DIRECT DEMONSTRATION OF THE CATALYTIC ROLE OF BINDING INTERACTIONS INAN ENZYMATIC-REACTION

It has been suggested that the fundamental feature that distinguishes enzymes from simple chemical catalysts is the ability of enzymes to us e binding interactions for catalysis. Results with the Tetrahymena gro up I RNA enzyme described herein directly demonstrate the catalytic co ntributions of binding interactions. With wild-type ribozyme, specific functional groups at a distance from the site of chemical transformat ion facilitate substrate binding without accelerating reaction of boun d substrate; with modified ribozymes, these functional groups provide the same overall energetic effect but instead accelerate reaction of b ound substrate without increasing binding, These observations are quan titatively described by a structural framework that was established by previous results. The P1 duplex between the substrate and the ribozym e's recognition sequence exists in two states, the open complex, in wh ich the substrate is localized to the ribozyme solely by base-pairing interactions, or the closed complex, in which the duplex is docked int o tertiary interactions and positioned with respect to the catalytic g roups in the active site. In the absence of sufficient binding energy to ensure stable docking in the ground state, added P1 functional grou ps accelerate reaction of the bound substrate by helping to overcome t he energetic barrier for docking into the reactive, closed complex. Wh en the functional groups present on the P1 duplex are sufficient to en sure stable docking in the closed complex, added functional groups giv e stronger binding without accelerating reaction of the bound substrat e. This behavior is a manifestation of the inextricable link between b inding interactions and catalysis. The conclusions also have implicati ons for interpreting the effects of site-directed mutagenesis and for the evolution of active site interactions.