Proton Donor in Yeast Pyruvate Kinase: Chemical and Kinetic Properties of the Active Site Thr 298 to Cys Mutant

Abstract

The active site T298 residue of yeast pyruvate kinase (YPK), located in a position to serve potentially as the proton donor, was mutated to cysteine. T298C YPK was isolated and purified, and its enzymatic properties were characterized. Fluorescence and CD spectra indicate minor structural perturbations. A kinetic analysis of the Mg(2+)-activated enzyme demonstrates no catalytic activity in the absence of the heterotropic activator fructose 1,6-bisphosphate (FBP). In the presence of Mg(2+) and FBP, T298C has approximately 20% of the activity of wild-type (wt) YPK. The activator constant for FBP increases by 1 order of magnitude compared to this constant with the wt enzyme. T298C shows positive cooperativity by FBP with a Hill coefficient of 2.6 (wt, n(H,FBP) = 1). Mn(2+)-activated T298C behaves like Mn(2+)-activated wt YPK with a V(max) that is 20% of that for the wt enzyme with or without FBP. A pH-rate profile of T298C relative to that for wt YPK shows that pK(a,2) has shifted from 6.4 in wt to 5.5, indicating that the thiol group elicits an acidic pK shift. Inactivation of both wt and T298C by iodoacetate elicits a pseudo-first-order loss of activity with T298C being inactivated from 8 to 100 times faster than wt YPK. A pH dependence of the inactivation rate constant for T298C gives a value of 8.2, consistent with the pK for a thiol. Changes in fluorescence indicate that the T298C-Mg(2+) complex binds PEP, ADP, and both ligands together. This demonstrates that the lack of activity is not due to the loss of substrate binding but to the lack of ability to induce the proper conformational change. The mutation also induces changes in binding of FBP to all the relevant complexes. Binding of the metal and binding of PEP to the enzyme complexes are also differentially altered. Solvent isotope effects are observed for both wt and T298C. Proton inventory studies indicate that k(cat) is affected by a proton from water in the transition state and the effects are metal ion-dependent. The results are consistent with water being the active site proton donor. Active site residue T298 is not critical for activity but plays a role in the activation of the water and affects the pK that modulates catalytic activity.