7Li, 31P, and 1H NMR studies of interactions between ATP, monovalent cations, and divalent cation sites on rabbit muscle pyruvate kinase.

Abstract

The interactions between ATP, monovalent cations, and divalent cations on rabbit muscle pyruvate kinase have been examined using 7Li, 31P, and 1H nuclear magnetic resonance. Water proton nuclear relaxation studies are consistent with the binding of Li+ to the K+ site on pyruvate kinase with an affinity of 120 mM in the absence of substrates and 16 mM in the presence of P-enolpyruvate. Titrations with pyruvate demonstrate that pyruvate binds to the enzyme with an affinity of 0.65 mM in the presence of Li+ and 0.4 mM in the presence of K+. 7Li+ nuclear relaxation rates in solutions of pyruvate kinase are increased upon titration with the metal-nucleotide analogue, Cr(H2O)4ATP. Mn2+ EPR spectra were used to determined the distribution of the enzyme between the so-called isotropic and anisotropic conformations of the enzyme (Ash, D. E., Kayne, F., and Reed, G.H. Arch. Biochem. Biophys. (1978) 190, 571-577). Li-Cr distances of 5.6 and 11.0 A were calculated for the anisotropic and isotropic forms, respectively, in the absence or presence of pyruvate. When the divalent cation site on the enzyme was saturated with Mg2+, these distances increased to 6.7 and 9.5 A, respectively, regardless of the presence or absence of pyruvate. 31P nuclear relaxation studies with the diamagnetic metal-nucleotide analogue, Co(NH3)4ATP, indicated that addition of Mn2+ ion to the divalent cation site on the enzyme increased the longitudinal relaxation rates of all three phosphorus nuclei of the analogue. The 31P data indicate that the presence of pyruvate at the active site effects a decrease in the Mn-P distances, bringing Mn2+ and Co(NH3)4ATP closer together at the active site. The data also permit an evaluation of the role of the metal coordinated to the beta-P and gamma-P of ATP at the active site.