Part of the MsrPQ system that repairs oxidized periplasmic proteins containing methionine sulfoxide residues (Met-O), using respiratory chain electrons. Thus protects these proteins from oxidative-stress damage caused by reactive species of oxygen and chlorine generated by the host defense mechanisms. MsrPQ is essential for the maintenance of envelope integrity under bleach stress, rescuing a wide series of structurally unrelated periplasmic proteins from methionine oxidation, including the primary periplasmic chaperone SurA and the lipoprotein Pal. The catalytic subunit MsrP is non-stereospecific, being able to reduce both (R-) and (S-) diastereoisomers of methionine sulfoxide. a quinone + H2O + L-methionyl-[protein] = a quinol + L-methionyl-(S)-S-oxide-[protein] a quinone + H2O + L-methionyl-[protein] = a quinol + L-methionyl-(R)-S-oxide-[protein] Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Heterodimer of a catalytic subunit (MsrP) and a heme-binding subunit (MsrQ). Is attached to the inner membrane when interacting with the MsrQ subunit. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the MsrP family.