Thioredoxin reductase amino acid composition

Thioredoxin from yeast has been obtained in two forms (I and II) of which thioredoxin II has been purified to homogeneity (43). Both yeast thioredoxins are able to serve as hydrogen donors for the ribonucleotide reductase from E. coli. The molecular weight (12,600) of thioredoxin II is similar to that of thioredoxin from E. coli. Although both yeast thioredoxins contain only one tryptophan residue and although their amino acid compositions differ markedly from that of E. coli thioredoxin, the amino acid sequences around the disulfide bridge of these three thioredoxins are identical ... 

A thioredoxin has also been partially purified from L. leichtnannii (39). Although this thioredoxin is similar in size (MW approximately 12,000) to the E. coli thioredoxin, it is not able to function as a substrate for the E. coli thioredoxin reductase. More recently thioredoxins have been purified to homogeneity from rat Novikoff ascites hepatoma (127) and from calf liver (45). The properties of these two thioredoxins are quite similar (MW 11,400 and 12,000 respectively). The amino acid compositions however, are different. For instance the Novikoff tumor thioredoxin contains six half-cysteine residues, whereas calf liver thioredoxin has only four. Both these thioredoxins have a tendency to aggregate in the oxidized form. This aggregation is probably due to mixed sulfide formation between the additional sulfhydryl groups. Like yeast thioredoxin II, calf liver thioredoxin contains only one tryptophan residue unfortunately the tryptophan content of Novikoff tumor thioredoxin was not determined. 

Thioredoxin reductases have also been purified from yeast (43, 134) and from rat fiver (135). Thioredoxin reductase from yeast is a flavo-protein with a molecular weight of approximately 75,000 and consists of two subunits, each containing one molecule of FAD. Although the amino acid composition of this thioredoxin reductase is quite different from that of E. coli, both enzymes contain 5 half-cystine residues and have almost identical absorption spectra. Like the E. coli enzyme, thioredoxin reductase from yeast is completely inhibited by p-chloro-mercuriphenylsulfonate (PCMS) only in the presence of NADPH suggesting that the yeast enzyme also contains a disulfide bridge at the catalytic site. 

The E. coli enzyme has been purified to homogeneity the molecular weight is about 66,000 and the enzyme consists of two identical or very similar polypeptide chains. The amino acid composition of the enzyme has been determined (13). It has been proposed that each of the two thioredoxin reductase subunits has an active center containing one FAD molecule and one disulfide linkage, both of which act as oxidation-reduction acceptors during catalysis (13). As well, thioredoxin reductase contains four additional sulfhydiyl groups per molecule (presumably, two per subunit), but these do not participate in the catalytic function. The amino acid sequence at the active site of the E. coii thioredoxin reductase has been determined to be as follows (14) ... 

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