Harpers

VITAMINS & MINERALS / 495OOCHNFigure 45–17.OSNHOHN NHSON NHSBiotinCOOBiotinyl-lysine (biocytin)COCHNCarboxy-biocytinOHNCCHNHCCHNHBiotin, biocytin, and carboxy-biocytin.Biotin also has a role in regulation of the cell cycle,acting to biotinylate key nuclear proteins.AS PART OF CoA AND ACP,PANTOTHENIC ACID ACTS ASA CARRIER OF ACYL RADICALSPantothenic acid has a central role in acyl group metabolismwhen acting as the pantetheine functional moietyof coenzyme A or acyl carrier protein (ACP) (Figure45–18). The pantetheine moiety is formed after combinationof pantothenate with cysteine, which providesOOthe ⎯SH prosthetic group of CoA and ACP. CoAtakes part in reactions of the citric acid cycle, fatty acidsynthesis and oxidation, acetylations, and cholesterolsynthesis. ACP participates in fatty acid synthesis. Thevitamin is widely distributed in all foodstuffs, and deficiencyhas not been unequivocally reported in humanbeings except in specific depletion studies.ASCORBIC ACID IS A VITAMINFOR ONLY SOME SPECIESVitamin C (Figure 45–19) is a vitamin for human beingsand other primates, the guinea pig, bats, passerine birds,and most fishes and invertebrates; other animals synthesizeit as an intermediate in the uronic acid pathway ofglucose metabolism (Chapter 20). In those species forwhich it is a vitamin, there is a block in that pathway dueto absence of gulonolactone oxidase. Both ascorbic acidand dehydroascorbic acid have vitamin activity.Vitamin C Is the Coenzymefor Two Groups of HydroxylasesAscorbic acid has specific roles in the copper-containinghydroxylases and the α-ketoglutarate-linked iron-containinghydroxylases. It also increases the activity of anumber of other enzymes in vitro, though this is a nonspecificreducing action. In addition, it has a number ofnonenzymic effects due to its action as a reducing agentand oxygen radical quencher.Dopamine -hydroxylase is a copper-containingenzyme involved in the synthesis of the catecholaminesnorepinephrine and epinephrine from tyrosine in theadrenal medulla and central nervous system. During hydroxylation,the Cu + is oxidized to Cu 2+ ; reduction backOCCH 2OHOCNHCH 2CH 2CH 2SHCH 2CH 2NHNHCOCOH 3 CCHOHCCH 3CH 2 OHH 3 CCHOHCCH 3CH 2 OOPOOOPOONN NHNCH 2NH 2OFigure 45–18. Pantothenic acid andcoenzyme A. * Shows the site of acylationby fatty acids.Pantothenic acidCoenzyme A (CoASH)OO OHP OO