Ligand binding and protein relaxation in heme proteins: a room temperature analysis of nitric oxide geminate recombination

Ultrafast absorption spectroscopy is used to study heme-NO recombination at room temperature in aqueous buffer on time scales where the lign=and cannot leave its cage environment. While a single barrier is observed for the cage recombination of NO with heme in the absence of globin, recombination in hemoglobin and myoglobin is nonexponential. Examination of hemoglobin with and without inositol hexaphosphate points to proximal constraints as important determinants of the geminate rebinding kinetics. Molecular dynamics simulations of myoglobin and hem-imidazole subsequent to ligand dissociation were used to investigate the transient behavior of the Fe-proximal histidine coordinate and its possible involvement in geminate recombination