STABILIZATION OF THE TRANSITION-STATE OF THE CHORISMATE-PREPHENATE REARRANGEMENT - AN AB-INITIO STUDY OF ENZYME AND ANTIBODY CATALYSIS

The Claisen rearrangement of chorismate to prephenate and models for the catalysis of this reaction by the enzyme chorismate mutase were studied using Hartree-Fock and density functional theories. Substituent effects on the reaction are studied using several simple model systems. Whereas carboxylic acid or carboxylate substituents in the 2-position or a carboxylate in the 6-position of allyl vinyl ether leads to a lower activation energy, substitution by a carboxylic acid in the 6-position increases the activation energy. Upon 2,6-disubstitution, there is an increase in activation energy due to electrostatic repulsion between the carboxylates in the transition state. Similar results were obtained for substituted vinyl cyclohexadienyl ethers. Secondary kinetic isotope effects and substituent effects on reactant and transition state geometries are discussed. The catalysis of the reaction by the amino acid side chains in the enzyme was studied by calculation of the interaction of various functional groups that mimic the active site of chorismate mutase from Bacillus subtilis with substituted allyl vinyl ethers. Selective transition state binding by appropriately positioned hydrogen bond donors is the most important factor for catalysis, lowering the activation energy by 6 kcal/mol in the case of allyl vinyl ether-2,6-dicarboxylate. Charge complementarity to a 2-carboxylate and increased hydrogen-bonding to the ether oxygen lower the activation energy by 1.7 and 2 kcal/mol, respectively. Stabilization of the positive partial charge in the allyl part of the transition state has no significant catalytic effect. Electrophilic catalysis involving strong binding to the ether oxygen leads to a dissociative mechanism. The results are discussed with respect to the catalytic mechanism of the native enzymes and the antibody 1F7.