Modulating carbon monoxide binding affinity and kinetics in myoglobin: The roles of the distal histidine and the heme pocket docking site

Myoglobin has long served as a model system for understanding the relations between protein structure, dynamics, and function. Its ability to discriminate between toxic CO and vital O₂, two small ligands that are almost equivalent in size and dipole moment, has attracted much attention. To understand discrimination and reversible ligand-binding in Mb, both the bound state and the 'docked' state that leads to binding need to be studied. We have reported previously the nearly linear Fe-C-O geometry of bound CO and the nearly orthogonal geometry of docked CO [Lim et al. (1995), Science 269:962]. With the exception of X-ray structures, a preponderance of evidence points to a nearly linear Fe-C-O geometry and calls into question the proposal that the highly conserved distal histidine forces CO to bind in a nonoptimal geometry. The differences between the bound CO structures determined using IR and X-ray methods might arise from a water molecule hydrogen bonded to the distal histidine in some of the unit cells. Discrimination by Mb is manifested not only thermodynamically but also kinetically. Time-resolved CO rebinding studies that compare Mb with microperoxidase suggest that the heme pocket docking site in Mb exerts steric control of the ligand rebinding rate, slowing the rate of CO binding by a factor of more than 10⁴.