Cystine residues lipoamide dehydrogenase

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). 

E. coli lipoamide dehydrogenase has a valine residue preceding the first half-cystine, whereas the eukaryote enzymes have a threonine residue. Chemically this is a relatively conservative change since the side chains of these amino acids are virtually identical in volume. 

The total half-cystine content of pig heart lipoamide dehydrogenase is 10 per FAD 6S). The basis for the protein quantitation has been discussed in Section II,C. Older data suggested that there were 2 cystine residues and 6 cysteine residues 83), but more recent data 61, 63) give strong evidence that the active center cystine residue is the only cystine residue. Only 7 thiols react with DTNB under denaturing conditions (dS) however, recalculation of data of Brown and Perham 86), taking... 

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