Energetics of proline racemase: transition-state fractionation factors for the two protons involved in the catalytic steps.

The isotope effects for the interconversion of L-proline and D-proline, catalyzed by proline racemase, have been determined in the saturated region with both [2-2H]proline and [2-3H]proline. The deuterium fractionation factors for each of the protons in flight have been obtained from two kinds of experiment: by measuring the rate of racemization of one [2-2H]proline enantiomer as it racemizes into an equilibrated pool of unlabeled proline and by measuring the deuterium content of a proline sample at the optical rotation maximum that occurs when an equimolar mixture of one deuterium-labeled enantiomer and the other unlabeled enantiomer runs to equilibrium. The tritium fractionation factors for each of the protons in flight have been determined from measurements of the rate of loss of tritium to the solvent as one [2-3H]proline enantiomer runs to equilibrium. Good agreement is found among the fractionation factors determined by each method. The deuterium fractionation factors for the two protons are not identical: that for the proton derived from L-proline is 0.375 and that for the proton derived from D-proline is 0.44. This difference has been confirmed by a double-competition experiment in which the optical rotation of a mixture of DL-[2-2H]proline and unlabeled DL-proline is followed with time. The rotation (initially zero) passes through a maximum, from which the ratio of the two fractionation factors (0.86) is obtained. These data, coupled with the equilibrium fractionation factor for the 2-position of proline (which has been determined to be 1.17), provide the transition-state factors for each of the in-flight protons, and delineate the nature of the transition state(s) for the enzyme-catalyzed racemization.