Field‐Assisted Substrate Ionization as a Plausible Initiating Step in Enzyme Catalysis

ABSTRACT Enzymes are widely understood to accelerate chemical reactions through transition‐state stabilization and optimizing the orientation of reactants within their active sites. A growing body of work highlights the central role of strong electrostatic fields in these processes with magnitudes reaching ∼10 8 –10 10 V/m in enzyme active sites. Here, we explore the hypothesis that such fields may, in some cases, facilitate catalysis by inducing field‐assisted electron transfer or partial ionization of bound substrates. To assess the plausibility of this mechanism, a semiclassical tunneling expression in the quasistatic limit was applied to two datasets of enzymatic substrates: a curated set of 54 substrates with experimentally known ionization potentials and an extended set of 3168 enzymatic substrates with machine‐learning‐predicted ionization potentials matched to 5272 enzymes. At field strengths of an order ≳ 10 10 V/m, estimated ionization rates span the nanosecond‐to‐picosecond regime for a subset of substrates, suggesting that field‐induced tunneling may render electron‐transfer events statistically accessible within the residence time of enzyme–substrate complexes. These results are recast in probabilistic terms, showing that even modest tunneling rates can lead, under certain circumstances, to non‐negligible ionization probabilities over biologically relevant timescales. Electronic‐structure calculations on the ketosteroid isomerase substrate (5‐androstene‐3,17‐dione) indicate that one‐electron oxidation leads to a redistribution of electron density consistent with the enhancing of the electrophilic character of the molecule's reactive site. While the tunneling expressions employed here are not intended to provide quantitatively predictive rates in condensed enzymatic environments, they offer a physically motivated metric for assessing the sensitivity of ionization barriers to strong electrostatic fields. Taken together, these results suggest that field‐assisted substrate ionization or oxidation may represent a plausible, though not yet established, mechanistic component in certain enzyme‐catalyzed reactions. This hypothesis remains qualitative and is intended to stimulate further theoretical and experimental testing.