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Vitamins & Coenzymes.docx

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Xudong Zhu

Unit 6 - Vitamins & Coenzymes There are other groups that contribute to the reactivity of enzymes beside amino acid residues. These groups are called cofactors - chemicals required by apoenzymes (inactive) to become holoenzymes (active). There are two types of cofactors: 1) essential ions - metal ions -inorganic 2) coenzymes - +rganic mo-ecules that act as group-transfer reagents (accept or donate groups)- can also be H and/or e Both provide reactive groups not found on a.a. side chains. Coenzymes can be either cosubstrates (loosely bound to enzyme; is altered, then regenerated) or prosthetic groups (tightly bound to enzyme). Coenzymes can be classified by their source: 1) metabolite coenzymes  synthesized by common metabolites  include nucleoside triphosphates  most abundant is ATP, but also include uridine diphosphate glucose (UDP-glucose) and S-adenosylmethionine  ATP can donate all of its three phosphoryl groups in group-transfer reactions  S-adenosylmethionine can donate its methyl group in biosynthetic reactions.  UDP-glucose is a source of glucose for synthesis of glycogen in animals and starch in plants. 2) vitamin-derived coenzymes  Vitamins are required for coenzyme synthesis and must be supplied in the diet  Lack of particular vitamins causes disease  There are two catagories of vitamins: 1) water-soluble - B vitamins and vit. C required daily in diet excess excreted in urine 2) lipid-soluble - vitamins A, D, E, K Intake must be limited Stored in fat B vitamins and their coenzymes Niacin (nicotinic acid)  nicotinamide   Get niacin in enriched cereals, meat, legumes. + +  NAD an+ NADP are the coenzymes (cosubstrates).  NAD consists of 2 5’ribonucleotides (AMP and nicotinamide monomucleotide) joined by a phosphoanhydride linkage. +  For NADP , have a phosphoryl group on 2’-oxygen.  Both coenzymes act as cosubstrates for dehydrogenases  catalyze the oxidation of substrates by transfer of 2e and 1H  NADH and NADPH. Vitamin B (2iboflavin)  Coenzymes are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).  Riboflavin found in milk, whole grains, liver.  The coenzymes serve as prosthetic groups involved in 1e or 2e transfers. - + FAD + 2e + 2H ---> FADH 2 FMN + 2e + 2H --> FMNH 2  Enzymes that require FAD or FMN are called flavoenzymes or flavoproteins. - -  Can actually donate 1 or 2 e at a time  form partially oxidized compound when only 1e is donated  relatively stable. Vitamin B (1hiamine)  Structure: pyrimidine ring and positively charged thiazolium ring.  Found in husks of rice and other cereals, liver, meat, particularly pork.  Deficiency in thiamine causes beriberi - extensive nervous system and circulatory system damage, muscle wasting, edema.  Coenzyme form is thiamine pyrophosphate (TPP) - synthesized by transfer of pyrophosphoryl group from ATP via thiamine pyrophosphate synthetase.  Used primarily in decarboxylases as a coenzyme. Vitamin B6 fa
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