SPECIFICITIES OF ENZYMES CORRECTED FOR SOLVATION DEPEND ON THE CHOICEOF THE STANDARD STATE

The observed specificity of enzymes is affected by the reaction medium and can be, at least partly, explained by a change in substrate solva tion. By using a transfer free energy method, enzyme kinetics in an or ganic solvent can be corrected to a chosen standard state. The choice of standard state does not affect conclusions about a single substrate . However, when two or more substrates are compared, the ''corrected s pecificity'' becomes a function of the standard state. To illustrate t his, we have studied the esterification of sulcatol and several satura ted fatty acids catalyzed by Candida rugosa lipase. The observations i n toluene (based on V-m/K-m) show a preference for C4, C8, C10, and C1 2 fatty acids. Correction to the pure liquid standard state suggests m ore or less the same specificity. But corrections to the gas phase or dilute aqueous standard states would suggest a strong preference for l onger-chain fatty acids. Hence, such corrected specificities should be used and interpreted only with great care.