The amino acid sequence of a peptide containing the active center disulfide of thioredoxin reductase from Escherichia coli.

Thioredoxin is a low molecular weight protein. In the oxidized form, thioredoxin-S2, it contains a single disulfide bridge formed from the 2 half-cystine residues in the molecule (1). The reduced or dithiol form of thioredoxin, thioredoxin-(SH)z, was first identified as the hydrogen donor in the reduction of ribonucleotides to deoxyribonucleotides in Escherichia coli (2). It has now been shown to also act as hydrogen donor in the enzymatic reduction of sulfate and methionine sulfoxide (3). Thioredoxin and thioredoxin reductase have been purified from E. coli (4, 5), Lactobacillus Zeichmannii (6), bakers’ yeast (7), and rat tumor (8). In addition it has recently been shown that a phagespecific thioredoxin is induced in E. coli after infection with bacteriophage T4 (9). Thioredoxin reductase has been obtained as a homogeneous protein of molecular weight 66,000 (lo), and its amino acid composition has been determined (11). The enzyme consists of two identical or very similar polypeptide chains held together by noncovalent bonds (11). There are 2 FAD molecules (10) and 2 disulfides per enzyme molecule, all of which were shown to act as oxidation-reduction acceptors during enzyme catalysis (11, 12) ; in this respect thioredoxin reductase is similar