Deuterium Kinetic Isotope Effects Resolve Low-Temperature Substrate Radical Reaction Pathways and Steps in...

The first-order reaction kinetics of the cryotrapped 1,1,2,2-²H₄-aminoethanol substrate radical intermediate state in the adenosylcobalamin (B₁₂)-dependent ethanolamine ammonia-lyase (EAL) from Salmonella enterica serovar Typhimurium are measured over the range of 203-225 K by using time-resolved, full-spectrum electron paramagnetic resonance spectroscopy. The studies target the fundamental understanding of the mechanism of EAL, the signature enzyme in ethanolamine utilization metabolism associated with microbiome homeostasis and disease conditions in the human gut. Incorporation of ²H into the hydrogen transfer that follows the substrate radical rearrangement step in the substrate radical decay reaction sequence leads to an observed ¹H/²H isotope effect of approximately 2 that preserves, with high fidelity, the idiosyncratic piecewise pattern of rate constant versus inverse temperature dependence that was previously reported for the ¹H-labeled substrate, including a monoexponential regime (T ≥ 220 K) and two distinct biexponential regimes (T = 203-219 K). In the global kinetic model, reaction at ≥220 K proceeds from the substrate radical macrostate, S, and at 203-219 K along parallel pathways from the two sequential microstates, S and S, that are distinguished by different protein configurations. Decay from S, or S and S, is rate-determined by radical rearrangement (¹H) or by contributions from both radical rearrangement and hydrogen transfer (²H). Non-native direct decay to products from S is a consequence of the free energy barrier to the native S → S protein configurational transition. At physiological temperatures, this is averted by the fast protein configurational dynamics that guide the S → S transition.