PRIMARY STRUCTURES OF ARENICOLA-MARINA ISOMYOGLOBINS - MOLECULAR-BASIS FOR FUNCTIONAL-HETEROGENEITY

The primary structures of isomyoglobins MbI and MbII from the body wal l musculature of the polychaete Arenicola mat-inn were investigated, a iming to trace the molecular basis for their functional differentiatio n. Unexpectedly, five chains, MbI(a), MbI(b), MbII(a), MbII(b) and MbI I(c), each consisting of 145 amino-acid residues and occurring in a ra tio of approximate to 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 th e 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 chain s indicate the following sequence of decreasing identities: Aplysia (m ollusc) Mb, Chironomus (insect) CTT III hemoglobin, whale Mb and Ascar is (nematode) Mb. The marked difference in O-2 affinities between MbI and MbII appears attributable to 62Pro, which distorts the E helix aro und E6 and occurs in all MbII chains,but in only 33% of the MbI chains (MbI(b)). (C) 1998 Elsevier Science B.V.