Abstract
The rate of CO binding to myoglobin increases 4-fold, from 5 X 10(5) M-1 s-1 to 2 X 10(6) M-1 s-1, in going from 0 to 80% glycerol in phosphate buffer at pH 7, 20 degrees C. Under the same conditions, the rate of CO binding to protoheme decreases monotonically from about 1 X 10(8) M-1 s-1 to 2 X 10(7) M-1 s-1. The kinetic behavior of protoheme at neutral pH is that expected for a diffusion-controlled reaction. Increasing solvent viscosity causes a decrease in the observed second order rate constant. In contrast, the behavior of the myoglobin indicates quite clearly that internal, nondiffusive processes are limiting the speed of the reaction. The rate enhancement is due to an increase in the standard chemical potential of the ligand molecule as the polyalcohol concentration is increased. Both types of behavior are observed for ligand binding to protoheme in 0.1 N NaOH; first an increase and then a decrease in rate is observed as the concentration of glycerol is increased. At low glycerol concentrations, the reaction rate is limited by a first order process. At high concentrations, the rate becomes diffusion-controlled and exhibits a dependence on the reciprocal of the solvent viscosity. The data for all these conditions have been analyzed empirically in terms of a single free energy barrier and more specifically in terms of a consecutive reaction scheme.
Highlights
Under the same conditions, the rate of CO binding to protoheme decreases monotonicallyfrom about 1 X lo8 M" s-l to 2 x lo7 M" s-'
The solvent composition must be affecting the results were obtained by rapid mixing and flash photolysis equilibrium chemical potentialsof the protein and theligand techniques
In the viscosity and changing chemical potentials, by comparing the latter case, CO is driven out of the protein by photolysis, so thermodynamic parametersfor CO binding to myoglobin with that theunliganded protein conformation has existed for only the solubility properties of the gas in various glycerol-buffer about 0.5 ms prior to rebinding
Summary
The rate of CO binding to protoheme decreases monotonicallyfrom about 1 X lo8 M" s-l to 2 x lo7 M" s-'. When the solvent viscosity is increased by adding polyethylene glycol polymerslittle or no change in the rate ofCO binding to myoglobin is observed.
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