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 rela tions between protein structure, dynamics, and function. Its ability t o discriminate between toxic CO and vital O-2, two Small ligands that are almost equivalent in size and dipole moment, has attracted much at tention. To understand discrimination and reversible ligand-binding in Mb, both the bound state and the ''docked'' state that leads to bindi ng need to be studied. We have reported previously the nearly linear F e-C-O geometry of bound CO and the nearly orthogonal geometry of docke d 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 cons erved 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 ma nifested not only thermodynamically but also kinetically. Time-resolve d CO rebinding studies that compare Mb with microperoxidase suggest th at the heme pocket docking site in Mb exerts steric control of the lig and rebinding rate, slowing the rate of CO binding by a factor of more than 10(4).