Ligand diffusion in the distal heme pocket of myoglobin. A primary determinant of geminate rebinding

There are at least two picosecond kinetic components in the rebinding of NO to native sperm whale myoglobin. Petrich et al. (Petrich, J. W., Lambry, J.-C., Kuczera, K., Karplus, M., Poyart, C., and Martin, J.-L. (1991) Biochemistry 30, 3975-3987) attribute the slowing of the reaction to a movement of the iron atom out of the plane of the heme following ligand dissociation. In contrast, Gibson et al. (Gibson, Q. H., Regan, R., Elber, R., Olson, J. S., and Carver, T. E. (1992) J. Biol. Chem. 267, 22022-22034) have explained multiphasic geminate reactions by diffusion of NO into the distal heme pocket as determined by its detailed structure. O2 and NO rebinding to iron and cobalt derivatives of native, V68F, and V68I sperm whale myoglobin has been examined. Each iron protein shows a biphasic time course of NO rebinding reactions with widely different rates and amplitudes. Although cobalt does not move out of the plane of the porphyrin on ligand removal, the reactions of the iron and cobalt derivatives of each protein were closely similar. The time course of O2 rebinding to cobalt was also similar to that of NO rebinding to iron. These results are consistent with a primary role for the structure of the distal pocket in determining diffusion of ligands away from the metal atom and as a result the time course of picosecond ligand rebinding.