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. MsrQ provides electrons for reduction to the reductase catalytic subunit MsrP, using the quinone pool of the respiratory chain (By similarity). Probably involved in protection against reactive chlorine species (RCS) generated by chlorite and hypochlorite (PubMed:25968643). Heterodimer of a catalytic subunit (MsrP) and a heme-binding subunit (MsrQ). Belongs to the MsrQ family.