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Different solvent and conformational entropy contributions to the allosteric activation and inhibition mechanisms of yeast chorismate mutase.

Allosteric regulation is important in many biological processes, including cell signaling, gene regulation and metabolism. Saccharomyces cerevisiae chorismate mutase (ScCM) is a key homodimeric enzyme in the shikimate pathway responsible for the generation of aromatic amino acids, where it is allosterically inhibited and activated by Tyr and Trp, respectively. Our previous studies indicated that binding of both allosteric effectors is negatively cooperative, that is binding at one allosteric binding site discourages binding at the other, due to the entropic penalty of binding the second allosteric effector. We utilized variable temperature isothermal titration calorimetry (ITC) and nuclear magnetic resonance (NMR) experiments to better understand the entropic contributions to allosteric effector binding, including changes to solvent entropy and protein conformational entropy. Upon binding either Tyr or Trp, ScCM experiences a quenching of motions on the picosecond-to-nanosecond timescale, which we could relate to a loss of protein conformational entropy. Further ITC and NMR studies were consistent with the Tyr-bound form of ScCM being associated with more water molecules compared to the Trp-bound form, and Tyr binding being associated with a less positive solvent entropy change. These studies provide insight into the role of structural dynamics in ScCM function and highlight the importance of solvent entropy changes in allosteric regulation, a historically underappreciated concept.