Reactivity of ferrous heme proteins at low pH

The pH rate profile of sperm whale myoglobin (Mb) reacting with CO has been confirmed to follow the behavior previously reported (Giacometti, G.M., Traylor, T.G., Ascenzi, P., Brunori, M., and Antonini, E. (1977) J. Biol. Chem. 252, 7447-7448), and appears to be different from that obtained by others. The pH investigation has been extended to the CO-binding rates of Chironomus thummi thummi erythrocruorin and Aplysia limacina Mb, whose pH rate profile is different from that of sperm whale Mb. Besides the "base dissociation" mechanism previously invoked, the iron atom-heme plane distance is discussed as a possible determinant of the CO reactivity in these monomeric heme proteins.