Secondary tritium isotope effects as probes of the enzymic and nonenzymic conversion of chorismate to prephenate

Abstract

To obtain information about the degree of concert of both the nonenzymic and the enzyme-catalyzed rearrangement of chorismate to prephenate, the secondary tritium isotope effects at the bond-making position (C-9) and the bond-breaking position (C-5) of chorismate were determined. The isotope effects were determined by the competitive method, using either [5-3H,7-14C] chorismate or [9-3H,7-14C]chorismate as the substrate. In the nonenzymic reaction (pH 7.5, 60.degree. C), kH/kT is 1.149 .+-. 0.012 for bond breaking (C-9) and 0.992 .+-. 0.012 for bond making (C-5). This indicates an asymmetric transition state in which the new bond is hardly, if at all, formed, while the bond between C-5 and oxygen is substantially broken. In the enzymic reaction (pH 7.5, 30.degree. C) [chorismate mutase-prephenate dehydrogenase from Escherichia coli], the values of kH/kT in both positions are unity within experimental error. It is most likely that the isotope effects are suppressed in the enzymic process and that the rate-limiting transition state occurs before the rearrangement itself. The kinetically significant transition state presumably involves either the binding step of the small equilibrium proportion of the axial conformer of the substrate or an isomerization of enzyme-bound chorismate from the more stable conformer in which the carboxyvinyloxy group is equatorial to that in which this group is axial. Rearrangement would then proceed relatively rapidly from the higher energy axial conformer.