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Fig 7.2 Mechanism of carbonic anhydrase

Fig 7.2 Mechanism of carbonic anhydrase. Action of carbonic anhydrase, a metalloenzyme Zinc ion promotes the ionization of bound H 2 O. Resulting nucleophilic OH - attacks carbon of CO 2. (continued next slide). Fig. 7.2 (continued). Iron in metalloenzymes.

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Fig 7.2 Mechanism of carbonic anhydrase

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  1. Fig 7.2 Mechanism of carbonic anhydrase • Action of carbonic anhydrase, a metalloenzyme • Zinc ion promotes the ionization of bound H2O. Resulting nucleophilic OH- attacks carbon of CO2 (continued next slide) Chapter 7

  2. Fig. 7.2 (continued) Chapter 7

  3. Iron in metalloenzymes • Iron undergoes reversible oxidation and reduction: • Fe3++ e- (reduced substrate) • Fe2+ + (oxidized substrate) • Enzyme hemegroups and cytochromes contain iron • Nonhemeiron exists in iron-sulfur clusters (iron is bound by sulfide ions and S- groups from cysteines) • Iron-sulfur clusters can accept only one e- in a reaction Chapter 7

  4. Fig 7.3 Iron-sulfur clusters • Iron atoms are complexed with an equal number of sulfide ions (S2-) and with thiolate groups of Cys side chains Chapter 7

  5. Reactions of ATP, a metabolite coenzyme • ATP is a versatile reactant that can donate its: • (1) Phosphoryl group (g-phosphate) • (2) Pyrophosphoryl group (g,b phosphates) • (3) Adenylyl group (AMP) • (4) Adenosyl group Fig 7.4 Chapter 7

  6. Methionine + ATP S-Adenosylmethionine + Pi + PPi SAM synthesis • ATP is also a source of other metabolite coenzymes such as S-adenosylmethionine (SAM) • SAM donates methyl groups in many biosynthesis reactions Fig 7.5 S-Adenosylmethionine • Activated methyl group in red Chapter 7

  7. S-Adenosylmethionine (SAM) is a methyl donor in many biosynthetic reactions • SAM donates the methyl group for the synthesis of the hormone epinephrine from norepinephrine Chapter 7

  8. Vitamin-Derived Coenzymes and Nutrition • Vitamins are required for coenzyme synthesis. Animals must obtain vitamins from diet. (Plants, microorganisms, meat) • Most vitamins are enzymatically transformed to the coenzyme Table 7.1 Vitamins, nutritional deficiency diseases VitaminDisease Ascorbate (C) Scurvy Nicotinic acid Pellagra Riboflavin (B2) Growth retardation Pantothenate (B3) Dermatitis in chickens Thiamine (B1) Beriberi Pyridoxal (B6) Dermatitis in rats Biotin Dermatitis in humans Folate Anemia Cobalamin (B12) Pernicious anemia Chapter 7

  9. Box 7.2 Vitamin C: a vitamin but not a coenzyme • A reducing reagent for hydroxylation of collagen • Deficiency leads to the disease scurvy • Most animals (not primates) can synthesize Vit C Chapter 7

  10. NAD+ and NADP+ • Nicotinic acid (niacin) is precursor of NAD+ and NADP+ • Lack of niacin causes the disease pellagra • Humans obtain niacin from cereals, meat, legumes Chapter 7

  11. Fig 7.8 Oxidized, reduced forms of NAD+ (NADP+) Chapter 7

  12. NAD+ and NADP+ are cosubstrates for dehydrogenases • Oxidation by NAD+ and NADP+ occurs two electrons at a time • Dehydrogenases transfer a hydride ion (H:-) from a substrate to pyridine ring C-4 of NAD+ or NADP+ • The netreaction is: NAD(P)+ + 2e- + 2H+ NAD(P)H + H+ Fig 7.9 Catalysis by lactate dehydrogenase Chapter 7

  13. FAD and FMN • Flavin adenine dinucleotide (FAD) and Flavin mono-nucleotide (FMN) are derived from riboflavin (Vitamin B2) • Flavin coenzymes are involved in oxidation-reduction reactions for many enzymes (flavoenzymes or flavoproteins) • FAD and FMN catalyze oneortwo electron transfers Chapter 7

  14. Fig 7.11 Riboflavin and its coenzymes (a) Riboflavin, (b) FMN (black), FAD (black/blue) Chapter 7

  15. Fig 7.12 Reduction, reoxidation of FMN or FAD Chapter 7

  16. Coenzyme A (CoA or HS-CoA) • Derived from the vitamin pantothenate (Vit B3) • Participates in acyl-grouptransferreactions with carboxylic acids and fatty acids • CoA-dependent reactions include oxidation of fuel molecules and biosynthesis of carboxylic acids and fatty acids • Acyl groups are covalentlyattached to the -SH of CoA to form thioesters Chapter 7

  17. Fig 7.13 (a) Coenzyme A Chapter 7

  18. Thiamine Pyrophosphate (TPP) • TPP is a derivative of thiamine (Vitamin B1) • TPP participates in reactions of: (1) Decarboxylation(2) Oxidative decarboxylation Fig 7.14 Thiamine (Vitamin B1) and TPP Chapter 7

  19. Pyridoxal Phosphate (PLP) • PLP is derived from Vit B6 family of vitamins • Vitamin B6 is phosphorylated to form PLP • PLP is a prosthetic group for enzymes catalyzing reactions involving aminoacidmetabolism (isomerizations, decarboxylations, side chain eliminations or replacements) Fig 7.16 B6 Vitamins and pyridoxal phosphate (PLP) Chapter 7

  20. Fig 7.18 Mechanism of transaminases Chapter 7

  21. Biotin(Why you shouldn’t eat raw eggs!) • Biotin is required in very small amounts because it is available from intestinal bacteria. Avidin (egg protein) binds biotin very tightly and may lead to a biotin deficiency (cooking eggs denatures avidin so it does not bind biotin) • Enzymes using biotin as a prosthetic group catalyze : • (1) Carboxyl-group transfer reactions • (2) ATP-dependent carboxylation reactions Chapter 7

  22. Fig 7.21 Pterin, folate and tetrahydrofolate (THF) Chapter 7

  23. Fig 7.24 Abbreviated structure of cobalamin coenzymes Chapter 7

  24. Fig 7.25 Intramolecular rearrangements catalyzed by adenosylcobalamin enzymes (a) Rearrangement of an H and substituent X on an adjacent carbon Chapter 7

  25. Fig 7.27 Formation of vitamin A from b-carotene Chapter 7

  26. Retinoic acid is a hormone that regulates gene expression in skin Chapter 7

  27. Vitamin D • A group of related lipids involved in control of Ca2+utilization in humans • Fig 7.28 Vitamin D3 and 1,25-dihydroxycholecalciferol Chapter 7

  28. Vitamin D deficiency causes rickets Chapter 7

  29. Vitamin E (a-tocopherol) • A reducingreagent that scavenges oxygen and free radicals • May prevent damage to fatty acids in membranes • Fig 7.29 Vitamin E (a-tocopherol) Chapter 7

  30. Fig 7.30 (a) Structure of vitamin K (b) Vit K-dependent carboxylation Chapter 7

  31. Warfarin is an anticoagulant Chapter 7

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