BINDING OF A HIGH-ENERGY SUBSTRATE CONFORMER IN ANTIBODY CATALYSIS

Enzymes can substantially increase the probability of a reaction by ex ploiting binding energy to preorganize their substrates into reactive conformations. Similar effects are likely to be important in a wide va riety of designed catalysts, including catalytic antibodies. Transferr ed nuclear Overhauser effects have been used here to investigate how a n antibody possessing chorismate mutase activity binds its flexible su bstrate molecule chorismate. The conversion of chorismate to prephenat e by way of a Claisen rearrangement requires the substrate to adopt an energetically disfavored diaxial conformation in which the enolpyruvy l side chain is positioned over the six-membered ring. The antibody, w hich was elicited by a conformationally restricted transition state an alog for this reaction, appears to bind this high-energy substrate con former preferentially, as judged by diagnostic intramolecular transfer red nuclear Overhauser effects. Inhibitor studies with the transition state analog confirm that preorganization takes place exclusively at t he antibody active site. These results thus provide strong physical ev idence for a direct relationship between the properties of a catalytic antibody and the structure of the transition state analog originally used to elicit the immune response.