SOLVENT VARIATION INVERTS SUBSTRATE-SPECIFICITY OF AN ENZYME

The substrate specificity of the serine protease subtilisin Carlsberg in the transesterification reaction of N-Ac-L-Ser-OEt and N-Ac-L-Phe-OEt with 1-propanol was examined in 20 anhydrous solvents. The serine substrate was strongly favored in some solvents, while the phenylalanine substrate was greatly preferred in others. A thermodynamic model was derived which correctly predicted the substrate specificity as a function of the solvent-to-water partition coefficients of the substrates and the substrate specificity of the enzyme-catalyzed hydrolysis of the esters in water. This model is independent of the enzyme and the substrate, so long as the latter is removed from the solvent in the transition state.