Mechanism of chorismate mutase: Contribution of conformational restrictionto catalysis in the Claisen rearrangement

The mechanism of the enzyme- and antibody-catalyzed Claisen rearrangement o f chorismate to prephenate was investigated experimentally on model compoun ds and by using quantum chemistry calculations at the Becke3LYP/6-31G* leve l of theory. Conformational restriction of the allyl vinyl ether fragment t o the reactive chairlike conformation in 1 induces a 2 x 10(5)-fold rate ac celeration (Delta Delta G(double dagger) = 7.3 kcal/mol) of the Claisen rea rrangement in C6D6 relative to an unrestricted analogue 3. A direct relatio nship between activation barrier lowering and the distance between reactive termini has been observed in additional model systems. Compression of the reactive centers from 4.0 to 3.0 Angstrom results in a barrier lowering fro m 24 kcal/mol to 12 kcal/mol in one conformationally restricted model. Furt her compression reduces the activation barrier to a mere 4 kcal/mol. Rearra ngement rate increases via conformational restriction and reactive center c ompression derive mainly from ground-state destabilization in which entropi c factors do not contribute significantly. The chorismate mutase mechanism is rationalized as a series of three steps involving (1) capture of the uns table pseudo-diaxial conformer of chorismate in a chairlike geometry (<3 kc al/mol contribution to barrier lowering); (2) further confinement of the re acting termini with a potential for >10 kcal/mol barrier reduction; and (3) rearrangement accompanied by additional transition-state stabilization fro m ionic I-I-bonding at the ether oxygen. Since the total barrier lowering i s greater than that required to account for the observed 3 x 10(6)-fold enz ymatic catalysis, the rearrangement itself is probably not the rate-determi ning step. The major contribution to catalysis could, in principle, come fr om confining the reactive centers to contact distances, a conclusion consis tent with the spatiotemporal precept.