Thioredoxin and related proteins in procaryotes

Thioredoxin is a small ( M r 12,000) ubiquitous redox protein with the conserved active site structure: ‐Trp‐Cys‐Gly‐Pro‐Cys‐. The oxidized form (Trx‐S 2 ) contains a disulfide bridge which is reduced by NADPH and thioredoxin reductase; the reduced form [Trx(SH) 2 ] is a powerful protein disulfide oxidoreductase. Thioredoxins have been characterized in a wide variety of prokaryotic cells, and generally show about 50% amino acid homology to Escherichia coli thioredoxin with a known three‐dimensional structure. In vitro Trx‐(SH) 2 serves as a hydrogen donor for ribonucleotide reductase, an essential enzyme in DNA synthesis, and for enzymes reducing sulfate or methionine sulfoxide. E. coli Trx‐(SH) 2 is essential for phage T7 DNA replication as a subunit of T7 DNA polymerase and also for assembly of the filamentous phages f1 and M13 perhaps through its localization at the cellular plasma membrane. Some photosynthetic organisms reduce Trx‐S 2 by light and ferrodoxin; Trx‐(SH) 2 is used as a disulfide reductase to regulate the activity of enzymes by thiol redox control. Thioredoxin‐negative mutants ( trxA ) of E. coli are viable making the precise cellular physiological functions of thioredoxin unknown. Another small E. coli protein, glutaredoxin, enables GSH to be hydrogen donor for ribonucleotide reductase or PAPS reductase. Further experiments with molecular genetic techniques are required to define the relative roles of the thioredoxin and glutaredoxin systems in intracellular redox reactions.