Hemoglobin I from Lucina pectinata: A model for distal heme-ligand control

Lucina pectinata hemoglobin I (HbI), which is a ferric sulfide-reactive hemeprotein, contains a distal pocket characterized by the presence of GlnE7 and PheB10. To elucidate the structural-functional properties of HbI, oxygen binding kinetics and FTIR studies with recombinant HbI (rHbI) and a set of mutants were conducted using CO and CN− as sensors of the hemeprotein environment. Three νCO modes were observed for rHbI at 1936 cm−1 (A3, closed conformer) 1950 cm−1 (A1,2, closed conformer) and 1960 cm−1(A0, open conformer). These νCO were affected by substitution of GlnE7 and PheB10 in the CO complexes. The contribution of GlnE7 is demonstrated when this residue is replaced with Asn, Val or His. For instance, decreasing the positive electrostatic environment with GlnE7Val, causes an increase of 65% in the population of A0 and the disappearance and 55% reduction of the population of the A1,2 and A3 respectively. The contribution of PheB10 to the stabilization of ligands is also observed in the Leu and Tyr mutants. The PheB10Leu mutation produced an 8% decrease in the population of the A3 conformer while that of the A1,2 configuration increased by 30%. This suggests that GlnE7 and PheB10 contribute to the A3 conformer stabilizing the CO in a closed configuration. With CN− as probe no substantial differences in the νCN was observed upon substitution of GlnE7 by Val while a slight down shift in the νCN from 2120 cm−1 to 2117 cm−1 was observed in the PheB10Leu mutant. This implies that in HbICN GlnE7 moves away from the binding site while PheB10 remains in the vicinity of the bound CN−. Here, a mechanism in which the flexibility of the distal protein matrix coupled with hemeporphyrin movement toward a different configuration is suggested as an important process in the H2S transport and delivery in hemoglobin I.