The rate enhancement produced by the ribosome: an improved model.

As a model for mechanistic comparison with peptidyl transfer within the ribosome, the reaction of aqueous glycinamide with N-formylphenylalanine trifluoroethyl ester (fPhe-TFE) represents an improvement over earlier model reactions involving Tris. The acidity of trifluoroethanol (pKa 12.4) resembles that of tRNA (12.98) more closely than do the acidities of model reactants described earlier, and the reactivity of the simple nucleophile glycinamide is free of potential complications that arise from alternative reaction pathways available to Tris. At 25 degrees C, the uncatalyzed reaction of glycinamide with fPhe-TFE proceeds with a second-order rate constant of 3 x 10(-5) M-1 s-1; DeltaH(++) = +7.8 kcal/mol; TDeltaS(++)= -15.7 kcal/mol. The ribosomal reaction of puromycin with fMet-tRNA proceeds 3 x 107-fold more rapidly, with a second-order rate constant (kcat/Km) of 1 x 10(3) M-1 s-1; DeltaH(++) = +16.0 kcal/mol; TDeltaS(++)= +2.0 kcal/mol. That rate enhancement, an order of magnitude larger than estimated earlier, is fully explained by the more favorable entropy of activation of the ribosomal reaction. Experiments involving ethylene glycol esters suggest that neighboring -OH group effects are negligible in the presence of solvent water, which itself acts as a general base catalyst. In the desolvated interior of the ribosome, the vicinal 2'-OH group of aminoacyl-tRNA probably replaces water as a general base catalyst. But the catalytic effect of the ribosome itself is overwhelmingly entropic in origin, suggesting that the ribosome achieves its effect by physical desolvation and/or juxtaposition of the reactants in a manner conducive to peptidyl transfer.