Thiol groups glutathione reductase

Thiols are also important protection against lipid peroxidation. Glutathione (7-Glu-Cys-Gly) is used by several glutathione-dependent enzymes such as free-radical reductase (converts vitamin E radical to vitamin E), glutathione peroxidase (reduces hydrogen peroxide and lipid hydroperoxides to water and to the lipid alcohol, respectively), and others. In addition, the thiol group of many proteins is essential for function. Oxidation of the thiol of calcium ATPases impairs function and leads to increased intracellular calcium. Thiol derivatives such as the ovothiols (l-methyl-4-mercaptohistidines) (Shapiro, 1991) have been explored as therapeutics. 

This enzyme [EC 1.8.4.2], also known as glutathione insulin transhydrogenase and insulin reductase, catalyzes the reaction of two glutathione with a disulfide bond in a protein to produce glutathione disulfide and a protein with two new thiol groups. The enzyme can reduce insulin and a number of other proteins. 

As already discussed in chapter 4, reactive intermediates can react with reduced GSH either by a direct chemical reaction or by a GSH transferase-mediated reaction. If excessive, these reactions can deplete the cellular GSH. Also, reactive metabolites can oxidize GSH and other thiol groups such as those in proteins and thereby cause a change in thiol status. When the rate of oxidation of GSH exceeds the capacity of GSH reductase, then oxidized glutathione (GSSG) is actively transported out of the cell and thereby lost. Thus, reduced GSH may be removed reversibly by oxidation or formation of mixed disulfides with proteins and irreversibly by conjugation or loss of the oxidized form from the cell. Thus, after exposure of cells to quinones such as menadione, which cause oxidative stress, GSH conjugates, mixed disulfides, and GSSG are formed, all of which will reduce the cellular GSH level. 

In neural cells, the redox status is controlled by the thioredoxin (Trx) and glutathione (GSH) systems that scavenge harmful intracellular ROS. Thioredoxins are antioxidants that serve as a general protein disulphide oxidoreductase (Saitoh et al., 1998). They interact with a broad range of proteins by a redox mechanism based on the reversible oxidation of 2 cysteine thiol groups to a disulphide, accompanied by the transfer of 2 electrons and 2 protons. These proteins maintain their reduced state through the thioredoxin system, which consists of NADPH, thioredoxin reductase (TR), and thioredoxin (Trx) (Williams, Jr. et al., 2000 Saitoh et al., 1998). The thioredoxin system is a system inducible by oxidative stress that reduces the disulfide bond in proteins (Fig. 7.4). It is a major cellular redox system that maintains cysteine residues in the reduced state in numerous proteins. 

Glutathione reductase from yeast and from E. coli contains 4 and 6 thiol groups per FAD, respectively (Table I), in addition to the active center disulfide. Peptides containing 3 of the 4 thiols have been isolated and sequenced Cys-AsnAsp Lys-Ile-Ala-Cys-Pro-Gly-Asn-Val-Gln-Lys Asp-Thr-Ile-Tyr- (His,Glx) -Val-Cys-Lys- (Thr,Gly,Ala,Leu2) (35). The thiols in the yeast enzyme, like those in lipoamide dehydrogenase, are relatively unreactive. Reactivity with phenyl mercuric acetate... 

Glutathione reductase amino acid composition, 102,104,105 cystine residues, 104 kinetic studies, 138-141 mechanism, 94, 97-98,134 metabolic functions, 129-133 reaction catalyzed, 92 reduction of, 112, 113 specificity of, 92-93 coenzymes and, 94 thiol groups, 141-142 two-electron-reduced enzyme, properties, 133-138... 

Oxidized glutathione (GSSG) is harmful to a cell because it can form disulfides with proteins or oxidize thiol groups, thus forming disulfide bonds in proteins. GSSG is reduced back to GSH by glutathione reductase. 

Crucial thiols such as cysteine and coen me A and the numerous cellular enzymes requiring thiol groups for proper function are kept reduced by the higli glutathione levels within cells. Oxidized glutathione in turn is reduced by glutathione reductase and NADPH-generating systems. 

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