Identification of the Six Ligands to Manganese(II) in Transition-State-Analog Complexes of Creatine Kinase: Oxygen-17 Superhyperfine Coupling from Selectively Labeled Ligands

The complete coordination scheme for Mn(II) in transition-state-analogue complexes with creatine kinase has been determined by electron paramagnetic resonance (EPR) spectroscopy. Perturbations in the EPR spectra for Mn(II) due to superhyperfine coupling to¹⁷O of selectively labeled ligands have been used to identify oxygen ligands in the first coordination sphere of the metal ion. The results show that in the complex of enzyme-MnADP-formate-creatine, Mn(II) is bound to oxygen ligands from both the α- and β-phosphate groups of ADP, to an oxygen from the carboxylate group of formate, and to three water molecules. In the complex with thiocyanate replacing formate as the stabilizing anion, previous infrared experiments [Reed, G. H., Barlow, C. H., & Burns, R. A., Jr. (1978) J. Biol. Chem. 253, 4153-4158] indicated that the nitrogen from thiocyanate was bound to the Mn(II). The magnitudes of the¹⁷O superhyperfine coupling constants from the O^-ligands of the ADP phosphate groups and from the formate carboxylate are approximately equal and are larger than that for the water ligands. The symmetry of the zerofield- splitting tensor for Mn(II) indicates that the oxygens from the α- and β-phosphate groups of ADP and the ligand donor atom from the anion occupy mutually cis positions in the octahedral coordination geometry. Water proton relaxation time measurements show that the three water molecules which are bound to Mn(II) are not in free exchange with the bulk solvent. Hence, an enclosed structure at the active site is indicated. The results suggest that for creatine kinase the activating metal ion is bound to all three phosphate groups in the transition state of the reaction.