The effect of water on enzyme action in organic media

Three model, unrelated enzymes (yeast alcohol oxidase, mushroom polyphenol oxidase, and horse liver alcohol dehydrogenase) were found to be catalytically active in a variety of organic solvents. For all enzymes and solvents tested, the enzymatic activity greatly increased upon an increase in the water content in the solvents (which always remained below the solubility limit). Much less water was required to reach the maximal activity in hydrophobic solvents than in their hydrophilic counterparts. However, when the catalytic activity was plotted versus the amount of water bound to the enzymes, a common pattern emerged for different solvents. These data suggest that the effect of organic solvents on an enzyme is primarily due to interactions with the enzyme-bound, essential layer of water rather than with the enzyme itself. At optimal water contents, enzymatic activities in organic solvents were in the range from 20 to 40% of those in aqueous solutions. From experiments on (i) replacement of water with other hydrogen bond-forming additives and (ii) titration of enzyme amino groups in an organic medium, as well as the literature data on dehydrated enzymes, it is concluded that the water required by enzymes in nonaqueous solvents provides them with sufficient conformational flexibility needed for catalysis.