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. Binds 1 FMN per subunit. Binds 1 heme b (iron(II)-protoporphyrin IX) group per subunit. Heterodimer of a catalytic subunit (MsrP) and a heme-binding subunit (MsrQ). Belongs to the MsrQ family.