The reaction occurs in the reverse direction to that shown above. The enzyme exhibits high specificity for reduction of the R-form of methionine S-oxide, with higher activity being observed with L-methionine S-oxide than with D-methionine S-oxide . While both free and protein-bound methionine (R)-S-oxide act as substrates, the activity with the peptide-bound form is far greater . The enzyme plays a role in preventing oxidative-stress damage caused by reactive oxygen species by reducing the oxidized form of methionine back to methionine and thereby reactivating peptides that had been damaged. In some species, e.g. Neisseria meningitidis, both this enzyme and EC 188.8.131.52, peptide-methionine (S)-S-oxide reductase, are found within the same protein whereas in other species, they are separate proteins [3,5]. The reaction proceeds via a sulfenic-acid intermediate [5,10]. For MsrB2 and MsrB3, thioredoxin is a poor reducing agent but thionein works well . The enzyme from some species contains selenocysteine and Zn2+.