Tyrosine B10 and heme-ligand interactions of Lucina pectinata hemoglobin II: Control of heme reactivity

The distal pocket of hemoglobin II (HbII) from Lucina pectinata is characterized by the presence of a GlnE7 and a TyrB10. To elucidate the functional properties of HbII, biophysical studies were conducted on HbII and a HbI PheB10Tyr site-directed mutant. The pH titration data at neutral conditions showed visible bands at 486, 541, 577 and 605 nm for both proteins. This suggests the possible existence of a conformational equilibrium between an open and closed configuration due to the interactions of the TyrB10, ligand, and heme iron. The kinetic behavior for the reaction of both ferric proteins with H ₂O₂ indicates that the rate for the formation of the ferryl intermediates species varies with pH, suggesting that the reaction is strongly dependent on the conformational states. At basic pH values, the barrier for the reaction increases as the tyrosine adopts a closed conformation and the ferric hydroxyl replaces the met-aquo species. The existence of these conformers is further supported by resonance Raman (RR) data, which indicate that in a neutral environment, the ferric HbII species is present as a possible mixture of coordination and spin states, with values at 1558 and 1580 cm ^-1 for the ν₂ marker, and 1479, 1492, and 1503 cm ^-1 for the ν₃ mode. Moreover, the presence of the A₃ and A_o conformers at 1924 and 1964 cm^-1 in the HbII-CO infrared spectra confirms the existence of an open and closed conformation due to the orientation of the TyrB10 with respect to the heme active center.