Thermodynamic and extrathermodynamic requirements of enzyme catalysis.

An enzyme's affinity for the altered substrate in the transition state (symbolized here as S) matches the value of k(cat)/K(m) divided by the rate constant for the uncatalyzed reaction in water. The validity of this relationship is not affected by the detailed mechanism by which any particular enzyme may act, or on whether changes in enzyme conformation occur on the path to the transition state. It subsumes potential effects of substrate desolvation, H-bonding and other polar attractions, and the juxtaposition of several substrates in a configuration appropriate for reaction. The startling rate enhancements that some enzymes produce have only recently been recognized. Direct measurements of the binding affinities of stable transition-state analog inhibitors confirm the remarkable power of binding discrimination of enzymes. Several parts of the enzyme and substrate, that contribute to S binding, exhibit extremely large connectivity effects, with effective relative concentrations in excess of 10(8) M. Exact structures of enzyme complexes with transition-state analogs also indicate a general tendency of enzyme active sites to close around S in such a way as to maximize binding contacts. The role of solvent water in these binding equilibria, for which Walter Kauzmann provided a primer, is only beginning to be appreciated.