1. Prentice Hall c2002Chapter 71 Chapter 7 Coenzymes and Vitamins

2. Prentice Hall c2002Chapter 72 Coenzyme, p192-193 Cofactors: nonprotein components Cofactors may be metal ions or organic molecules (coenzyme) Cofactor: metal ion + coenzyme Prosthetic groups: tightly bound coenzymes

3. Prentice Hall c2002Chapter 73 Holoenzyme and Apoenzyme Holoenzyme –Complex of protein and prosthetic groups –Catalytically active Apoenzyme –The enzyme without the prosthetic groups –Catalytically inactive

4. Prentice Hall c2002Chapter 74 Apoenzyme + Cofactor Holoenzyme (protein only)(active) (inactive) Some enzymes require cofactors for activity (1) Essential ions (mostly metal ions) (2) Coenzymes (organic compounds)

5. Prentice Hall c2002Chapter 75 Coenzymes, p192-193 Group-transfer reagents Transfer hydrogen, electrons, or other groups Reactive center of the coenzyme Fig 7.1 Types of cofactors, p192

6. Prentice Hall c2002Chapter 76 7.1 Many Enzymes Require Inorganic Cations, p193 Enzymes requiring metal ions for full activity: (1) Metal-activated enzymes (2) Metalloenzymes

7. Prentice Hall c2002Chapter 77 Fig 7.2 Mechanism of carbonic anhydrase, p193 A metalloenzyme Zinc ion promotes the ionization of bound H 2 O. Resulting nucleophilic OH - attacks carbon of CO 2

8. Prentice Hall c2002Chapter 78 Iron in metalloenzymes, p193 Fe 3+ + e - (reduced substrate)  Fe 2+ + (oxidized substrate) Heme groups, heme protein Cytochromes contain iron Nonheme iron: iron-sulfur clusters Iron-sulfur clusters can accept only one e - in a reaction

9. Prentice Hall c2002Chapter 79 7.2 Coenzyme Classification, p193-194 (1) Cosubstrates (2) Prosthetic groups - Vitamin-derived coenzymes

10. Prentice Hall c2002Chapter 710 7.3 ATP and other nucleotidecosubstrate, p196 Nucleoside triphosphates act as cosubstrate Fig 7.4 ATP Donate (1) Phosphoryl group (  -phosphate) (2) Pyrophosphoryl group ( ,  -phosphates) (3) Adenylyl group (AMP) (4) Adenosyl group

11. Prentice Hall c2002Chapter 711 S-adenosylmethionine synthesis, p196 ATP is also a source of other metabolite coenzymes such as S-adenosylmethionine Equation 7.1 S-adenosylmethionine donates methyl groups in many biosynthesis reactions –Synthesis of the hormone epinephrine from norepinephrine –Equation 7.2

12. Prentice Hall c2002Chapter 712 Nucleotide-sugar coenzymes are involved in carbohydrate metabolism UDP-Glucose is a sugar coenzyme Fig 7.6, p197

13. Prentice Hall c2002Chapter 713 Vitamin-Derived Coenzymes and Nutrition, p194 Animals rely on plants and microorganisms for vitamin sources (meat supplies vitamins also) Most vitamins must be enzymatically transformed to the coenzyme Table 7.1 Vitamins, nutritional deficiency diseases, p194

14. Prentice Hall c2002Chapter 714 Box 7.1 Vitamin C: a vitamin but not a coenzyme, p195 A reducing reagent for hydroxylation of collagen Deficiency leads to the disease scurvy Most animals (not primates) can synthesize Vit C Anti-oxidant

15. Prentice Hall c2002Chapter 715 7.4 NAD + and NADP +, p197 Vitamin: Nicotinic acid (niacin) Coenzyme:NAD + and NADP + Lack of niacin causes the disease pellagra Humans obtain niacin from cereals, meat, legumes Fig 7.8 Dehydrogenases transfer a hydride ion (H:-, one proton and two electrons) from a substrate to pyridine ring C-4 of NAD + or NADP + The net reaction is: NAD(P)+ + 2e- + 2H+  NAD(P)H + H+

16. Prentice Hall c2002Chapter 716 Reaction of lactate dehydrogenase Equation 7.3 Fig 7.9 Mechanism of lactate dehydrogenase, p200

17. Prentice Hall c2002Chapter 717 7.5 FAD and FMN, p200-201 Flavin adenine dinucleotide (FAD) Flavin mono-nucleotide (FMN) Derived from riboflavin (Vit B 2 ) In oxidation-reduction reactions One or two electron transfers Fig 7.10, Fig 7.11

18. Prentice Hall c2002Chapter 718

19. Prentice Hall c2002Chapter 719 7.6 Coenzyme A (CoA or HS-CoA) p201-202 Derived from the vitamin pantothenate (Vit B 3 ) Acyl-group transfer reactions Acyl groups are covalently attached to the -SH of CoA to form thioesters Fig 7.12, Fig. 7.13

20. Prentice Hall c2002Chapter 720 7.7 Thiamine Pyrophosphate (TPP) p202-203 TPP is a derivative of thiamine (Vit B 1 ) Reactive center: thiazolium ring Fig 7.14 TPP participates in reactions of: (1) Decarboxylation (2) Oxidative decarboxylation of  -keto acids (3) Transketolase enzyme reactions

21. Prentice Hall c2002Chapter 721 Yeast pyruvate decarboxylase, p203 Pyruvate  acetaldehyde  acetyl CoA TPP Fig 7.15

22. Prentice Hall c2002Chapter 722 7.8 Pyridoxal Phosphate (PLP), p203-206 Derived from Vit B 6 Vitamin B 6 (Pyridoxine) is phosphorylated to form PLP Involving amino acid metabolism (isomerizations, decarboxylations, side chain eliminations or replacements) The reactive center is the aldehyde group Fig 7.16, Fig 7.17 Fig 7.18 TPP in transaminase action

23. Prentice Hall c2002Chapter 723 7.9 Biotin, p207 Available from intestinal bacteria Avidin (raw egg protein) binds biotin very tightly and may lead to a biotin deficiency (cooking eggs denatures avidin so it does not bind biotin) Biotin (a prosthetic group) enzymes catalyze: (1) Carboxyl-group transfer reactions (2) ATP-dependent carboxylation reactions

24. Prentice Hall c2002Chapter 724 Fig 7.19 Enzyme-bound biotin, p207 Biotin is linked by an amide bond to the e-amino group of a lysine residue of the enzyme The reactive center of biotin is the N-1 Fig 7.20 Reaction catalyzed by pyruvate carboxylase, p207

25. Prentice Hall c2002Chapter 725 7.10 Tetrahydrofolate (THF) p208, Fig 7.21, 7.22 From vitamin folate: in green leaves, liver, yeast The coenzyme THF is a folate derivative where positions 5,6,7,8 of the pterin ring are reduced (Equation 7.4). THF contains 5-6 glutamate residues which facilitate binding of the coenzyme to enzymes Transfers of one carbon units at the oxidation levels of methanol (CH 3 OH), formaldehyde (HCHO), formic acid (HCOOH)

26. Prentice Hall c2002Chapter 726 1-7

27. Prentice Hall c2002Chapter 727 Fig. 7.23 5,6,7,8, Tetrahydrobiopterin, a pterin coenzyme, p210 Coenzyme has a 3-carbon side chain at C-6 Not vitamin-derived, but synthesized by some organisms

28. Prentice Hall c2002Chapter 728 7.11 Cobalamin (Vitamin B 12 ), p210-211 Coenzymes: methylcobalamin, adenosylcobalamin Cobalamin contains a corrin ring system and a cobalt (it is synthesized by only a few microorganisms) Humans obtain cobalamin from foods of animal origin (deficiency leads to pernicious anemia) Coenzymes participate in enzyme-catalyzed molecular rearrangements Fig. 7.24 Fig 7.25 Intramolecular rearrangements catalyzed by adenosylcobalamin enzymes, p211

29. Prentice Hall c2002Chapter 729 Methylcobalamin participates in the transfer of methyl groups, p211 Equation 7.5

30. Prentice Hall c2002Chapter 730 7.12 Lipoamide, p212 From lipoic acid Coenzyme: lipoamide Animals can synthesize lipoic acid, it is not a vitamin Lipoic acid is an 8-carbon carboxylic acid with sulfhydryl groups on C-6 and C-8 Lipoamide functions as a “swinging arm” that carries acyl groups between active sites in multienzyme complexes

31. Prentice Hall c2002Chapter 731 Fig 7.26 Lipoamide, p212 Lipoic acid is bound via an amide linkage to the  - amino group of an enzyme lysine Transfer of an acyl group between active sites - Equation 7.6

32. Prentice Hall c2002Chapter 732 Pyruvate dehydrogenase complex p385-386 Equation 13.1 Conversion of pyruvate to acetyl CoA Pyruvate dehydrogenase complex (PDH complex) is a multienzyme complex containing: 3 enzymes + 5 coenzymes + other proteins (+ ATP coenzyme as a regulator) E1 = pyruvate dehydrogenase E2 = dihydrolipoamide acetyltransferase E3 = dihydrolipoamide dehydrogenase

33. Prentice Hall c2002Chapter 733 Fig 13.1 Reactions of the PDH complex, p388

34. Prentice Hall c2002Chapter 734 7.13 Lipid Vitamins- p212-213 Vitamin A, D, E, K All contain rings and long, aliphatic side chains Highly hydrophobic

35. Prentice Hall c2002Chapter 735 A. Vitamin A (Retinol), p213 Vit A exists in 3 forms: alcohol (retinol), aldehyde and retinoic acid Retinol and retinoic acid are signal compounds Rentinal (aldehyde) is a light-sensitive compound with a role in vision Fig 7.27

36. Prentice Hall c2002Chapter 736

37. Prentice Hall c2002Chapter 737 B. Vitamin D, p213, Fig 7.28 Control of Ca 2+ utilization in humans Regulates intestinal absorption of calcium and its deposition in bones. Active form: 1, 25-hydroxyvitamin D 3 Under the sunlight, vitamin D 3 (cholecalciferol) is formed nonenzymatically in the skin from the steroid 7-dehydrocholesterol. Vitamin D deficiency –Ricket in children, osteomalacia in adults – 軟骨病骨質軟化症

38. Prentice Hall c2002Chapter 738 Absorbed in the intestine or photosynthesized in the skin, cholecalciferol is transported to the liver by vitamin D- binding protein (DBP, or transcalciferin). In the liver, cholecalciferol is 25- hydroxylated by mixed-function oxidase to form 25-hydroxyvitamin D 3 Vitamin D, p213

39. Prentice Hall c2002Chapter 739 Vitamin D, p213 25-hydroxyvitamin D is the mayor circulating form of vitamin D in the body, but the biological activity is far less than the final active form, 1, 25-hydroxyvitamin D 3 In the kidney, a mitochondrial mixed- function oxidase hydroxylates 25- hydroxyvitamin D to 1, 25-hydroxyvitamin D 3 (Active form)

40. Prentice Hall c2002Chapter 740 C. Vitamin E (  -tocopherol), p213 A reducing reagent that scavenges oxygen and free radicals May prevent damage to fatty acids in membranes Fig 7.29

41. Prentice Hall c2002Chapter 741 D.Vitamin K (phylloquinone), p214 Fig 7.29 Required for synthesis of blood coagulation proteins A coenzyme for mammalian carboxylases that convert glutamate to  -carboxyglutamate Equation 7.7 Vit K-dependent carboxylation, p214 Calcium binds to the  -carboxyGlu residues of these coagulation proteins which adhere to platelet surfaces Vitamin K analogs (used as competitive inhibitors to prevent regeneration of dihydrovitamin K) are given to individuals who suffer excessive blood clotting

42. Prentice Hall c2002Chapter 742

43. Prentice Hall c2002Chapter 743 7.14 Ubiquinone (Coenzyme Q), p214 Electrons transfer Plastoquinone (ubiquinone analog) functions in photosynthetic electron transport Hydrophobic tail: repeat of five-carbon isoprenoid units Fig 7.30, p215 Fig 7.31, p215

44. Prentice Hall c2002Chapter 744 7.15 Protein Coenzymes, p215 Protein coenzymes (group-transfer proteins) Participate in: (1) Group-transfer reactions (2) Oxidation-reduction reactions: transfer a hydrogen or an electron Metal ions, iron-sulfur clusters and heme groups are commonly found in these proteins Fig 7.32 Thioredoxin, p216

45. Prentice Hall c2002Chapter 745 7.16 Cytochromes, p216 Heme-containing coenzymes Fe(III) undergoes reversible one-electron reduction Cytochromes a,b and c have different visible absorption spectra and heme prosthetic groups Electron transfer potential varies among different cytochromes due to the different protein environment of each prosthetic group Fig 7.33 Heme group of cyt a,b, and c p217 Fig 7.34 Absorption spectra of oxidized and reduced cytochrome c, p218