ENERGETIC EFFECTS OF MULTIPLE HYDROGEN-BONDS - IMPLICATIONS FOR ENZYMATIC CATALYSIS

The rate enhancements obtained by enzymes often require greater than 2 0 kcal/mol of stabilization that is specific to the transition state. Inspection of active sites suggests that enzymes often use multiple in teractions with substrate groups undergoing charge rearrangement to he lp provide this preferential stabilization. We have analyzed the depro tonation, in DMSO, of benzoic acids with one or two hydrogen bond dona ting substituents ortho to the carboxylic acid. This provides a model for the ability of multiple hydrogen bonds to facilitate reactions at enzymatic active sites. The simplicity of this model has allowed the e nergetic contributions of the hydrogen bonds to be isolated and assess ed quantitatively. It was found that one and two hydrogen bonds from t he hydroxyl group(s) ortho to the carboxyl group provide specific stab ilization of the carboxylate relative to the carboxylic acid by 7.9 an d 14.4 kcal/mol, respectively. The energetic contribution of the two h ydrogen bonds is nearly additive. This provides a quantitative demonst ration of the basic principle that the energy of multiple interactions , each of moderate strength, can be combined to make a significant con tribution to enzymatic catalysis.