NITRIC-OXIDE RECOMBINATION TO DOUBLE MUTANTS OF MYOGLOBIN - ROLE OF LIGAND DIFFUSION IN A FLUCTUATING HEME POCKET

Picosecond recombination of nitric oxide to the double mutants of myoglobin, His64Gly.Val68Ala and His64Gly.Val68Ile, at E7 and E11, has been studied experimentally and by computation. It is shown that distal residues have a profound effect on NO recombination. Recombination in the mutants may be explained in terms of fluctuating free volume and structure of the heme pocket. The double mutants provide insight into the effects of free volume and steric hindrance on rates of ligand rebinding following photolysis. Water molecules of the first solvation shell replace surface residues deleted by mutation and can block apparent holes in the protein structure. Thus, water molecules extend the time required for ligands to escape significantly to a nanosecond time scale, which is much longer than would be expected for an open heme pocket. Both nearly exponential (G64A68) and markedly nonexponential (native and G64I68) kinetics are observed, a result at variance with expectation from the model of Petrich et al. [Petrich, J. W., Lambry, J. C., Kuczera, K., Karplus, M., Poyart, C., and Martin, J. L. (1991) Biochemistry 30, 3975-3987], which attributes nonexponential kinetics to proximal effects.