Field dependence of solvent proton and deuteron NMR relaxation rates of the manganese(II) binding site of chloroplast coupling factor 1

Abstract

The high-affinity Mn(II) binding site of chloroplast coupling factor 1 (CF{sub 1}), the ATP synthase of higher plants, was studied by field-dependent water proton and deuteron NMR spin-lattice relaxation analysis. Magnetic field dependence of the solvent {sup 1}H relaxation rate (R{sub 1}) contains information about the number (q) and the residence lifetime ({tau}{sub m}) of water molecules within the inner coordination sphere of bound Mn(II) and the spectral density function of the magnetic dipole interaction of the water protons and the Mn(II) ion. Chemical analysis of endogenous nucleotides and metals in the purified enzyme revealed about 1.1-1.3 mol of ADP, 0.1-0.2 mol of ATP, and 1.3 mol of Mg(II) per mole of CF{sub 1}; no Mn(II) was detected. The enzyme bound 0.7 molar equiv of Mn(II) in nonlabile sites and at least 1 additional molar equiv of Mn(II) in a labile site. {sup 1}H NMR relaxation studies were conducted on solutions of purified CF{sub 1} and MnCl{sub 2} at binding ratios of 0.47, 0.89, and 1.3 and {sup 1}H Larmor frequencies between 4 and 62 MHz. The Mn(II) site appears to have approximate tetrahedral symmetry with three protein ligands and one exchangeable water ligand. This Mn(II) binding site is filledmore » when the divalent metal concentration exceeds the substrate concentration, and it appears to have an inhibitory effect on ATPase activity.« less