Activation and Inhibition of Pyruvate Carboxylase from <i>Rhizobium etli</i>

While crystallographic structures of the <i>R. etli</i> pyruvate carboxylase (PC) holoenzyme revealed the location and probable positioning of the essential activator, Mg<sup>2+</sup>, and nonessential activator, acetyl-CoA, an understanding of how they affect catalysis remains unclear. The current steady-state kinetic investigation indicates that both acetyl-CoA and Mg<sup>2+</sup> assist in coupling the MgATP-dependent carboxylation of biotin in the biotin carboxylase (BC) domain with pyruvate carboxylation in the carboxyl transferase (CT) domain. Initial velocity plots of free Mg<sup>2+</sup> vs pyruvate were nonlinear at low concentrations of Mg<sup>2+</sup> and a nearly complete loss of coupling between the BC and CT domain reactions was observed in the absence of acetyl-CoA. Increasing concentrations of free Mg<sup>2+</sup> also resulted in a decrease in the <i>K</i><sub>a</sub> for acetyl-CoA. Acetyl phosphate was determined to be a suitable phosphoryl donor for the catalytic phosphorylation of MgADP, while phosphonoacetate inhibited both the phosphorylation of MgADP by carbamoyl phosphate (<i>K</i><sub>i</sub> = 0.026 mM) and pyruvate carboxylation (<i>K</i><sub>i</sub> = 2.5 mM). In conjunction with crystal structures of T882A <i>R. etli</i> PC mutant cocrystallized with phosphonoacetate and MgADP, computational docking studies suggest that phosphonoacetate could coordinate to one of two Mg<sup>2+</sup> metal centers in the BC domain active site. Based on the pH profiles, inhibition studies, and initial velocity patterns, possible mechanisms for the activation, regulation, and coordination of catalysis between the two spatially distinct active sites in pyruvate carboxylase from <i>R. etli</i> by acetyl-CoA and Mg<sup>2+</sup> are described.