The First Experimental Test of the Hypothesis that Enzymes Have Evolved To Enhance Hydrogen Tunneling

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Abstract
The literature hypothesis that “the optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of quantum-mechanical tunneling” is experimentally tested herein for the first time. The system employed is the key to being able to provide this first experimental test of the “enhanced hydrogen tunneling” hypothesis, one that requires a comparison of the three criteria diagnostic of tunneling (vide infra) for the same, or nearly the same, reaction with and without the enzyme. Specifically, studied herein are the adenosylcobalamin (AdoCbl, also known as coenzyme B12)-dependent diol dehydratase model reactions of (i) H(D)• atom abstraction from ethylene glycol-d0 and ethylene glycol-d4 solvent by 5‘-deoxyadenosyl radical (Ado•) and (ii) the same H• abstraction reactions by the 8-methoxy-5‘-deoxyadenosyl radical (8-MeOAdo•). The Ado• and 8-MeOAdo• radicals are generated by Co−C thermolysis of their respective precursors, AdoCbl and 8-MeOAdoCbl. Deuterium kinetic isotope effects (KIEs) of the H•(D•) abstraction reactions from ethylene glycol have been measured over a temperature range of 80−120 °C: KIE = 12.4 ± 1.1 at 80 °C for Ado• and KIE = 12.5 ± 0.9 at 80 °C for 8-MeOAdo• (values ca. 2-fold that of the predicted maximum primary times secondary ground-state zero-point energy (GS-ZPE) KIE of 6.4 at 80 °C). From the temperature dependence of the KIEs, zero-point activation energy differences ([ED − EH]) of 3.0 ± 0.3 kcal mol-1 for Ado• and 2.1 ± 0.6 kcal mol-1 for 8-MeOAdo• have been obtained, both of which are significantly larger than the nontunneling, zero-point energy only maximum of 1.2 kcal mol-1. Pre-exponential factor ratios (AH/AD) of 0.16 ± 0.07 for Ado• and 0.5 ± 0.4 for 8-MeOAdo• are observed, both of which are significantly less than the 0.7 minimum for nontunneling behavior. The data provide strong evidence for the expected quantum mechanical tunneling in the Ado• and 8-MeOAdo•-mediated H• abstraction reactions from ethylene glycol. More importantly, a comparison of these enzyme-free tunneling data to the same KIE, (ED − EH) and AH/AD data for a closely related, Ado•-mediated H• abstraction reaction from a primary CH3- group in AdoCbl-dependent methylmalonyl-CoA mutase shows the enzymic and enzyme-free data sets are identical within experimental error. The Occam's Razor conclusion is that at least this adenosylcobalamin-dependent enzyme has not evolved to enhance quantum mechanical tunneling, at least within the present error bars. Instead, this B12-dependent enzyme simply exploits the identical level of quantum mechanical tunneling that is available in the enzyme-free, solution-based H• abstraction reaction. The results also require a similar, if not identical, barrier width and height within experimental error for the H• abstraction both within, and outside of, the enzyme.