Activation volumes for horseradish peroxidase compound ii reactions

The activation volumes for the reactions of horseradish peroxidase compound II with L-tyrosine. 3-iodo-L-tyrosine. p-aminobenzoic acid and ferrocyanide were determined by using a high-pressure stopped-flow technique at 25°C and pH 7. For the tyrosines, the solvent electrostriction accompanying substrate ionization and H⁺ transfer from the substituted phenol to a basic group of the enzyme can account for the observed negative activation volumes. For p-aminobenzoic acid a simple electron transfer without H⁺ transfer appears to occur. The positive activation volume for ferrocyanide may be explained in terms of electron transfer associated with a large change in electrostriction of the inorganic redox couple.