Crystal structure of the Escherichia coli peptide methionine sulphoxide reductase at 1.9 angstrom resolution

Background: Peptide methionine sulphoxide reductases catalyze the reduction of oxidized methionine residues in proteins. They are implicated in the de fense of organisms against oxidative stress and in the regulation of proces ses involving peptide methionine oxidation/reduction. These enzymes are fou nd in numerous organisms, from bacteria to mammals and plants. Their primar y structure shows no significant similarity to any other known protein. Results: The X-ray structure of the peptide methionine sulphoxide reductase from Escherichia coli was determined at 3 Angstrom resolution by the multi ple wavelength anomalous dispersion method for the selenomethionine-substit uted enzyme, and it was refined to 1.9 Angstrom resolution for the native e nzyme. The 23 kDa protein is folded into an alpha/beta roll and contains a large proportion of coils. Among the three cysteine residues involved in th e catalytic mechanism, Cys-51 is positioned at the N terminus of an a helix , in a solvent-exposed area composed of highly conserved amino acids. The t wo others, Cys-198 and Cys-206, are located in the C-terminal coil. Conclusions: Sequence alignments show that the overall fold of the peptide methionine sulphoxide reductase from E. coli is likely to be conserved in m any species. The characteristics observed in the Cys-51 environment are in agreement with the expected accessibility of the active site of an enzyme t hat reduces methionine sulphoxides in various proteins. (Cys-51 could be ac tivated by the influence of an a helix dipole. The involvement of the two o ther cysteine residues in the catalytic mechanism requires a movement of th e C-terminal coil. Several conserved amino acids and water molecules are di scussed as potential participants in the reaction.