ANATOMY AND DYNAMICS OF A LIGAND-BINDING PATHWAY IN MYOGLOBIN - THE ROLES OF RESIDUE-45, RESIDUE-60, RESIDUE-64, AND RESIDUE-68

In order for diatomic ligands to enter and exit myoglobin, there must be substantial displacements of amino acid side chains from their posi tions in the static X-ray structure. One pathway, involving Arg/Lys45, His64, and Val68, has been studied in greatest detail. In an earlier study (Lambright et al., 1989) we reported the surprising result that mutation of the surface residue Lys45 to arginine lowers the inner bar rier to CO rebinding. Until then, it had been thought that this barrie r primarily involves interior distal pocket residues such as His64 and Val68. In this report, we present a detailed study of the CO rebindin g kinetics in aqueous solution of a series of single- and double-site mutants of human myoglobin at positions 64, 68, 45, and 60. On the bas is of the observed kinetics, we propose that the effect of surface res idue 45 on the inner barrier can be explained by a chain of interactio ns between surface and pocket residues. Very large, and in some cases unexpected, changes are observed in the kinetics of recombination and in the partitioning between geminate and bimolecular recombination.