ENZYMATIC TRANSITION-STATES AND INHIBITOR DESIGN FROM PRINCIPLES OF CLASSICAL AND QUANTUM-CHEMISTRY

A procedure is described which leads to experimentally based models fo r the transition-state structures of enzyme-catalyzed reactions. Subst rates for an enzymic reaction are synthesized with isotopically enrich ed atoms at every position in which bonding changes are anticipated at the enzyme-enforced transition state. Kinetic isotope effects are mea sured for each atomic substitution and corrected for diminution of the isotope effects from nonchemical steps of the enzymic mechanism. A tr uncated geometric model of the transition-state structure is fitted to the kinetic isotope effects using bond-energy bond-order vibrational analysis. Full molecularity is restored to the transition state while maintaining the geometry of the bonds which define the transition stat e. Electronic wave functions are calculated for the substrate and the transition-state molecules. The molecular electrostatic potential ener gies are defined for the van der Waal surfaces of substrate and transi tion state and displayed in numerical and color-coded constructs. The electronic differences between substrate and transition state reveal c haracteristics of the transition state which permits the extraordinary binding affinity of enzyme-transition state interactions. The informa tion has been used to characterize several enzymatic transition states and to design powerfully inhibitory transition-state analogues. Enzym atic examples are provided for the reactions catalyzed by AMP deaminas e, nucleoside hydrolase, purine nucleoside phosphorylase, and for seve ral bacterial toxins. The results demonstrate that the combination of experimental, classical, and quantum chemistry approaches is capable o f providing reliable transition-state structures and sufficient inform ation to permit the design of transition-state inhibitors. (C) 1996 Jo hn Wiley & Sons, Inc.