MGADP AND FREE PI AS THE SUBSTRATES AND THE MG2+ REQUIREMENT FOR PHOTOPHOSPHORYLATION

Previous studies have not provided definitive information about whether ADP or P(i) or their complexes with Mg2+ serve as substrates for photophosphorylation and whether free Mg2+ or ADP is required. Results presented show MgADP, MgGDP, or MgUDP are substrates. At variable Mg2+ concentrations, observed velocities are determined by MgADP and not the free ADP concentration. The approximate K(m) for MgADP with spinach chloroplasts is about 30-mu-M, for MgGDP 260-mu-M, and for MgUDP above 5 mM. The apparent K(m) values for added ADP or Mg2+ are decreased to constant low values near 30-mu-M as the added Mg2+ or ADP concentrations, respectively, are increased to the millimolar range. With 100-mu-M added Mg2+, near-maximal velocities can be obtained with excess ADP, but not with excess GDP or UDP. This is explainable by the apparent K(m) values for MgGDP and MgUDP being well above 100-mu-M. High phosphorylation rates with excess of either Mg2+ or ADP present show that little or no (< 2-3-mu-M) free Mg2+ or ADP is required. In addition, the results show that during rapid photophosphorylation, when one or more catalytic sites are always filled with nucleotide, free ADP does not combine and block the combination of MgADP to catalytic sites that become vacant. This is in contrast to the ability of free ADP to combine tightly with one catalytic site when all catalytic sites are empty. The apparent K(m) for added ADP above a few micromolar concentration, and with excess Mg2+ present, results from binding of MgADP at a second catalytic site. In contrast to the behavior of ADP, the apparent K(m) for P(i) is independent of the Mg2+ concentration, showing that free P(i) is the substrate. In addition, some deviations of photophosphorylation rates from simple Michaelis-Menten relationships are noted and discussed.