InterPro domain: IPR002579

Peptide methionine sulphoxide reductase (Msr) reverses the inactivation of many proteins due to the oxidation of critical methionine residues by reducing methionine sulphoxide, Met(O), to methionine [ 1 ]. It is present in most living organisms, and the cognate structural gene belongs to the so-called minimum gene set [ 2 , 3 ].

The domains: MsrA and MsrB, reduce different epimeric forms of methionine sulphoxide. This group represents MsrB, the crystal structure of which has been determined to 1.8A [ 4 ]. The overall structure shows no resemblance to the structures of MsrA ( IPR002569 ) from other organisms; though the active sites show approximate mirror symmetry. In each case, conserved amino acid motifs mediate the stereo-specific recognition and reduction of the substrate. Unlike the MsrA domain, the MsrB domain activates the cysteine or selenocysteine nucleophile through a unique Cys-Arg-Asp/Glu catalytic triad. The collapse of the reaction intermediate most likely results in the formation of a sulphenic or selenenic acid moiety. Regeneration of the active site occurs through a series of thiol-disulphide exchange steps involving another active site Cys residue and thioredoxin.

In a number of pathogenic bacteria, including Neisseria gonorrhoeae, the MsrA and MsrB domains are fused; the MsrA being N-terminal to MsrB. This arrangement is reversed in Treponema pallidum. In N. gonorrhoeae and Neisseria meningitidis, a thioredoxin domain is fused to the N terminus. This may function to reduce the active sites of the downstream MsrA and MsrB domains.

1. Thiol-disulfide exchange is involved in the catalytic mechanism of peptide methionine sulfoxide reductase. Proc. Natl. Acad. Sci. U.S.A. 97, 6463-8
2. Complete genome sequences of cellular life forms: glimpses of theoretical evolutionary genomics. Curr. Opin. Genet. Dev. 6, 757-62
3. A minimal gene set for cellular life derived by comparison of complete bacterial genomes. Proc. Natl. Acad. Sci. U.S.A. 93, 10268-73
4. The mirrored methionine sulfoxide reductases of Neisseria gonorrhoeae pilB. Nat. Struct. Biol. 9, 348-52