Electron paramagnetic resonance studies of the coordination schemes and site selectivities for divalent metal ions in complexes with pyruvate kinase

Abstract

Electron paramagnetic resonance (EPR) spectroscopy has been used to investigate the properties of the binuclear divalent metal center at the active site of pyruvate kinase. The preferred binding sites for different types of divalent cation in complexes of the enzyme with ATP and oxalate were determined in hybrid metal complexes with Mn(II). Superhyperfine coupling between the unpaired electron spin of Mn(II) and the nuclear spin of 17O in isotopically enriched forms of oxalate and ATP was used to determine the position of Mn(II) at the binuclear metal center. When Mn(II) is present in combination with Zn(II), Ni(II), or Co(II), Mn(II) binds predominantly at the site defined by ligands from the protein, oxalate, and the gamma-phosphate of ATP. In contrast, EPR data of samples with mixtures of Mn(II)/Ca(II) or Mn(II)/Cd(II) reveal signals of two distinct hybrid-metal complexes. In one species, Mn(II) binds at the oxalate/gamma-phosphate site, and Ca(II) or Cd(II) binds at the ATP site. In the other species, the positions of Mn(II) and the second metal ion are reversed. The results indicate that, in enzymic complexes with ATP and oxalate, the relative size of the cation is a major factor controlling site selectivity. Metal ions that have ionic radii smaller than Mn(II) bind preferentially at the site occupied by ATP whereas metal ions that have ionic radii larger than Mn(II) bind preferentially at the site occupied by oxalate. EPR data of one of the hybrid complexes formed by Cd(II) and Mn(II) show that an alpha,beta,gamma-tridentate species of MnIIATP binds to the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)