Select Configurational Dynamics in Ethanolamine Ammonia-Lyase Radical Enzyme Catalysis

Contributions of protein and coupled solvent configurational fluctuations to the molecular mechanism of enzyme catalysis are addressed in the adenosylcobalamin (coenzyme B₁₂)-dependent ethanolamine ammonia-lyase (EAL) enzyme from <i>Salmonella enterica</i> serovar Typhimurium. Full-spectrum, time-resolved electron paramagnetic resonance (EPR) spectroscopy is used to measure the temperature dependence of the first-order kinetics of the substrate radical reaction in EAL surrounded by successive hydration and aqueous-cosolvent (aminoethanol; added dimethyl sulfoxide, glycerol, and sucrose) layers in frozen solution. At different temperature (<i>T</i>) values in each system, the piecewise-continuous Arrhenius relation displays the characteristic bifurcation, from high-<i>T</i> monoexponential dependence (reaction from substrate radical macrostate, <b>S</b> ^<b>•</b> ) to the low-<i>T</i> biexponential dependence (reaction from sequential substates, <b>S</b> _<b>1</b> ^<b>•</b> and <b>S</b> _<b>2</b> ^<b>•</b> ). Parallel measurements of the solvent dynamics around EAL by using EPR spin probe mobility or electric permittivity detect a dynamical transition from collective cluster to individual fluctuations of coupled protein surface groups and hydration water, with decreasing <i>T</i>, that precisely coincides with the kinetic bifurcation <i>T</i> in each solvent system. When shifted along their common monotonic high-<i>T</i> relation, the Arrhenius dependences collapse onto a single, universal pattern. The results indicate that specific, or select, collective fluctuations in the EAL protein hydration layer are coupled to active-site configurational fluctuations, providing low-barrier portals through the configuration space. The direct mechanistic link between solvent dynamics and turnover expands the understanding of radical-mediated reactions in EAL and supports the model that select collective configurational dynamics are a fundamental feature of enzyme catalysis.