Interaction of ADP and ATP with noncatalytic sites of isolated and membrane-bound chloroplast coupling factor CF1.

This study of ATP and ADP binding to noncatalytic sites of membrane-bound CF1 (ATP synthase) revealed two noncatalytic sites with different specificities and affinities for nucleotides. One of these is characterized by a high affinity and specificity to ADP (Kd=2.6+/-0.3 microM). However, a certain increase in ADP apparent dissociation constant at high ATP/ADP ratio in the medium allows a possibility that ATP binds to this site as well. The other site displays high specificity to ATP. When the ADP-binding site is vacant, it shows a comparatively low affinity for ATP, which greatly increases with increasing ADP concentration accompanied by filling of the ADP-binding site. The reported specificities of these two sites are independent of thylakoid membrane energization, since both in the dark and in the light the ratios of ATP/ADP tightly bound to the noncatalytic sites were very close. The difference in noncatalytic site affinity for ATP and ADP is shown to depend on the amount of delta subunit in a particular sample. Thylakoid membrane ATP synthase, with stoichiometric content of delta-subunit (one delta-subunit per CF1 molecule), showed the maximal difference in ADP and ATP affinities for the noncatalytic sites. For CF1, with substoichiometric delta subunit values, this difference was less, and after delta subunit removal it decreased still more.