Structure and ligand selection of hemoglobin II from Lucina pectinata

Lucina pectinata ctenidia harbor three heme proteins: sulfide-reactive hemoglobin I (HbI_Lp) and the oxygen transporting hemoglobins II and III (HbII_Lp and HbIII_Lp) 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 HbII_Lp shows a dimeric oxyHbII_Lp 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 HbII_Lp than in HbI_Lp. 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 HbII_Lp 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 HbI_Lp Phe(B10) → Tyr and HbII_Lp 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 Fe^II to Fe^III.