Activation volumes in enzymic catalysis: their sources and modification by low-molecular-weight solutes.

Changes in enzyme conformation are often accompanied by large changes in volume. Model compound studies suggest that these volume changes may derive from two sources: (i) "hydration density" effects due to changes in the exposure to solvent of protein groups which modify water density, and (ii) "structural" contributions arising from changes in the volume of the protein itself. An experimental approach was developed to test the validity of the predictions based on model compound studies for catalytic conformational changes. By examining the effects of different solutes on the activation volumes of different enzymic reactions, we show that both sources of volume change provide significant contributions to the activation volume.