Cystine reductase

Alanine. Transamination of alanine forms pyruvate. Perhaps for the reason advanced under glutamate and aspartate catabolism, there is no known metabolic defect of alanine catabolism. Cysteine. Cystine is first reduced to cysteine by cystine reductase (Figure 30-7). Two different pathways then convert cysteine to pyruvate (Figure 30-8). 

The enzyme, found in a number of plant tissues, reduces the disulfide bonds in a number of proteins. Its activity is strongly inhibited by sulfhydryl reagents. The enzyme is different from cystine reductase and glutathione reductase which do not reduce protein disulfide bonds. 

The generally accepted dietary equivalency of cystine and cysteine indicates that tissues must contain an efficient enzyme system which catalyzes the interconversion of the thiol and disulfide forms of this amino acid. The biological role of cystine reductase may be further emphasized by the current view that reduction of the disulfide to the thiol is a necessary intermediary step for its further metabolism. 

There is a correlation between the backbone conformations which commonly flank disulfides and the frequency with which disulfides occur in the different types of overall protein structure (see Section III,A for explanation of structure types), although it is unclear which preference is the cause and which the effect. There are very few disulfides in the antiparallel helical bundle proteins and none in proteins based on pure parallel /3 sheet (except for active-site disulfides such as in glutathione reductase). Antiparallel /3 sheet, mixed /8 sheet, and the miscellaneous a proteins have a half-cystine content of 0-5%. Small proteins with low secondary-structure content often have up to 15-20% half-cystine. Figure 52 shows the structure of insulin, one of the small proteins in which disulfides appear to play a major role in the organization and stability of the overall structure. 

This reaction is accomplished by several types of iron ribonucleoside reductase (RNR) [1,5,7,13,118], One of the best-characterized RNRs (from E. coli) contains two homodimeric protein components, R1 and R2. The R2 protein comprises an oxygen bridged dinuclear Fe(III) in its oxidized form [119], All RNRs promote the formation of a stable organic radical, which, eventually, leads to the abstraction of a hydrogen atom from the ribose. In the case of E. coli RNR, the latter is accomplished by the R1 protein, specifically by a cysteinyl residue a redox active cystine, also part of Rl, provides the required reducing equivalents (Figure 25). 

The third system is that of thioltransferase, which may not be distinct from glutaredoxin in some tissues. The thioltransferase system is composed of thioltransferase, GSH, glutathione reductase and NADPH. All thioltransferases which have been found in mammalian cells have molecular weights which are close to 11 KDa. Sub-millimolar concentrations of either cystamine or cystine inactivate GST-IT from human placenta with concomitant formation of enzyme mixed-disulfides [282]. Thioltransferase from human placenta specifically reactivates GST-IT in the presence of 10-100 pM GSH while thioredoxin is inactive [282], Thioltransferase-mediated cleavage of mixed disulfides was also shown to restore enzyme activities in phosphofructokinase [283] and pyruvate kinase [284]. 

Electron transfer between pyridine nucleotides and disulfide compounds is catalyzed by several fiavoproteins and three of these are well characterized. Lipoamide dehydrogenase functions in the oxidative decarboxylation of a-keto acids catalyzing the reoxidation of reduced lipoate by NAD+ (18, 19). Glutathione reductase catalyzes electron transfer between NADPH and glutathione ZO-22). Thioredoxin reductase catalyzes the reduction of thioredoxin by NADPH (5) thioredoxin is a protein of 12,000 molecular weight containing a single cystine residue which is the electron acceptor S3). 

It is not surprising that enzymes catalyzing such similar chemical reactions should bear striking similarity to one another both structurally and mechanistically. Lipoamide dehydrogenase (34-38), glutathione reductase (39), and thioredoxin reductase (SO, 31) contain, in addition to FAD, a reactive disulfide which is functional in catalysis. These fiavoproteins consist of two identical or near identical polypeptide chains, each with a reactive cystine residue, and two molecules of FAD (31-36). 

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... 

Certain oxidation products of proteins, such as oxidation of Cys to cystine, and Met residues to methionine sulfoxide can be repaired by enzymes such as glutathione reductase and methionine sulfoxide (Gabbita et al., 1999 Moskovitz et al.,... 

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