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Enzymatic Redox Chemistry 123 O L-lactate dehydrogenase H OH CO 2 − NADH NAD + CO 2 − (1) NAD + + H + + 2e − NADH − 0.32 V (2) CH 3 COCO 2 H + 2H + + 2e − CH 3 CH(OH)CO 2 H − 0.19 V (2) - (1) CH 3 COCO 2 H + NADH CH 3 CH(OH)CO 2 H + NAD + + 0.13 V Figure 6.2 Redox potentials in the lactate dehydrogenase reaction. potential of 0.19 V. We simply subtract the redox potentials as shown in Figure 6.2, giving an overall redox potential diVerence for the pyruvateto-lactate reaction of þ0.13 V. If the redox potential diVerence is above zero, then the reaction is thermodynamically favourable. The more positive the redox potential diVerence is, the more favourable the reaction is and the greater the equilibrium constant for the reaction (the equilibrium constant can be calculated from the redox potential diVerence using the Nernst equation – see physical chemistry texts). Remember that a highly positive redox potential means a strong oxidising agent, whilst a highly negative redox potential means a strong reducing agent. One Wnal point is that an enzyme cannot change the equilibrium constant of a chemical reactionthat is thermodynamically unfavourable, howeveritcan modify the redox potential of a cofactor bound at its active site by selectively stabilising either the oxidised or the reduced form of the cofactor. For example, the redox potential for free oxidised riboXavin is 0.20 V, whereas redox potentials of between 0.45 and þ0.15 V have been measured for Xavo-enzymes in general. Justas with amino acid side chain pK a values,enzymes areableto modify chemical reactivity at their active sites using subtle changes in micro-environment. 6.2 Nicotinamide adenine dinucleotide-dependent dehydrogenases The Wrst class of redox enzymes that we shall meet are the dehydrogenases (Table 6.1). These enzymes transfer two hydrogen atoms from a reduced substrate, which is usually an alcohol, to an electron acceptor. The electron acceptor in these enzymes is the coenzyme NAD, whose oxidised and reduced forms are shown in Figure 6.3. The redox-active part of this coenzyme is the nicotinamide heterocyclic ring. In the oxidised form NAD þ this is a pyridinium salt, which is reduced to a 1,4- dihydro-pyridine in the reduced form NADH. A phosphorylated version of the cofactor is also found, bearing a 2 0 -phosphate on the adenosine portion of the structure, which is written as NADP þ in the oxidised form and NADPH
Table 6.1 Classes of redox enzymes. H C H X Oxidases (1) NAD þ (X ¼ O, NH) (2) Xavin/1e acceptors (X ¼ O, NH, CHR) Flavin / O2 (X ¼ NH) H hydroxylases Aromatic C2H: (1) Flavin/O2/NADPH (2) Pterin/O2/NADPH C C Aliphatic or aromatic C2H: (1) P450=O2=NADH (2) Non-haem metal/O2=e donor (3) Fe 2þ =O2=a-ketoglutarate C C HO OH Dioxygenases (dihydroxylating) C C Dioxygenases (C2C cleaving) Fe 2þ =O2=flavin=NADH H C O O C 124 Chapter 6 C X + 2H + + 2e − Dehydrogenases OH Mono-oxygenases, Fe 2þ or Fe 3þ =O2 A H H A + 2H + + 2e − Peroxidases (1) P450=H2O2 (2) Non-haem metal/H2O2 H 2H + + 2e − Hydrogenases Ni 2þ /factor F420 Transformation Enzyme class Redox cofactors
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Introduction to Enzyme and Coenzyme
Page 4 and 5:
Introduction to Enzyme and Coenzyme
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Contents Preface Representation of
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Contents vii 8 Enzymatic Addition/E
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Representation of Protein Three-Dim
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2 Chapter 1 H 2 N O NH 2 + H 2 O Ja
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4 Chapter 1 Table 1.1 The vitamins.
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6 Chapter 1 has very diVerent biolo
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2 All Enzymes are Proteins 2.1 Intr
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10 Chapter 2 (Gln). There are three
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12 Chapter 2 AAA Lys ACA Thr AGA Ar
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14 Chapter 2 H N O H N O Figure 2.8
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16 Chapter 2 (a) (b) Figure 2.12 St
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18 Chapter 2 Figure 2.14 Structure
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20 Chapter 2 (a) X Y Enzyme + Subst
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22 Chapter 2 maintaining protein te
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24 Chapter 2 surface glycoprotein g
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26 Chapter 2 O-linked glycosylation
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28 Chapter 2 Metalloproteins I. Ber
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30 Chapter 3 O N H R CO 2 H acylase
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32 Chapter 3 (a) Free energy uncata
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34 Chapter 3 Ester k rel Effective
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36 Chapter 3 3.4 The importance of
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38 Chapter 3 pKa Tyrosine CH 2 O H
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40 Chapter 3 glycoside hydrolysis (
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42 Chapter 3 O O Glu 143 O − R H
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44 Chapter 3 the hydroxyl groups of
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46 Chapter 3 E + S E + S ES' ES ‘
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48 Chapter 3 Examination of protein
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50 Chapter 3 (Note: Similar results
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52 Chapter 4 (1) Direct UV (2) Radi
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54 Chapter 4 Figure 4.3 PuriWcation
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56 Chapter 4 The two kinetic consta
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58 Chapter 4 Lineweaver−Burk Plot
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60 Chapter 4 E + S K i EI+ I ES E +
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62 Chapter 4 (2) by treatment with
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64 Chapter 4 In general the stereoc
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H D T CO 2 H CO − 2 T HO D H −
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68 Chapter 4 4.5 The existence of i
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70 Chapter 4 18O 18O 18 O O P O −
Page 83 and 84: 72 Chapter 4 If a reaction involves
Page 85 and 86: 74 Chapter 4 O D H ketosteroid isom
Page 87 and 88: 76 Chapter 4 Table 4.3 Group speciW
Page 89 and 90: 78 Chapter 4 [S] (mM) Rate of produ
Page 91 and 92: 80 Chapter 4 Enzyme kinetics I.H. S
Page 93 and 94: 82 Chapter 5 O = C NHR peptidase H
Page 95 and 96: 84 Chapter 5 non-speciWc protease c
Page 97 and 98: 86 Chapter 5 His 57 His 57 Asp 102
Page 99 and 100: 88 Chapter 5 Other serine proteases
Page 101 and 102: 90 Chapter 5 Figure 5.8 Structure o
Page 103 and 104: 92 Chapter 5 now predominate Perhap
Page 105 and 106: OH R O Zn 2+ O O O Ph Glu 270 H 2 N
Page 107 and 108: 96 Chapter 5 CASE STUDY: HIV-1 prot
Page 109 and 110: 98 Chapter 5 HO Transition state pe
Page 111 and 112: 100 Chapter 5 The aminoacyl group o
Page 113 and 114: 102 Chapter 5 5.5 Enzymatic phospho
Page 115 and 116: 104 Chapter 5 Figure 5.29 Structure
Page 119 and 120: 108 Chapter 5 Adenosine 5 0 -tripho
Page 121 and 122: 110 Chapter 5 functional group. Gly
Page 123 and 124: 112 Chapter 5 CH 2 OH O RO HO OH O
Page 125 and 126: 114 Chapter 5 to these atoms that t
Page 127 and 128: 116 Chapter 5 Problems (1) Using pa
Page 129 and 130: 118 Chapter 5 (6) Retaining glycosi
Page 131 and 132: 120 Chapter 5 J.R. Knowles (1980) E
Page 133: 122 Chapter 6 +1.0 Redox potential
Page 137 and 138: 126 Chapter 6 An important point is
Page 139 and 140: 128 Chapter 6 O H − OH − O OH H
Page 141 and 142: 130 Chapter 6 one-electron transfer
Page 143 and 144: 132 Chapter 6 (a) R N H + N O R N H
Page 145 and 146: 134 Chapter 6 Inactivation by trany
Page 149 and 150: 138 Chapter 6 (a) (b) Figure 6.23 S
Page 151 and 152: 140 Chapter 6 glutathione called tr
Page 153 and 154: 142 Chapter 6 neither has a stable
Page 155 and 156: 144 Chapter 6 of the haem cofactor
Page 157 and 158: 146 Chapter 6 Figure 6.33 Active si
Page 159 and 160: 148 Chapter 6 HO H N N O O H N prol
Page 161 and 162: 150 Chapter 6 R R R O 2 , Fe 2+ O O
Page 163 and 164: 152 Chapter 6 CoA. Explain these re
Page 165 and 166: 154 Chapter 6 NADH models O. Almars
Page 167 and 168: 7 Enzymatic Carbon-Carbon Bond Form
Page 169 and 170: 158 Chapter 7 O H − OH O − O H
Page 173 and 174: 162 Chapter 7 Figure 7.6 Active sit
Page 175 and 176: 164 Chapter 7 7.3 Claisen enzymes I
Page 177 and 178: 166 Chapter 7 CoAS O EnzB − H D T
Page 179 and 180: 168 Chapter 7 onto the acetyl-thioe
Page 181 and 182: 170 Chapter 7 In the case of erythr
Page 183 and 184: 172 Chapter 7 O HO O − O ATP ADP
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174 Chapter 7 CO 2 − vitamin K-de
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176 Chapter 7 7.8 Thiamine pyrophos
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178 Chapter 7 HN N + N H NH S O OPP
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180 Chapter 7 O OH OH camphor geran
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182 Chapter 7 CH 3 * OPP (−)pinen
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184 Chapter 7 Figure 7.36 Structure
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186 Chapter 7 C C C C C O OCH 3 C H
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188 Chapter 7 AcO HO HO NHAc O OH +
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190 Chapter 7 (9) Usnic acid is a n
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192 Chapter 7 Radical couplings W.M
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194 Chapter 8 C-O cleavage H E1 H C
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196 Chapter 8 leads to the formatio
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198 Chapter 8 aconitase citrate cis
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200 Chapter 8 *H R H H CO 2 − NH
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202 Chapter 8 EnzB − 2 H − O 2
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204 Chapter 8 involves no change in
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206 Chapter 8 Glyphosate H H N CO
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208 Chapter 8 (5) Chorismic acid (s
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9 Enzymatic Transformations of Amin
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CO − − 2 CO 2 H H CO 2 − N +
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214 Chapter 9 - BEnz CO − 2 Cl H
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216 Chapter 9 Arg 292 Arg 292 Lys 2
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218 Chapter 9 Arg 292 Asp 292 HN H
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220 Chapter 9 S − B 1 Enz H − C
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222 Chapter 9 9.6 N-Pyruvoyl-depend
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224 Chapter 9 The biosyntheses of n
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226 Chapter 9 Further reading Gener
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228 Chapter 10 B 1 - H + NH 3 CO
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230 Chapter 10 ‘low-barrier’ +
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232 Chapter 10 H S C 6 H 13 HN His
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234 Chapter 10 O H NH 3 CO 2 − CO
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236 Chapter 10 sub-units. Each sub-
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238 Chapter 10 Further reading Gene
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11 Radicals in Enzyme Catalysis 11.
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242 Chapter 11 CH 2 Co III Ad H OH
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244 Chapter 11 − O 2 C − O CO
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246 Chapter 11 H N H O 734 H N H PF
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248 Chapter 11 H H* + H + H 3 N CO
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250 Chapter 11 O HN NH + H 3 N H 3
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252 Chapter 11 cofactor is involved
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254 Chapter 11 Protein Radicals J.
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256 Chapter 12 self-splicing reacti
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258 Chapter 12 Figure 12.2 Structur
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260 Chapter 12 antigen combining si
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262 Chapter 12 R O OR' OH − R O
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264 Chapter 12 CO 2 − − O 2 C O
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266 Chapter 12 (1) Selective substr
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268 Chapter 12 HO O HO OH HO OH O O
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270 Chapter 12 Problems (1) How rev
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Appendix 1: Cahn-Ingold-Prelog Rule
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Appendix 2: Amino Acid Abbreviation
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276 Appendix 3 Preparation of orang
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278 Appendix 4 (2) Intramolecular a
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280 Appendix 4 from water at a-posi
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282 Appendix 4 Chapter 8 (1) Treat
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284 Appendix 4 (b) Formation of rad
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286 Index b-hydroxydecanoyl thioest
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288 Index glycosylation 26-7 glysop
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290 Index phenylalanine ammonia lya
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292 Index transition states 31-2 an
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