Specificity of nucleotide binding sites in isolated chloroplast coupling factor (CF1)

The binding of various nucleotides to chloroplast coupling factor CF1 was studied by two dialysis techniques. It was found that the number of nucleoside diphosphate sites and their specificities for the base moiety is dependent on the magnesium concentration. In the presence of 50 micrometer added MgCl2, the protein has a single strong site/mol protein with Kd = 0.5 micrometer for ADP and high specificity (Kd greater than 20 micrometer for epsilonADP, GDP, CDP). In the presence of 5 mM MgCl2, the protein has two independent tight ADP sites (Kd = 0.4 micrometer) of low specificity (Kd approximately 0.8, 2, and 2 micrometer, respectively for episilonADP, GDP, and CDP). These results are compared with the specificity of the partial reactions for photophosphorylation.