Hemoglobins of the Lucina pectinata/bacteria symbiosis. I. Molecular properties, kinetics and equilibria of reactions with ligands.

摘要: Three hemoglobins have been isolated from the symbiont-harboring gill of bivalve mollusc Lucina pectinata. Oxyhemoglobin I (Hb I), which may be called sulfide-reactive hemoglobin, reacts with hydrogen sulfide to form ferric hemoglobin in a reaction that proceed by nucleophilic displacement bound superoxide anion hydrosulfide anion. Hemoglobins II and II, oxygen-reactive hemoglobins, remain oxygenated presence sulfide. Hemoglobin is monomeric; Hb III self-associate concentration-dependent manner tetramer when mixed. Oxygen binding not cooperative. affinities are all nearly same, P50 = 0.1 0.2 Torr, independent pH. Combination oxygen fast; k'on (estimated) 100-200 x 10(6) M-1 s-1. slow: 0.4 0.3 s-1, respectively. Dissociation fast relative myoglobin: koff 61 0.11 0.08 These large differences rates together reactions carbon monoxide suggest configuration distal heme pocket. The comparable those lack histidine residues. Slow dissociation residue interact strongly ligand. We infer facilitate delivery chemoautotrophic bacterial symbiont oxygen. midpoint oxidation-reduction potential ferrous/ferric couple I, 103 +/- 8 mV, was Potentials were pH-dependent. At neutral pH three similar potentials. rate constant for combination increases 3000-fold 10.5 5.5, apparent pK 7.0, suggesting undissociated attacking acid limit sulfide, 2.3 10(5) 40-fold faster than or myoglobin.(ABSTRACT TRUNCATED AT 400 WORDS)