Amino acid sequence of the cooperative homodimeric hemoglobin from the mollusc Scapharca inaequivalvis and topology of the intersubunit contacts.

The dimeric hemoglobin (HbI) from Scapharca inaequivalvis is highly homologous to the other known dimeric Acid hemoglobins. The sequence has a distinctive hydrophobicity profile in the region corresponding to the E and F helices with respect to both the hemoglobin and myoglobin chains from vertebrates due to the presence of several additional hydrophobic residues. The characteristic topology of the E and F helices is conserved in all the known sequences of Arcid hemoglobins including that of the so-called alpha chain of the tetrameric component from Anadara trapezia. The rationale for this conservation lies in the unusual assembly of Arcid hemoglobins where the E and F helices are involved in the interdimeric contact. It is suggested that the extra hydrophobic residues play a major role in the assembly of the basic dimeric unit in these hemoglobins.