Dissecting NO Association and Dissociation Dynamics of Myoglobin, Hemoglobin, and Heme-Model Compounds

Nitric oxide (NO) binding to heme-proteins is a physiologically relevant molecular process, contributing to regulate blood pressure and flow to provide an adequate O₂ supply to all body compartments. This appears to be relevant for energy-demanding tissues, characterized by high O₂-consuming rates, such as the eye's retina and striated muscles under physical effort. The reaction of NO with the ferrous heme-Fe atom (heme-Fe(II)) is characterized by very fast association rate constants and very slow dissociation kinetics, which drastically differ from those observed for other gaseous ligands, such as O₂ and CO. Since the heme-Fe(II)-NO bond is photosensitive, it is possible to investigate both the bimolecular dynamics of the NO pathway from the bulk solution to the metal center and ligand trapping within cavities, located in the protein matrix and affecting heme-Fe(II)-NO complex formation and dissociation. Here, dynamics of NO binding to human hemoglobin and sperm whale and horse heart myoglobin, both involved in the supply of O₂ to tissues and organs, are reported and compared with heme-model compounds, dissecting the various steps of the ligand migration from the bulk solution through the protein matrix to the heme active center, correlating dynamics with structural features to clarify the structural determinants of heme-Fe(II) nitrosylation steps.