Effect of Ionic Strength on Chloroplast ATP Synthase

ATP synthase is a membrane protein complex that plays a crucial role in cellular bioenergetics. The formation of I-shaped dimers of spinach chloroplast ATP synthase in a model system (detergent micelles) with a high concentration of sodium chloride has been reported in the literature, but the presence and functional role of this mechanism in vivo remains unclear. We studied the impact of ionic strength on the thermostability and activity of spinach ATP synthase in liposomes. We measured ATP synthesis in the presence of NaCl, NaNO₃, and Na₂SO₄ at various concentrations and found that high ionic strength (~1 M) reduced ATP synthase activity by approximately 50%. Additionally, we determined the melting temperature of ATP synthase in the presence of NaCl and KCl, observing an increase from ~60.5 to ~62.0°C in solutions with high ionic strength. Our results demonstrate that salt ions affect ATP synthesis and thermostability in a non-specific manner. These findings provide support for the hypothesis that ATP synthase dimerization in vivo may serve as a regulatory mechanism for controlling its activity.