InterPro domain: IPR044192

Thioredoxins [ 1 , 2 , 3 , 4 ] are small disulphide-containing redox proteins that have been found in all the kingdoms of living organisms. Thioredoxin serves as a general protein disulphide oxidoreductase. It interacts with a broad range of proteins by a redox mechanism based on reversible oxidation of two cysteine thiol groups to a disulphide, accompanied by the transfer of two electrons and two protons. The net result is the covalent interconversion of a disulphide and a dithiol. In the NADPH-dependent protein disulphide reduction, thioredoxin reductase (TR) catalyses the reduction of oxidised thioredoxin (trx) by NADPH using FAD and its redox-active disulphide; reduced thioredoxin then directly reduces the disulphide in the substrate protein [ 5 ].

Thioredoxin is present in prokaryotes and eukaryotes and the sequence around the redox-active disulphide bond is well conserved. All thioredoxins contain a cis-proline located in a loop preceding beta-strand 4, which makes contact with the active site cysteines, and is important for stability and function [ 5 ]. Thioredoxin belongs to a structural family that includes glutaredoxin, glutathione peroxidase, bacterial protein disulphide isomerase DsbA, and the N-terminal domain of glutathione transferase [ 6 ]. Thioredoxins have a beta-alpha unit preceding the motif common to all these proteins.

A number of eukaryotic proteins contain domains evolutionary related to thioredoxin, most of them are protein disulphide isomerases (PDI). PDI ( 5.3.4.1 ) [ 6 , 7 , 8 ] is an endoplasmic reticulum multi-functional enzyme that catalyses the formation and rearrangement of disulphide bonds during protein folding [ 9 ]. All PDI contains two or three (ERp72) copies of the thioredoxin domain, each of which contributes to disulphide isomerase activity, but which are functionally non-equivalent [ 10 ]. Moreover, PDI exhibits chaperone-like activity towards proteins that contain no disulphide bonds, i.e. behaving independently of its disulphide isomerase activity [ 11 ]. The various forms of PDI which are currently known are:

• PDI major isozyme; a multifunctional protein that also function as the beta subunit of prolyl 4-hydroxylase ( 1.14.11.2 ), as a component of oligosaccharyl transferase ( 2.4.1.119 ), as thyroxine deiodinase ( 3.8.1.4 ), as glutathione-insulin transhydrogenase ( 1.8.4.2 ) and as a thyroid hormone-binding protein
• ERp60 (ER-60; 58 Kd microsomal protein). ERp60 was originally thought to be a phosphoinositide-specific phospholipase C isozyme and later to be a protease.
• ERp72.
• ERp5.

Bacterial proteins that act as thiol:disulphide interchange proteins that allows disulphide bond formation in some periplasmic proteins also contain a thioredoxin domain. These proteins include:

• Escherichia coli DsbA (or PrfA) and its orthologs in Vibrio cholerae (TtcpG) and Haemophilus influenzae (Por).
• E. coli DsbC (or XpRA) and its orthologues in Erwinia chrysanthemi and H. influenzae.
• E. coli DsbD (or DipZ) and its H. influenzae orthologue.
• E. coli DsbE (or CcmG) and orthologues in H. influenzae.
• Rhodobacter capsulatus (Rhodopseudomonas capsulata) (HelX), Rhiziobiacae (CycY and TlpA).

Thioredoxins (Trxs) are ubiquitous enzymes with a CXXC active site that catalyses the reduction of disulfide bonds. This entry represents a group of plant thioredoxin-like proteins, including AtCDSP32 from Arabidopsis and OsCDSP32 from rice. AtCDSP32 includes two Trx modules with one potential active site (219)CGPC(222) and three extra Cys, this region is responsible for the insulin reduction activity of the protein [ 12 ]. It forms a heterodimeric complex with MSRB1 (methionine sulfoxide reductases B 1) via reduction of the sulfenic acid formed on MSRB1 catalytic Cys after methionine sulfoxide reduction [ 13 ].

1. Thioredoxin. Annu. Rev. Biochem. 54, 237-71
2. Thioredoxin and glutaredoxin systems. J. Biol. Chem. 264, 13963-6
3. Thioredoxin structure and mechanism: conformational changes on oxidation of the active-site sulfhydryls to a disulfide. Structure 3, 239-43
4. Thioredoxin--a fold for all reasons. Structure 3, 245-50
5. Crystal structure of thioredoxin-2 from Anabaena. Structure 3, 1097-108
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7. Protein hydroxylation: prolyl 4-hydroxylase, an enzyme with four cosubstrates and a multifunctional subunit. FASEB J. 3, 1609-17
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9. The role of the thiol/disulfide centers and peptide binding site in the chaperone and anti-chaperone activities of protein disulfide isomerase. J. Biol. Chem. 269, 19128-35
10. Mutations in the thioredoxin sites of protein disulfide isomerase reveal functional nonequivalence of the N- and C-terminal domains. J. Biol. Chem. 269, 30946-52
11. Chaperone-like activity of protein disulfide-isomerase in the refolding of rhodanese. Eur. J. Biochem. 231, 312-6
12. Rice NTRC is a high-efficiency redox system for chloroplast protection against oxidative damage. Plant Cell 18, 2356-68
13. Plant thioredoxin CDSP32 regenerates 1-cys methionine sulfoxide reductase B activity through the direct reduction of sulfenic acid. J. Biol. Chem. 285, 14964-72