Elusive transition state of alcohol dehydrogenase unveiled

For several decades the hydride transfer catalyzed by alcohol dehydrogenase has been difficult to understand. Here we add to the large corpus of anomalous and paradoxical data collected for this reaction by measuring a normal (> 1) 2 degrees kinetic isotope effect (KIE) for the reduction of benzaldehyde. Because the relevant equilibrium effect is inverse (< 1), this KIE eludes the traditional interpretation of 2 degrees KIEs. It does, however, enable the development of a comprehensive model for the "tunneling ready state" (TRS) of the reaction that fits into the general scheme of Marcus-like models of hydrogen tunneling. The TRS is the ensemble of states along the intricate reorganization coordinate, where H tunneling between the donor and acceptor occurs (the crossing point in Marcus theory). It is comparable to the effective transition state implied by ensemble-averaged variational transition state theory. Properties of the TRS are approximated as an average of the individual properties of the donor and acceptor states. The model is consistent with experimental findings that previously appeared contradictory; specifically, it resolves the long-standing ambiguity regarding the location of the TRS (aldehyde-like vs. alcohol-like). The new picture of the TRS for this reaction identifies the principal components of the collective reaction coordinate and the average structure of the saddle point along that coordinate.