Effect of commitments to catalysis on the degree of curvature in proton inventories of the kinetic parameters for enzyme-catalyzed reactions: Application to tryptophan indole-lyase

Article abstract of DOI:10.1021/ja00022a043

A relatively simple method for obtaining ^Dk, the intrinsic solvent deuterium isotope effect for an enzyme-catalyzed reaction, is presented and discussed. Steady-state and pre-steady-state determinations of the solvent deuterium and substrate deuterium isotope effects, the later obtained in both H₂O and D₂O, enable the development of a catalytic mechanism. The interrelatedness of the isotope effect equations for the enzyme system allows calculation of ^Dk. As a result, the variation of l/^Dk_a, the reciprocal of the partial intrinsic solvent deuterium isotope effect, with n(D₂O), the atom fraction of deuterium in mixed isotopic waters, has been determined for the tryptophan indole-lyase enzyme-catalyzed reaction, and is compared to the variation of (V/K)_n/(V/K)₀ with n(D₂O). Comparison of the proton inventories indicates that without knowledge of the commitments to catalysis underestimates of the number of protons calculated from a fit of the kinetic parameter data to the Gross-Butler equation can occur. A series of curves is presented which demonstrate that the degree of curvature in a bowl-shaped proton inventory decreases as the commitment to catalysis increases, and that ultimately, as the relative size of the commitment continues to increase, a dome-shaped proton inventory will result.