Binding and Exchange of Nucleotides on the Chloroplast Coupling Factor CF1

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Abstract
On the soluble part of the coupling factor (CF1), extracted from spinach chloroplasts, 3 nucleotide-binding sites are identified. Three ADP are bound per CF1 when the enzyme is incubated with ADP either with or without Mg2+. Two ADP and one ATP are bound per CF1 when the enzyme is incubated with a limiting concentration of ATP, in the presence of Mg2+. At high ATP concentration, in the presence of Mg2+, one free ATP exchanges with 1 bound ADP and 2 ATP and 1 ADP remain bound per CF1. When Mg2+ is omitted from the incubation medium of ATP and CF1, only 2 ADP and around 0.5 ATP are bound per CF1. The 3 nucleotide binding sites of CF1 fall into 2 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 2 sites, the bound nucleotides, ADP or ATP, are readily exchangeable. The 2 exchangeable sites apparently form the catalytic part of the enzyme where ATP is hydrolyzed. When ATP concentration is high enough, in the presence of Mg2+, one ATP displaces 1 bound ADP and allows the ATP hydrolysis to proceed. 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 CF1 in vivo when the thylakoid membranes are energized by light, as stressed by Bickel-Sandkotter and Strotman.