Oxygen and Nitric Oxide Rebinding Kinetics in Nonsymbiotic Hemoglobin AHb1 from Arabidopsis thaliana

Type 1 nonsymbiotic hemoglobin from Arabidopsis thaliana (Alibi) shows a partial bis-histidyl hexacoordination but can reversibly bind diatomic ligands. The physiological function is still unclear, but the high oxygen affinity rules out a function related to O-2 sensing, carrying, or storing. To gain insight into its possible functional roles, we have investigated its O-2 and NO rebinding kinetics after laser flash photolysis. The rate constants of the rebinding from the primary docking site for both O-2 and NO are higher than CO, with lower photolysis yields. Moreover, the amplitude of the geminate phase increases and, as for CO, the numerical analysis of the experimental curves suggests the existence of an internal pathway leading, with high rate, to an additional docking site. However, the accessibility to this site seems to be strongly ligand-dependent, being lower for O-2 and higher for NO.(C) 2011 IUBMB IUBMB Life, 63(12): 1094-1100, 2011