The heme pocket geometry of Lucina pectinata hemoglobin II restricts nitric oxide and peroxide entry:: Model of ligand control for the design of a stable oxygen carrier

Blood pressure elevation has been attributed in large part to the consumption of nitric oxide (NO) by extracellular hemoglobin (Hb) therapeutics following infusion in humans. We studied NO and hydrogen peroxide (H2O2) oxidative reaction kinetics of monomeric Hbs isolated from the clam Lucina pectinata to probe the effects of their distinctive heme pocket chemistries on ligand controls and heme oxidative stability. HbI (Phe43(CD1), Gln64(E7), Phe29(B10), and Phe68(E11)) reacted with high avidity with NO (k'(ox,NO) = 91 mu M-1 s(-1)), whereas HbII (Phe44(CD1), Gln65(E7), Tyr30(B10), and Phe69(El 1)) reacted at a much slower rate (ko'(ox,NO) = 2.8 mu M-1 s(-1)). However, replacing B10 (Phe) by Tyr in recombinant HbI (HbI PheB10Tyr) produced only a 2-fold reduction in the NO-induced oxidation rate (k'(ox,NO) = 49.9 mu M-1 s(-1)). Among the clam Hbs, HbII exhibited the fastest NO dissociation and the slowest NO association with ferrous iron. Autoxidation, H2O2-mediated ferryl iron (Fe-IV) formation, and the subsequent heme degradation kinetics were much slower in HbII and HbI PheB10Tyr when compared to those of HbI. The Tyr(B10) residue appears to afford a greater heme oxidative stability advantage toward H2O2, whereas the close proximity of this residue together with Gln(E7) to the heme iron contributes largely to the distal control of NO binding. Engineering of second-generation Hb-based oxygen therapeutics that are resistant to NO/H2O2-driven oxidation may ultimately require further optimization of the heme pocket architecture to limit heme exposure to solvent.