MAGNESIUM ION-INDUCED CHANGES IN BINDING MODE OF ADENYLATES TO CHLOROPLAST COUPLING FACTOR-1

The effect of Mg2+ on the binding of adenylates to isolated chloroplast [spinach] coupling factor 1 (CF1) was studied using CD [circular dichroism] spectrometry and ultrafiltration. At adenylate concentrations smaller than 100 .mu.M, 1 mol of CF1 binds 3 mol of ATP (or ADP) regardless of the presence of Mg2+. In the presence of Mg2+, the first 2 ATP bind to CF1 independently with the same binding constant of 2.5 .times. 10-1 .mu.M-1, then the 3rd ATP binds with a much higher affinity of 10 .mu.M-1. In the absence of Mg2+, the 1st ATP binds to CF1 with a binding constant of 2.5 .times. 10-1 .mu.M-1, then the other 2 ATP bind less easily with the same binding constant of 4.0 .times. 10-2 .mu.M-1. The binding mode of ADP to CF1 is quite similar to that of ATP. In the presence of Mg2+, the binding constants of the first 2 ADP are both 7.6 .times. 10-2 .mu.M-1, that of the 3rd ATP being 4.0 .mu.M-1. In the absence of Mg2+, the binding constant of the 1st ADP is 7.6 .times. 10-2 .mu.M-1, the constants of the other 2 ADP both being 4.0 .times. 10-2 .mu.M-1. AMP caused a negligible change in CD.