Local Electric Fields Originate Unusual Kinetic Isotope Effects in the Hydrogen Abstraction Reactions of the Functionally Analogous P450 and TauD Enzymes

Cytochrome P450 enzymes (P450s) and their functionally analogous TauD enzyme (taurine D α-KG-dependent dioxygenase) oxidize alkanes by an initial hydrogen atom abstraction. We use molecular dynamics (MD) simulations and hybrid quantum mechanics/molecular-mechanics (QM/MM) calculations to comprehend the origins of the counterintuitive kinetic isotope effects (KIEs), which are exhibited by these two enzyme families. Thus, P450s exhibit low KIE values, which indicate the absence of quantum mechanical tunneling (QMT) contributions, whereas TauD exhibits high KIEs, which apparently include QMT. Furthermore, the calculations show that the P450 protein-folds compensate for the lack of QMT, by reactivity enhancement due to the favorable interactions of the substrate(s) with the corresponding local electric field (LEF) of these enzymes. By contrast, TauD exhibits higher hydrogen abstraction barriers as well as a significant QMT. Thus, the relative reactivities of the two enzyme-types, toward a given substrate, are determined by the larger LEF in P450s vis-à-vis the higher QMT in TauD. The present manuscript provides a comprehensive understanding of the root-causes of the LEF effects of the protein-folds vs the QMT factors, in modulating hydrogen-atom abstraction reactivity, in two distinct mechanistic strategies, for the functionally analogous P450 and TauD enzymes.