The widespread occurrence of enzymatic hydrogen tunneling,andits unique properties,lead to a new physical model for the origins of enzyme catalysis

The investigation of C-H bond activation by enzymes over the past several decades has revealed a plethora of deviations from semi-classical kinetic models. Although the early enzymatic results were interpreted in the context of a tunneling correction, the emergent properties are now seen to be largely incompatible with this type of analysis as well. This chapter introduces some of the experimental data that form the basis for our present understanding. A vibronically nonadiabatic model, that has a number of features in common with the Marcus treatment for electron transfer, offers a robust physical picture for the hydrogen tunneling behavior seen in both native enzymes and in enzymes that have been perturbed either by site-specific mutagenesis or by perturbation of the reaction conditions. Native enzymes under optimal conditions most commonly show behavior that requires a heavy atom donor-acceptor distance that is in the range of 2.7 Å. This compression beyond van der Waals distances is proposed to arise from the process of enzymatic conformational sampling. The absence of any evolutionary driving force to optimize tunneling for deuterium transfer (natural abundance < 0.02%), together with the frequent observation that the enthalpic barrier for deuterium transfer is the same or very similar to that for protium transfer, leads to the proposal that tunneling is a consequence of a generic property of enzyme function in which overall protein flexibility enables the generation of active sites that can be quite compressed.