Amino acid sequence of polypeptide chain IIB of extracellular hemoglobin from the polychaete Tylorrhynchus heterochaetus.

The giant extracellular hemoglobin from the polychaete Tylorrhynchus heterochaetus consists of two types of subunits: a "monomeric" chain (chain I) and a disulfide-bonded trimer of chains IIA, IIB, and IIC. The complete amino acid sequence of chain IIB was determined. This chain has 148 amino acid residues and a molecular weight of 17,236 including a heme group. Of the residues in chain IIB, 74 (50%) and 34 (30%) were found to be identical with those in the corresponding positions in Tylorrhynchus chains IIC and I, respectively (Suzuki, T., Furukohri, T., and Gotoh, T. (1985) J. Biol. Chem. 260, 3145-3154). Marked differences were found between the chains of Tylorrhynchus and Lumbricus in the COOH-terminal regions. Significant differences were predicted between the monomeric chain I and the "trimeric" chains (IIB and IIC) in the hydropathy profiles and alpha-helical contents.