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

Effects of the water activity (a(w)) and the solvent ordering, as determine d by the activity coefficient of water, were investigated on the enzyme kin etics of alcohol dehydrogenase, lysozyme, and beta-galactosidase in various aqueous solutions. The water activity and the solvent orderings were adjus ted by addition of electrolytes (NaCl, KCl, CsCl, etc.) or nonelectrolytes (sugars, alcohols, urea, etc.) at various concentrations. Although the enzy me kinetics were strongly dependent on a(w), a(w) was not a complete determ inant of the enzyme behavior in aqueous solutions. Enzyme kinetics were als o dependent on the solvent ordering. At a fixed a(w), all the enzyme kineti c parameters tested had a good correlation with the solvent ordering parame ter as represented by the parameter alpha, an index of the deviation of the water state from the ideal solution, determined from the activity coeffici ent of water in solutions. Solvent ordering was expected to affect the enzy me kinetics through its effect on the hydrophobic interaction between the e nzyme and the substrate and also on the thermal fluctuation. (C) 2000 Elsev ier Science Inc. All rights reserved.