P16: Thermodynamic Origins of Catalysis by a Newly Recruited Enzyme

When challenged for growth in the presence of a lethal concentration of the nonnatural toxin bromoacetate, Escherichia coli K-12 is capable of recruiting the YliJ gene product to afford survival. Recently, our laboratory demonstrated that yliJ encodes a glutathione transferase that catalyzes the dehalogenation of bromoacetate with a k_cat/K_m value of 5.3 x 10³ M^-1 s^-1. YliJ enhances the spontaneous rate of glutathione conjugation to bromoacetate by more than five orders of magnitude. In so doing, the enzyme reduces the activation barrier for dehalogenation by 7.7 kcal/mol. Here we report the enthalpies and entropies of activation for the spontaneous and enzyme-catalyzed decomposition of bromoacetate, as determined from the temperature dependencies of these reactions. The activation parameters demonstrate the thermodynamic basis for catalysis of a non-biological transformation within an extant active site. Such information provides insight into the contemporary and primordial evolution of new catalytic function via the process of enzyme recruitment.