Salt Inactivation as a Mechanistic Probe of Membrane‐Bound Chloroplast Coupling Factor 1

Conditions are reported under which ATP protects membrane‐bound coupling factor 1 against sodium bromide inactivation. The presence of Mg²⁺ was found to be obligatory for this protection. ADP and GTP also protected the enzyme against salt inactivation but to a much smaller extent. Other nucleotides tested were ineffective. At low ATP concentrations ADP prevented the effect of ATP and modified the saturation curve for ATP from hyperbolic to sigmoidal. Treatment of chloroplasts with 0.4 M MgCI₂ or 2 M LiCI resulted in inactivation of photophosphorylation. In contrast to NaBrdepleted particles the MgCl₂ or LiCl‐depleted chloroplasts can be reconstituted by purified coupling factor 1. A binding site for Mg²⁺ and two different sites for ATP upon the coupling factor 1 are suggested to explain the mechanism of their protection against salt inactivation.