EPR-kinetic isotope effect study of the mechanism of radical-mediated dehydrogenation of an alcohol by the radical SAM enzyme DesII

The radical S-adenosyl-L-methionine enzyme DesII from Streptomyces venezuelae is able to oxidize the C3 hydroxyl group of TDP-D-quinovose to the corresponding ketone via an alpha-hydroxyalkyl radical intermediate. It is unknown whether electron transfer from the radical intermediate precedes or follows its deprotonation, and answering this question would offer considerable insight into the mechanism by which the small but important class of radical-mediated alcohol dehydrogenases operate. This question can be addressed by measuring steady-state kinetic isotope effects (KIEs); however, their interpretation is obfuscated by the degree to which the steps of interest limit catalysis. To circumvent this problem, we measured the solvent deuterium KIE on the saturating steady-state concentration of the radical intermediate using electron paramagnetic resonance spectroscopy. The resulting value, 0.22 +/- 0.03, when combined with the solvent deuterium KIE on the maximum rate of turnover (V) of 1.8 +/- 0.2, yielded a KIE of 8 +/- 2 on the net rate constant specifically associated with the alpha-hydroxyalkyl radical intermediate. This result implies that electron transfer from the radical intermediate does not precede deprotonation. Further analysis of these isotope effects, along with the pH dependence of the steady-state kinetic parameters, likewise suggests that DesII must be in the correct protonation state for initial generation of the alpha-hydroxyalkyl radical. In addition to providing unique mechanistic insights, this work introduces a unique approach to investigating enzymatic reactions using KIEs.