Abstract

The chloroplast ATP synthase (CF1-F0) is a crucial enzyme embedded in the thylakoid membrane, responsible for converting the proton-motive force generated by light-driven electron transport into ATP. This study investigates the **effect of ionic strength** on the catalytic activity and structural stability of the CF1-F0 complex, a factor highly relevant to the dynamic environment of the chloroplast stroma. Using isolated thylakoid membranes and purified CF1-F0, we measured ATP synthesis and hydrolysis rates across a range of monovalent (K+) and divalent (Mg2+) cation concentrations. Our results demonstrate that the enzyme's activity is highly sensitive to ionic strength. Specifically, elevated concentrations of K+ significantly **inhibit ATP synthesis**, suggesting a disruption of electrostatic interactions critical for conformational changes in the CF1 headpiece. Furthermore, while a basal level of Mg2+ is essential for activity, concentrations exceeding 10 mM lead to a marked decrease in the maximum reaction velocity ($V_{max}$). These findings suggest that ionic strength acts as a critical regulatory mechanism for photosynthetic energy conversion, potentially modulating the coupling efficiency between proton translocation and ATP formation. This work provides new insights into the biophysical regulation of this vital membrane protein.