharmacol. Ther. 127: 19‐40 [PMID:20420853]

610Seiki S et al. (2009) Pharmacologic inhibition of squalene synthase and other downstream enzymes of the cholesterol synthesis pathway: a new therapeutic approach to treatment of hypercholesterolemia. Cardiol Rev 17: 70‐6 [PMID:19367148]

611Zhang H et al. (2014) Cholesterol and lipoprotein metabolism: Early Career Committee contribution. Arterioscler. Thromb. Vasc. Biol. 34: 1791‐4 [PMID:25142876]

612van der Burgh R et al. (2012) Mevalonate kinase deficiency, a metabolic autoinflammatory disease. Clin. Immunol. [PMID:23110805]

613The de novo synthesis and salvage of nucleosides have been targetted for therapeutic advantage in the treatment of particular cancers and gout. Dihydrofolate reductase produces tetrahydrofolate, a cofactor required for synthesis of purines, pyrimidines and amino acids. GART allows formylation of phosphoribosylglycinamide, an early step in purine biosynthesis. Dihydroorotate dehydrogenase produces orotate, a key intermediate in pyrimidine synthesis. IMP dehydrogenase generates xanthosine monophosphate, an intermediate in GTP synthesis.

614Thymidylate synthetase allows the interconversion of dUMP and dTMP, thereby acting as a crucial step in DNA synthesis. Purine nucleoside phosphorylase allows separation of a nucleoside into the nucleobase and ribose phosphate for nucleotide salvage. Xanthine dehydrogenase generates urate in the purine degradation pathway. Post‐translational modifications of xanthine dehydrogenase convert the enzymatic reaction to a xanthine oxidase, allowing the interconversion of hypoxanthine and xanthine, with the production (or consumption) of reactive oxygen species. Ribonucleotide reductases allow the production of deoxyribonucleotides from ribonucleotides.

615Battelli MG et al. (2014) Pathophysiology of circulating xanthine oxidoreductase: new emerging roles for a multi‐tasking enzyme. Biochim. Biophys. Acta 1842: 1502‐17 [PMID:24882753]

616Cantu‐Medellin N et al. (2013) Xanthine oxidoreductase‐catalyzed reduction of nitrite to nitric oxide: insights regarding where, when and how. Nitric Oxide 34: 19‐26 [PMID:23454592]

617Glander P et al. (2012) Inosine 5'‐monophosphate dehydrogenase activity as a biomarker in the field of transplantation. Clin. Chim. Acta 413: 1391‐7 [PMID:21889500]

618Munier‐Lehmann H et al. (2013) On dihydroorotate dehydrogenases and their inhibitors and uses. J. Med. Chem. 56: 3148‐67 [PMID:23452331]

619S1P (sphingosine 1‐phosphate) is a pro‐survival signal, in contrast to ceramide. It is formed by the sphingosine kinase‐catalysed phosphorylation of sphingosine. S1P can be released from cells to act as an agonist at a family of five G protein‐coupled receptors (S1P1‐5) but also has intracellular targets. S1P can be dephosphorylated back to sphingosine or hydrolysed to form hexadecanal and phosphoethanolamine. Sphingosine choline phosphotransferase (EC 2.7.8.10) generates sphingosylphosphocholine from sphingosine and CDP‐choline. Sphingosine β‐galactosyltransferase (EC 2.4.1.23) generates psychosine from sphingosine in the presence of UDP‐α‐D‐galactose. The molecular identities of these enzymes have not been confirmed.

620Chan H et al. (2013) Post‐translational regulation of sphingosine kinases. Biochim. Biophys. Acta 1831: 147‐56 [PMID:22801036]

621Khavandgar Z et al. (2015) Sphingolipid metabolism and its role in the skeletal tissues. Cell. Mol. Life Sci. 72: 959‐69 [PMID:25424644]

622Maceyka M et al. (2014) Sphingolipid metabolites in inflammatory disease. Nature 510: 58‐67 [PMID:24899305]

623Plano D et al. (2014) Importance of sphingosine kinase (SphK) as a target in developing cancer therapeutics and recent developments in the synthesis of novel SphK inhibitors. J. Med. Chem. 57: 5509‐24 [PMID:24471412]

624Pyne S et al. (2011) Translational aspects of sphingosine 1‐phosphate biology. Trends Mol Med 17: 463‐72 [PMID:21514226]

625Rosen H et al. (2013) Sphingosine‐1‐phosphate and its receptors: structure, signaling, and influence. Annu. Rev. Biochem. 82: 637‐62 [PMID:23527695]

626Schwalm S et al. (2014) Targeting the sphingosine kinase/sphingosine 1‐phosphate pathway to treat chronic inflammatory kidney diseases. Basic Clin. Pharmacol. Toxicol. 114: 44‐9 [PMID:23789924]

627Neubauer HA et al. (2013) Roles, regulation and inhibitors of sphingosine kinase 2. FEBS J. 280: 5317‐36 [PMID:23638983]

628Truman JP et al. (2014) Evolving concepts in cancer therapy through targeting sphingolipid metabolism. Biochim. Biophys. Acta 1841: 1174‐88 [PMID:24384461]

629Bigaud M et al. (2014) Second generation S1P pathway modulators: research strategies and clinical developments. Biochim. Biophys. Acta 1841: 745‐58 [PMID:24239768]

630Van Veldhoven PP et al. (2000) Human sphingosine‐1‐phosphate lyase: cDNA cloning, functional expression studies and mapping to chromosome 10q22(1). Biochim. Biophys. Acta 1487: 128‐34 [PMID:11018465]

631The thyroid hormones triiodothyronine and thyroxine, usually abbreviated as triiodothyronine and T4, respectively, are synthesized in the thyroid gland by sequential metabolism of tyrosine residues in the glycosylated homodimeric protein thyroglobulin (TG, P01266) under the influence of the haem‐containing protein iodide peroxidase. Iodide peroxidase/TPO is a haem‐containing enzyme, from the same structural family as eosinophil peroxidase (EPX, P11678), lactoperoxidase (LPO, P22079) and myeloperoxidase (MPO, P05164). Circulating thyroid hormone is bound to thyroxine‐binding globulin (SERPINA7, P05543).

632Tissue deiodinasesThese are 1 TM selenoproteins that remove an iodine from T4(3,3',5,5'‐tetraiodothyronine) to generate triiodothyronine(3,3',5‐triiodothyronine, a more potent agonist at thyroid hormone receptors) or rT3(rT3, 3,3',5'‐triiodothyronine, a relatively inactive analogue). DIO1 is also able to deiodinate RT3 to form 3,3'‐diidothyronine (T2). Iodotyrosine deiodinase is a 1TM homodimeric enzyme.

633Arrojo E Drigo R et al. (2013) Role of the type 2 iodothyronine deiodinase (D2) in the control of thyroid hormone signaling. Biochim. Biophys. Acta 1830: 3956‐64 [PMID:22967761]

634Darras VM et al. (2015) Intracellular thyroid hormone metabolism as a local regulator of nuclear thyroid hormone receptor‐mediated impact on vertebrate development. Biochim. Biophys. Acta 1849: 130‐41 [PMID:24844179]

635Darras VM et al. (2012) Iodothyronine deiodinase structure and function: from ascidians to humans. J. Endocrinol. 215: 189‐206 [PMID:22825922]

636Gereben B et al. (2008) Cellular and molecular basis of deiodinase‐regulated thyroid hormone signaling. Endocr. Rev. 29: 898‐938 [PMID:18815314]

637Waung JA et al. (2012) Thyroid hormone metabolism in skeletal development and adult bone maintenance. Trends Endocrinol. Metab. 23: 155‐62 [PMID:22169753]

638The hypoxia inducible factor (HIF) is a transcriptional complex that is involved in oxygen homeostasis [429]. At normal oxygen levels, the alpha subunit of HIF (HIF‐1α) is targeted for degradation by prolyl hydroxylation by the PHD proteins 1‐3 (HIF‐PHs) whch are 2‐oxoglutarate (2OG) oxygenases responsible for the post‐translational modification of a specific proline in each of the oxygen‐dependent degradation (ODD) domains of HIF‐1α. Hydroxylated HIFs are then targeted for proteasomal degradation via the von Hippel‐Lindau ubiquitination complex [232]. Under hypoxic conditions, the hydroxylation reaction is blunted which results in decreased HIF degradation. The surviving HIFs are then available to translocate to the nucleus where they heterodimerize with HIF‐1β, effecting increased expression of hypoxia‐inducible genes.

639HIF‐PH enzymes are being investigated as pharmacological targets as their inhibition mimics the hypoxic state and switches on transcription of genes associated with processes such as erythropoiesis and vasculogenesis [142]. Small molecule HIF‐PH inhibitors are in clinical trial as novel therapies for the amelioration of anemia associated with chronic kidney disease [46].

640The Poly ADP‐ribose polymerase family is a series of enzymes, where the best characterised members are nuclear proteins which are thought to function by binding to single strand breaks in DNA, allowing the recruitment of repair enzymes by the synthesis of NAD‐derived ADP‐ribose polymers, which are subsequently degraded by a glycohydrolase (PARG, Q86W56).

641Bürkle A et al. (2013) Poly(ADP‐ribose): PARadigms and PARadoxes. Mol. Aspects Med. 34: 1046‐65 [PMID:23290998]

642De Vos M et al. (2012) The diverse roles and clinical relevance of PARPs in DNA damage repair: current state of the art. Biochem. Pharmacol. 84: 137‐46 [PMID:22469522]

643Lord CJ et al. (2012) The DNA damage response and cancer therapy. Nature 481: 287‐94 [PMID:22258607]

644Sonnenblick A et al. (2015) An update on PARP inhibitors–moving to the adjuvant setting. Nat Rev Clin Oncol 12: 27‐41 [PMID:25286972]

645Farnesyltransferase is a member of the prenyltransferases family which also includes geranylgeranyltransferase types I (EC 2.5.1.59) and II (EC 2.5.1.60) [66]. Protein farnesyltransferase catalyses the post‐translational formation of a thioether linkage between the C‐1 of an isoprenyl group and a cysteine residue fourth from the C‐terminus of a protein (ie to the CaaX motif, where 'a' is an aliphatic amino acid and 'X' is usually serine, methionine, alanine or glutamine; leucine for EC 2.5.1.59) [156]. Farnesyltransferase is a dimer, composed of an alpha and beta subunit and requires Mg2+ and Zn2+ ions as cofactors. The active site is located between the subunits. Prenylation creates a hydrophobic domain on protein tails which acts as a membrane anchor.

646Substrates of the prenyltransferases include Ras, Rho, Rab, other Ras‐related small GTP‐binding proteins, G‐protein γ‐subunits, nuclear lamins, centromeric proteins and many proteins involved in visual signal transduction.

647In relation to the causative association between oncogenic Ras proteins and cancer, farnesyltransferase has become an important mechanistic drug discovery target.

648In humans, the peptidyl arginine deiminases (PADIs; HGNC family link) are a family of five enzymes, PADI1‐4 and PADI6. PADIs catalyze the deimination of protein L‐arginine residues to L‐citrulline and ammonia. The human isozymes exhibit tissue‐specific expression patterns [244].

649The RAS proteins (HRAS, NRAS and KRAS) are small membrane‐localised G protein‐like molecules of 21 kd. They act as an on/off switch linking receptor and non‐receptor tyrosine kinase activation to downstream cytoplasmic or nuclear events. Binding of GTP activates the switch, and hydrolysis of the GTP to GDP inactivates the switch.

650The RAS proto‐oncogenes are the most frequently mutated class of proteins in human cancers. Common mutations compromise the GTP‐hydrolysing ability of the proteins causing constitutive activation [457], which leads to increased cell proliferation and decreased apoptosis [550]. Because of their importance in oncogenic transformation these proteins have become the targets of intense drug discovery effort [23].

651Carbonic anhydrases facilitate the interconversion of water and carbon dioxide with bicarbonate ions and protons (EC 4.2.1.1), with over a dozen gene products identified in man. The enzymes function in acid‐base balance and the movement of carbon dioxide and water. They are targetted for therapeutic gain by particular antiglaucoma agents and diuretics.

652Alterio V et al. (2012) Multiple binding modes of inhibitors to carbonic anhydrases: how to design specific drugs targeting 15 different isoforms? Chem. Rev. 112: 4421‐68 [PMID:22607219]

653Cummins EP et al. (2014) Carbon dioxide‐sensing in organisms and its implications for human disease. Cell. Mol. Life Sci. 71: 831‐45 [PMID:24045706]

654Imtaiyaz Hassan M et al. (2013) Structure, function and applications of carbonic anhydrase isozymes. Bioorg. Med. Chem. 21: 1570‐82 [PMID:22607884]

655Sjöblom M. (2011) Duodenal epithelial sensing of luminal acid: role of carbonic anhydrases. Acta Physiol (Oxf) 201: 85‐95 [PMID:20632999]

656DNA topoisomerases regulate the supercoiling of nuclear DNA to influence the capacity for replication or transcription. The enzymatic function of this series of enzymes involves cutting the DNA to allow unwinding, followed by re‐attachment to reseal the backbone. Members of the family are targetted in anti‐cancer chemotherapy.

657Castelli S et al. (2012) Interaction between natural compounds and human topoisomerase I. Biol. Chem. 393: 1327‐40 [PMID:23109546]

658Chen SH et al. (2013) New mechanistic and functional insights into DNA topoisomerases. Annu. Rev. Biochem. 82: 139‐70 [PMID:23495937]

659Kathiravan MK et al. (2013) Topoisomerase as target for antibacterial and anticancer drug discovery. J Enzyme Inhib Med Chem 28: 419‐35 [PMID:22380774]

660Tomicic MT et al. (2013) Topoisomerase degradation, DSB repair, p53 and IAPs in cancer cell resistance to camptothecin‐like topoisomerase I inhibitors. Biochim. Biophys. Acta 1835: 11‐27 [PMID:23006513]