A Critical Test of the “Tunneling and Coupled Motion” Concept in Enzymatic Alcohol Oxidation
2013
Roston, Daniel | Kohen, Amnon
The physical mechanism of C–H bond activation by enzymes is the subject of intense study, and we have tested the predictions of two competing models for C–H activation in the context of alcohol dehydrogenase. The kinetic isotope effects (KIEs) in this enzyme have previously suggested a model of quantum mechanical tunneling and coupled motion of primary (1°) and secondary (2°) hydrogens. Here we measure the 2° H/T KIEs with both H and D at the 1° position and find that the 2° KIE is significantly deflated with D-transfer, consistent with the predictions of recent Marcus-like models of H-transfer. The results suggest that the fast dynamics of H-tunneling result in a 1° isotope effect on the structure of the tunneling ready state: the trajectory of D-transfer goes through a shorter donor–acceptor distance than that of H-transfer.
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