The magnitude of electrostatic interactions in inhibitor binding and during catalysis by ribonuclease A

It is demonstrated that a model of nucleotide binding to ribonuclease A similar to that proposed by Hammes and coworkers (G. G. Hammes (1968), Adv. Protein Chem. 23, 1) is at least, approximately applicable for both cyclic nucleotide substrates and mononucleotide inhibitors at pH values less than or equal to 6.5 and as a function of ionic strength. Calorimetric data on various inhibitors show that the binding reaction can be thermodynamically dissected into a contribution arising from van der Waal's interaction of the nucleoside moiety, characterized by a large negative enthalpy change, and a contribution arising from electrostatic interactions between the negatively charged phosphate group of the inhibitor and the positively charged protein fabric, characterized by a large positive unitary entropy change. Assuming a catalytic mechanism involving the formation of a dianionic pentacoordinated phosphate transition state intermediate, the magnitude of the effect of electrostatic interactions on the overall rate enhancement by the enzyme is estimated to be 2 times 10(2) to 10(6). It is suggested that this effect, along with substrate approximation effects, is sufficient to "explain" the catalytic behavior of the enzyme.