Bound adenosine 5'-triphosphate formation, bound adenosine 5'-diphosphate and inorganic phosphate retention, and inorganic phosphate oxygen exchange by chloroplast adenosinetriphosphatase in the presence of Ca2+ or Mg2+

When the heat-activated chloroplast F1 ATPase hydrolyzes 3H, gamma-32P]ATP, followed by the removal of medium ATP, ADP, and Pi, the enzyme has labeled ATP, ADP, and Pi bound to it in about equal amounts. The total of the bound 3H]ADP and 3H]ATP approaches 1 mol/mol of enzyme. Over a 30-min period, most of the bound 32P]Pi falls off, and the bound 3H]ATP is converted to bound 3H]ADP. Enzyme with such remaining tightly bound ADP will form bound ATP from relatively high concentrations of medium Pi with either Mg2+ or Ca2+ present. The tightly bound ADP is thus at a site that retains a catalytic capacity for slow single-site ATP hydrolysis (or synthesis) and is likely the site that participates in cooperative rapid net ATP hydrolysis. During hydrolysis of 50 microM 3H]ATP in the presence of either Mg2+ or Ca2+, the enzyme has a steady-state level of about one bound 3H]ADP per mole of enzyme. Because bound 3H]ATP is also present, the 3H]ADP is regarded as being present on two cooperating catalytic sites. The formation and levels of bound ATP, ADP, and Pi show that reversal of bound ATP hydrolysis can occur with either Ca2+ or Mg2+ present. They do not reveal why no phosphate oxygen exchange accompanies cleavage of low ATP concentrations with Ca2+ in contrast to Mg2+ with the heat-activated enzyme. Phosphate oxygen exchange does occur with either Mg2+ or Ca2+ present when low ATP concentrations are hydrolyzed with the octyl glucoside activated ATPase. Ligand binding properties of Ca2+ at the catalytic site rather than lack of reversible cleavage of bound ATP may underlie lack of oxygen exchange under some conditions.