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 [ 10841552]. It is present in most living organisms, and the cognate structural gene belongs to the so-called minimum gene set [8994848, 8816789].
The domains: MsrA and MsrB, reduce different epimeric forms of methionine sulphoxide. This group represent MsrA, the crystal structure of which has been determined in a number of organisms. In Mycobacterium tuberculosis, the MsrA structure has been determined to 1.5 Aż resolution [12837786].
In contrast to the three catalytic cysteine residues found in previously characterised MsrA structures, M. tuberculosis MsrA represents a class containing only two functional cysteine residues. The overall structure shows no resemblance to the structures of MsrB 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.
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. | 
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