THE AMINO-ACID-SEQUENCE OF HEMOGLOBIN-III FROM THE SYMBIONT-HARBORINGCLAM LUCINA-PECTINATA

The cytoplasmic hemoglobin III from the gill of the symbiont-harboring clam Lucina pectinata consists of 152 amino acid residues, has a calc ulated M(m) of 18,068, including heme, and has N-acetyl-serine as the N-terminal residue. Based on the alignment of its sequence with other vertebrate and nonvertebrate globins, it retains the invariant residue s Phe45 at position CD1 and His98 at the proximal position F8, as well as the highly conserved Trp16 and Pro39 at positions A12 and C2, resp ectively. The most likely candidate for the distal residue at position E7 is Gln66. Lucina hemoglobin III shares 95 identical residues with hemoglobin II (J. D. Hockenhull-Johnson et al., J. Prot. Chem. 10, 609 -622, 1991), including Tyr at position B10, which has been shown to be capable of entering the distal heme cavity and placing its hydroxyl g roup within a 2.8 angstrom of the water molecule occupying the distal ligand position, by modeling the hemoglobin II sequence using the crys tal structure of sperm whale metmyoglobin. The amino acid sequences of the two Lucina globins are compared in detail with the known sequence s of mollusc globins, including seven cytoplasmic and 11 intracellular globins. Relative to 75% homology between the two Lucina globins (cou nting identical and conserved residues), both sequences have percent h omology scores ranging from 36-49% when compared to the two groups of mollusc globins. The highest homology appears to exist between the Luc ina globins and the cytoplasmic hemoglobin of Busycon canaliculatum.