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Volume 271, Number 33, Issue of August 16, 1996 pp. 19976-19982
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Differences between Two Tight ADP Binding Sites of the Chloroplast Coupling Factor 1 and Their Effects on ATPase Activity

(Received for publication, March 20, 1996, and in revised form, May 21, 1996)

From the Department of Biology, The Johns Hopkins University, Baltimore, Maryland 21218

Purified chloroplast ATP synthase (CF₁) contains 1.2-2 mol of tightly bound ADP/mol of enzyme that resists removal by gel filtration or dialysis. CF₁ was depleted of its endogenous nucleotide by treatment with alkaline phosphatase. Tightly bound nucleotide was demonstrated not to have an essential structural role. CF₁ depleted of endogenous nucleotide retains its ability to catalyze Ca²⁺- and Mg²⁺-dependent ATPase activity and is not more sensitive to cold inactivation than untreated CF₁. 2(3)-O-Trinitrophenyladenosine 5-diphosphate (TNP-ADP) binds tightly to two sites on nucleotide-depleted CF₁, binding to either site at a faster rate than that of exchange of bound nucleotide for medium nucleotide. The nucleotide-depleted enzyme binds about one additional mol of TNP-ADP/mol of CF₁, indicating that there is a tight TNP-ADP binding site that does not exchange readily with medium nucleotide. It is MgADP in this nonexchanging site, not the easily exchanging ADP binding site, that is responsible for the MgADP-induced inhibition of the ATPase activity. The rate of exchange of tightly bound ADP from CF₁ matches the rate at which the Mg²⁺ATPase activity of CF₁ is activated but is not itself responsible for the activation.

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