Magnetic resonance and kinetic studies of pyruvate, phosphate dikinase. Interaction of oxalate with the phosphorylated form of the enzyme.

Pyruvate, orthophosphate dikinase (EC 2.7.9.1) carries out its catalytic function in three successive partial reactions, the final step being the reaction of pyruvate with a stable phosphoenzyme intermediate to give phosphoenolpyruvate and free enzyme (Evans, H.J., and Wood, H. G. (1968), Proc. Natl. Acad. Sci. U.S.A. 61, 1448). Interactions of oxalate, a structural analog of enolpyruvate, with the phosphorylated form of the enzyme have been investigated by kinetic inhibition measurements and by magnetic resonance studies of manganous ion complexes with the enzyme. Oxalate inhibits the reaction catalyzed by pyruvate, phosphate dikinase, and the inhibition is linearly competitive with respect to pyruvate. The inhibitor constant for oxalate of 25 mu-M is fourfold lower than the Michaelis constant for pyruvate. The enhancement in the longitudinal relaxation rate of water protons (PRR) which occurs upon binding of Mn(II) to the enzyme has been used to monitor binding of oxalate to Mn(II)-enzyme complexes. PRR titrations indicate that the dissociation constant of oxalate from the Mn(II) complex of the free form of the enzyme is an order of magnitude weaker than the kinetically determined Ki. On the other hand, titrations of solutions which contain the phosphorylated form of the enzyme reveal a much stronger binding of oxalate. Moreover, the strength of oxalate binding to the phosphorylated enzyme is a function both of the species and of the concentration of monovalent cations in the solution. In the presence of Tl+, which has the most favorable activator constant for the final partial reaction, the dissociation constant for oxalate from its complex with the phosphorylated enzyme is less than 1 mu-M. Electron paramagnetic resonance (EPR) spectra for the enzyme-bound Mn(II) are sensitive to structural perturbations which occur upon binding of substrates or of oxalate to the enzyme. The EPR spectrum for the Mn(II)-phosphoenzyme-oxalate species is distinguished from spectra for other complexes of the enzyme by unusually narrow line widths and consequent resolution of fine structure from electronic quadrupole splitting. The narrow lines in the EPR spectrum are indicative of a rigid, pseudocrystalline environment for the bound Mn(II). The magnitude and frequency dependence of the PRR for the Mn(II)-phosphoenzyme-oxalate complex indicate that if any water molecules are bound to the Mn(II), their exchange with the bulk water is severely retarded. The kinetic and magnetic resonance studies support the hypothesis that oxalate mimics the reactive intermediate, enolpyruvate, in a complex with the phosphorylated enzyme which may resemble the structure of the transition state of the final partial reaction.