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

M Ikeda-Saito,Y Dou,T Yonetani,J.S Olson,T Li,R Regan,Q.H Gibson

doi:10.1016/s0021-9258(18)53115-x

Abstract

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.

Highlights

This paper describes the effect of: ( a ) changing the metal, ( b )changing the diatomicligand, and (c) changing the structure of the distal pocket in spermwhale myoglobin
This is not with ligand or with the metal coordinated to theporphyrin
The cobalt atom does not move relative to the porphyrin relaxation toward the initial state during the 400 ps

Summary

A PRIMARYDETERMINANT OF GEMINATE REBINDING*

Kinetic studies of myoglobin have been extended to thepicosecond range and have shown multiple ratesin the geminate recombination reaction of NO during the first few hundred picoseconds after photolysis of ligand [2, 6, 7] This heterogeneity was at first attributed to ligands following multiple paths in the protein, presumably determinedby the detailed and fluctuating structure of the distal heme pocket [2, 6, 8, 9]. Petrich et al [3] have offered an alternative explanation based on molecular dynamicssimulations of movements of the iron atom in native sperm whale myoglobin They attributed thdeecrease in the ratoef picosecond rebinding of NO to iron to movemenotf the iron ouot f the planeof the heme, making it less accessible to ligand molecules. The single mutants V68F and V68I were selected for cobalt substitution because earlier work with the corresponding iron mutants had shown large differences in their geminate reactions [2]

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION