Primary structures of Arenicola marina isomyoglobins: Molecular basis for functional heterogeneity

Abstract The primary structures of isomyoglobins MbI and MbII from the body wall musculature of the polychaete Arenicola marina were investigated, aiming to trace the molecular basis for their functional differentiation. Unexpectedly, five chains, MbI a , MbI b , MbII a , MbII b and MbII c , each consisting of 145 amino-acid residues and occurring in a ratio of ≈33 : 17 : 25 : 12.5 : 12.5 were found. All substitutions can be explained by one-point mutations. With the exception of the 41(C6)Asn→Asp(MbI/MbII) exchange that appears to be the basis for the electrophoretic separation of MbI and MbII, the substitutions do not involve drastic changes in the character of the side-chains. Pairwise comparison of MbI a and MbII a with other invertebrate globin chains indicate the following sequence of decreasing identities: Aplysia (mollusc) Mb, Chironomus (insect) CTT III hemoglobin, whale Mb and Ascaris (nematode) Mb. The marked difference in O 2 affinities between MbI and MbII appears attributable to 62Pro, which distorts the E helix around E6 and occurs in all MbII chains, but in only 33% of the MbI chains (MbI b ).