Influence of water activity and aqueous solvent ordering on enzyme kinetics of alcohol dehydrogenase, lysozyme, and β-galactosidase

Effects of the water activity (aw) and the solvent ordering, as determined by the activity coefficient of water, were investigated on the enzyme kinetics of alcohol dehydrogenase, lysozyme, and β-galactosidase in various aqueous solutions. The water activity and the solvent ordering were adjusted by addition of electrolytes (NaCl, KCl, CsCl, etc.) or nonelectrolytes (sugars, alcohols, urea, etc.) at various concentrations. Although the enzyme kinetics were strongly dependent on aw, aw was not a complete determinant of the enzyme behavior in aqueous solutions. Enzyme kinetics were also dependent on the solvent ordering. At a fixed aw, all the enzyme kinetic parameters tested had a good correlation with the solvent ordering parameter as represented by the parameter α, an index of the deviation of the water state from the ideal solution, determined from the activity coefficient of water in solutions. Solvent ordering was expected to affect the enzyme kinetics through its effect on the hydrophobic interaction between the enzyme and the substrate and also on the thermal fluctuation.