α-Secondary isotope effects as probes of "tunneling-ready" configurations in enzymatic H-tunneling:: Insight from environmentally coupled tunneling models

Using alpha-secondary kinetic isotope effects (2 degrees KIEs) in conjunction with primary (1) KIEs, we have investigated the mechanism of environmentally coupled hydrogen tunneling in the reductive half-reactions of two homologous flavoenzymes, morphinone reductase (MR) and pentaerythritol tetranitrate reductase (PETNR). We find exalted 2 KIEs (1.17-1.18) for both enzymes, consistent with hydrogen tunneling. These 2 KIEs, unlike 1 KIEs, are independent of promoting motions-a nonequilibrium pre-organization of cofactor and active site residues that is required to bring the reactants into a "tunneling-ready" configuration. That these 2 KIEs are identical suggests the geometries of the "tunneling-ready" configurations in both enzymes are indistinguishable, despite the fact that MR, but not PETNR, has a clearly temperature-dependent 1 degrees KIE. The work emphasizes the benefit of combining studies of 1 and 2 KIEs to report on pre-organization and local geometries within the context of contemporary environmentally coupled frameworks for H-tunneling.