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Chapter 7 (part 1)

Chapter 7 (part 1). Cofactors. Cofactors. Cofactors are organic or inorganic molecules that are required for the activity of a certain conjugated enzymes Apoenzyme = enzyme (-) cofactor Holoenzyme = enzyme (+) cofactor Inorganic cofactors – essential ions Organic cofactors – coenzymes.

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Chapter 7 (part 1)

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  1. Chapter 7 (part 1) Cofactors

  2. Cofactors • Cofactors are organic or inorganic molecules that are required for the activity of a certain conjugated enzymes • Apoenzyme = enzyme (-) cofactor • Holoenzyme = enzyme (+) cofactor • Inorganic cofactors – essential ions • Organic cofactors – coenzymes

  3. Essential Ion Cofactors • Activator ions – bind reversibly to enzyme and often participate in substrate binding. • Metal ions of metalloenzymes – cations that are tightly bound to enzyme and participate directly in catalysis (Fe, Zn, Cu, Co). • Metal activated enzymes – require or are stimulated by addition of metal ions (i.e. Mg2+, is required by many ATP requiring enzymes)

  4. Metal ions can function as electrophiles in active site Zinc protease (angiotensin converting enzyme)

  5. Coenzymes Cosubstrates- - altered in rxn and regenerated to original structure in subsequent rxn - disassociated from active site - shuttle chemical groups among different enzyme rxns. Prosthetic groups- - remains bound to enzyme - must return to original form Both cosubstrates and prosthetic groups supply reactive groups not present on amino acid side chains

  6. Coenzymes • Metabolite coenzymes – synthesized from common metabolites • Nucleoside triphosphates – (ATP) can donate phosphates, pyrophosphates, adenosyl grroups • S-adenosylmethionine (SAM) – donates methyl groups • Nucleotide sugars (uridine diphosphate glucose = UDP-glucose) - transfer sugars in carbohydrate metabolism

  7. Vitamin derived coenzymes • Must be obtained from diet • Synthesized by microorganisms and plants • Vitamin deficiencies lead to disease state • Most vitamins must be enzymatically transformed to function as a coenzyme

  8. Vitamins VitaminCoenzyme Ascorbic acid (C) not a coenzyme Niacin NAD(P)+/NAD(P)H Riboflavin (B2) FMN & FAD Thiamin (B1) Thiamin-pyrophosphate Pyridoxal (B6) Pyridoxal phosphate Biotin Biotin Folate Tetrahydrafolate Cobalamin (B12) adenosyl-and methylcobalamin Vitamin A Retinal Vitamin K Vitamin K Pantothenate (B3) Coenzyme A

  9. Niacin (nicotinic acid) • Deficiencies lead to pellagra (dermatitis, diarrhea, dementia) • Required in relatively high amounts compared to other vitamins • Not true enzyme because can be synthesized from tryptophan in the liver

  10. Nicotinamide Coenzymes

  11. NAD+ / NADP+ • Serve as cofactors in oxidation/reduction reactions • Act as co-substrates for dehydrogenases • Reduction of NAD+/NADP+ and oxidation of NADH/NADPH occurs 2 e- at a time. • Function in hydride ion transfer • Rxns forming NADH/NADPH are catabolic • NADH is coupled with ATP production in mitochondria • NADPH is an impt reducing agent in biosynthetic reactions • Reduced forms (NADH/NADPH) absorb light at 340 nm, oxidized forms (NAD+/NADP+) do not

  12. Riboflavin (B2) • Water soluble vitamin • Severe deficiencies lead to growth retardation, reproductive problems and neural degeneration • Meat, dairy products and dark green vegetables, legumes and grains are good sources

  13. FMN/FAD

  14. FAD and FMN can transfer electrons one or two at a time Hydroquinone form Quinone form semiquinone form

  15. Thiamin • Thiamin is the first Vitamin discovered (Vital amine = Vitamin) • Deficiencies lead to disease called Beriberi (neurological disorders, heart problems, anorexia) • Beriberi prevealent in undeveloped countries where polished grains make up the majority of the diet. • Associated with alcohol related disorders (Wernickes-Korskofff syndrome – memory loss, unstable walk)

  16. Thiamin pyrophosphate • Serves as a cofactor in decarboxylation rxn of keto acids • Also functions as a prosthetic group in transketolases (catalyze the transfer of two carbon units in carbohydrate metabolism)

  17. Thiazolium ring is the chemically active part of TPP Ylid = a molecule with opposite charges on adjacent atoms

  18. Pyridoxal

  19. PYRIDOXAL-PHOSPHATE • Important in amino acid metabolism • Bound to enzyme as a Schiff base thru rxn with lysine • PLP functions in transamination, decarboxylation, racemization, isomerization, side-chain elimination rxns involving amino acids

  20. PLP in transamination reaction

  21. PLP in amino acid decarboxylation reaction

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