Structure and ligand selection of hemoglobin II from Lucina pectinata

Lucina pectinata ctenidia harbor three heme proteins: sulfide-reactive hemoglobin I (HbILp) and the oxygen transporting hemoglobins II and III (HbIILp and HbIIILp) that remain unaffected by the presence of H2S. The mechanisms used by these three proteins for their function, including ligand control, remain unknown. The crystal structure of oxygen-bound HbIILp shows a dimeric oxyHbIILp where oxygen is tightly anchored to the heme through hydrogen bonds with Tyr 30(B10) and Gln65(E7). The heme group is buried farther within HbIILp than in HbILp. The proximal His 97(F8) is hydrogen bonded to a water molecule, which interacts electrostatically with a propionate group, resulting in a Fe-His vibration at 211 cm-1. The combined effects of the HbIILp small heme pocket, the hydrogen bonding network, the His97 trans-effect, and the orientation of the oxygen molecule confer stability to the oxy-HbII Lp complex. Oxidation of HbILp Phe(B10) → Tyr and HbIILp only occurs when the pH is decreased from pH 7.5 to 5.0. Structural and resonance Raman spectroscopy studies suggest that HbII Lp oxygen binding and transport to the host bacteria may be regulated by the dynamic displacements of the Gln65(E7) and Tyr 30(B10) pair toward the heme to protect it from changes in the heme oxidation state from FeII to FeIII. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc