The amino acid sequence of hemoglobin II from the symbiont-harboring clam Lucina pectinata

The cytoplasmic hemoglobin II from the gill of the clam Lucina pectinata consists of 150 amino acid residues, has a calculated Mm of 17,476, including heme and an acetylated N-terminal residue. It retains the invariant residues Phe 44 at position CD1 and His 65 at the proximal position F8, as well as the highly conserved Trp 15 at position A12 and Pro 38 at position C2. The most likely candidate for the distal residue at position E7, based on the alignment with other globins, is Gln 65. However, optical and EPR spectroscopic studies of the ferri Hb II (Kraus, D. W., Wittenberg, J. B., Lu, J. F., and Peisach, J., J. Biol. Chem. 265, 16054-16059, 1990) have implicated a tyrosinate oxygen as the distal ligand. Modeling of the Lucina Hb II sequence, using the crystal structure of sperm whale aquometmyoglobin, showed that Tyr 30 substituting for the Leu located at position B10 can place its oxygen within 2.8 A of the water molecule occupying the distal ligand position. This structural alteration is facilitated by the coordinate mutation of the residue at position CD4, from Phe 46 in the sperm whale myoglobin sequence to Leu 47 in Lucina Hb II.