Vitamin sulfhydryl group

Wald s assumption that a more chemically active sulfhydryl group was probably required94. The discussion of the difference between the chromophores based on Vitamin A1 and A2 is superficial. None of the comments concerning the Schiffbase or the potential of the sulfhydryl group have been confirmed outside of the Wald laboratory. Most have not been confirmed at all. 

The carboxylation reaction is catalyzed by y-glutamylcarboxylase, whose substrates are peptide-bound glutamate, 02, C02, and reduced (hydroquinone form) vitamin K. The mechanism of action most likely involves activation of the y-methylene residue of glutamate to a carbanion, which then interacts with the COz. Vitamin K is oxidized in the process and is then reconverted to the hydroquinone form via an epoxide intermediate with the participation of NADPH, sulfhydryl groups, and various enzymes. These reactions are summarized in Figure 6.9. 

In an acidic environment, it is protonated, and occurs mainly as sulfurous acid. In an alkaline environment, the protons dissociate, and it occurs mainly as bisulfite. Sulfurous acid is in an equilibrium with sulfur dioxide, which can leave a solution of water to enter atmosphere. The toxic effects of sulfite arise from its reactions with sulfhydryl groups, aldehyde groups, and ketones. Sulfite can also react with enz5nne-bound NAD and FAD. It is well known that the sulfite added to foods can react with the thiamin in the food, destroying this vitamin. The reaction of sulfite with sulfhydryl groups (R— SH) results in its conversion to an S-sulfonate group (R—S—SO3-). 

How the above described vitamins influence in vitro 3H-tryptophan nuclear receptor binding is not clear. Based upon the experiments with added dithiothreitol, it appears that some vitamins act on the sulfhydryl groups of the receptor, which become modified, which interferes with 3H-tryptophan binding. Reviews of reports by others indicate that certain vitamins can bind to hepatic nuclei. Examples include (1) 3H-a-tocopherol, which has been reported to become incorporated into isolated rat liver nuclei in a nonspecific manner by binding to chromatin nonhistone chromosomal protein,196 and (2) rat liver nuclei, which contain receptors for a folate-binding protein.197 As yet, it is not known whether others act similarly or not. Thus, whether competitive binding to nuclei between vitamins and tryptophan occurs is not known. 

Coenzyme A (CoA), biotin, and pyridoxal phosphate are also activation-transfer coenzymes synthesized from vitamins. CoA (CoASH), which is synthesized from the vitamin pantothenate, contains an adenosine 3, 5 -bisphosphate which binds reversibly, but tightly, to a site on an enzyme (Fig. 8.12A). Its functional group, a sulfhydryl group at the other end of the molecule, is a nucleophile that always... 

Fig. 8.12. CoA and biotin, activation-transfer coenzymes. A. Coenzyme A (CoA or CoASH) and phosphopantetheine are synthesized from the vitamin pantothenate (pantothenic acid). The active sulfhydryl group, shown in blue, binds to acyl groups (e.g., acetyl, succinyl, or fatty acyl) to form thioesters. B. Biotin activates and transfers CO2 to compounds in car-boxylation reactions. The reactive N is shown in blue. Biotin is covalently attached to a lysine residue in the carboxylase enzyme.

Fig. 33.12. Phosphopantetheinyl residue of the fatty acid synthase complex. The portion derived from the vitamin, pantothenic acid, is indicated. Phosphopantetheine is covalently linked to a serine residue of the acyl carrier protein (ACP) segment of the enzyme. The sulfhydryl group reacts with malonyl CoA to form a thioester.

Coenzyme A is derived from the B vitamin pantothenic acid (B5) (see b Figure 12.17). It also contains two other components, a phosphate derivative of ADP and b-mercapto-ethylamine. A key feature in the structure of coenzyme A is the presence of the sulfhydryl group (—SH), which is the reactive portion of the molecule. For this reason, coenzyme A is often abbreviated as CoA—SH. 

K. Schwarz Vitamin E, Trace Elements and Sulfhydryl Groups in Respiratory... 

VITAMIN E, TRACE ELEMENTS, AND SULFHYDRYL GROUPS 473 TABLE IV Prevention op Decline Due to Arsbnite and Cd" by a-TocoPHEROL ... 

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