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Chapter 7 - Coenzymes and Vitamins

Chapter 7 - Coenzymes and Vitamins. Some enzymes require cofactors for activity (1) Essential ions (mostly metal ions) (2) Coenzymes (organic compounds) . Apoenzyme + Cofactor Holoenzyme (protein only) (active) (inactive) . Fig 7.1 Types of cofactors.

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Chapter 7 - Coenzymes and Vitamins

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  1. Chapter 7 - Coenzymes and Vitamins • Some enzymes requirecofactors for activity • (1) Essential ions (mostly metal ions) • (2) Coenzymes (organic compounds) Apoenzyme + CofactorHoloenzyme (protein only) (active) (inactive) Chapter 7

  2. Fig 7.1 Types of cofactors Chapter 7

  3. Many Enzymes Require Coenzymes • Coenzymes act as group-transferreagents • Hydrogen, electrons, or other groups can be transferred • Larger mobile metabolic groups can be attached at the reactive center of the coenzyme • Coenzyme reactions can be organized by their types of substrates and mechanisms Many Enzymes Require Inorganic Cations • Enzymes requiring metal ions for full activity: • (1) Metal-activated enzymes have an absolute requirement or are stimulated by metal ions (examples: K+, Ca2+, Mg2+) • (2) Metalloenzymescontain firmly bound metal ions at the enzyme active sites (examples: iron, zinc, copper, cobalt ) Chapter 7

  4. 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

  5. Fig. 7.2 (continued) Chapter 7

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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 and nutritional deficiency diseases Chapter 7

  12. 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

  13. 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

  14. Fig 7.8 Oxidized, reduced forms of NAD+ (NADP+)

  15. 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+ Catalysis by lactate dehydrogenase Chapter 7

  16. 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

  17. Fig 7.10 Riboflavin and its coenzymes (a) Riboflavin, (b) FMN (black), FAD (black/blue)

  18. Fig 7.11 Reduction, reoxidation of FMN or FAD Chapter 7

  19. Coenzyme A (CoA or HS-CoA) • Derived from the vitamin pantothenate • 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

  20. Fig 7.12 Coenzyme A Chapter 7

  21. 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

  22. 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

  23. Fig 7.18 Mechanism of transaminases Chapter 7

  24. 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

  25. Fig 7.24 Pterin, folate and tetrahydrofolate (THF) Chapter 7

  26. Abbreviated structure of cobalamin coenzymes Fig 7.27 Chapter 7

  27. Fig 7.28 Intramolecular rearrangements catalyzed by adenosylcobalamin enzymes (a) Rearrangement of an H and substituent X on an adjacent carbon Chapter 7

  28. Fig 7.30 Formation of vitamin A from b-carotene

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

  30. Vitamin D • A group of related lipids involved in control of Ca2+utilization in humans • Fig 7.31Vitamin D3 and 1,25-dihydroxycholecalciferol Chapter 7

  31. Vitamin D deficiency causes rickets Chapter 7

  32. Vitamin E (a-tocopherol) • A reducingreagent that scavenges oxygen and free radicals • May prevent damage to fatty acids in membranes • Fig 7.32 Vitamin E (a-tocopherol) Chapter 7

  33. Fig 7.32 (a) Structure of vitamin K (b) Vit K-dependent carboxylation Chapter 7

  34. Warfarin is an anticoagulant Chapter 7

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