Enzyme-catalyzed reactions sometimes display curvature in their Eyring plots in the absence of denaturation, indicative of a change in activation heat capacity. However, the effects of pH and (de)protonation on this phenomenon have remained unexplored. Herein, we report a kinetic characterization of the thermophilic pyrimidine nucleoside phosphorylase from across a two-dimensional working space covering 35 °C and 3 pH units with two substrates displaying different p values. Our analysis revealed the presence of a measurable activation heat capacity change Δ in this reaction system, which showed no significant dependence on medium pH or substrate charge. Our results further describe the remarkable effects of a single halide substitution that has a minor influence on Δ but conveys a significant kinetic effect by decreasing the activation enthalpy, causing a >10-fold rate increase. Collectively, our results present an important piece in the understanding of enzymatic systems across multidimensional working spaces where the choice of reaction conditions can affect the rate, affinity, and thermodynamic phenomena independently of one another.
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