Binding and Exchange of Nucleotides on the Chloroplast Coupling Factor CF 1

On the soluble part of the coupling factor (CF 1 ), extracted from spinach chloroplasts, three nucleotide‐binding sites are identified. Three ADP are bound per CF 1 when the enzyme is incubated with ADP either with or without Mg 2+ . Two ADP and one ATP are bound per CF 1 when the enzyme is incubated with a limiting concentration of ATP, in the presence of Mg 2+ . At high ATP concentration, in the presence of Mg 2+ , one free ATP exchanges with one bound ADP and two ATP and one ADP remain bound per CF 1 . When Mg 2+ is omitted from the incubation medium of ATP and CF 1 , only two ADP and around 0.5 ATP are bound per CF 1 . The three nucleotide binding sites of CF 1 fall into two different and independent categories according to the ability of the bound nucleotides to be exchanged with free nucleotides. On one site the bound ADP is difficult to exchange. On the other two sites, the bound nucleotides, ADP or ATP, are readily exchangable. We propose that the two exchangeable sites form the catalytic part of the enzyme where ATP is hydrolyzed. When ATP concentration is high enough, in the presence of Mg 2+ , one ATP displaces one bound ADP and allows the ATP hydrolysis to proceed. We propose too that the site where ADP is difficult to exchange may represent the ‘tight’ ADP‐binding site, different from the catalytic ones, which becomes exchangeable on the CF 1 in vivo when the thylakoid membranes are energized by light, as stressed by Bickel‐Sandkötter and Strotman [(1976) FEBS Lett. 65 , 102–106].