Effect of mixed solvents on the formation of horseradish peroxidase compound I. The importance of diffusion-controlled reactions

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Abstract
In aqueous glycerol solutions, the rate constant for the formation of compound I of horseradish peroxidase [EC 1.11.1.7] from hydrogen peroxide is independent of the viscosity. However, when m-chloroperbenzoic acid is used to form compound I, the rate is viscosity dependent, indicating that this reaction is diffusion controlled. The problems in obtaining accurate information about diffusion-controlled reactions and the possible significance of diffusion control are discussed. Ethanol was used in previous studies to decrease the dielectric constant of the medium but it was found that its dominant effect on horseradish peroxidase was to bind to the sixth coordination position of the heme ion. The stability of the native enzyme in different environments was tested by difference absorption and circular dichroic spectroscopy and new circular dichroic data for compound I are reported.