| Abstract
| - The important biosynthetic intermediate chorismate reacts thermally by two competitive pathways,one leading to 4-hydroxybenzoate via elimination of the enolpyruvyl side chain, and the other to prephenateby a facile Claisen rearrangement. Measurements with isotopically labeled chorismate derivatives indicatethat both are concerted sigmatropic processes, controlled by the orientation of the enolpyruvyl group. Inthe elimination reaction of [4-2H]chorismate, roughly 60% of the label was found in pyruvate after 3 h at 60°C. Moreover, a 1.846 ± 0.057 2H isotope effect for the transferred hydrogen atom and a 1.0374 ± 0.000518O isotope effect for the ether oxygen show that the transition state for this process is highly asymmetric,with hydrogen atom transfer from C4 to C9 significantly less advanced than C−O bond cleavage. In thecompeting Claisen rearrangement, a very large 18O isotope effect at the bond-breaking position (1.0482 ±0.0005) and a smaller 13C isotope effect at the bond-making position (1.0118 ± 0.0004) were determined.Isotope effects of similar magnitude characterized the transformations catalyzed by evolutionarily unrelatedchorismate mutases from Escherichia coli and Bacillus subtilis. The enzymatic reactions, like their solutioncounterpart, are thus concerted [3,3]-sigmatropic processes in which C−C bond formation lags behindC−O bond cleavage. However, as substantially larger 18O and smaller 13C isotope effects were observedfor a mutant enzyme in which chemistry is fully rate determining, the ionic active site may favor a somewhatmore polarized transition state than that seen in solution.
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