Properties of noncatalytic sites of thioredoxin-activated chloroplast coupling factor 1

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Nucleotide binding properties of two vacant noncatalytic sites of thioredoxin-activated chloroplast coupling factor 1 (CF 1) were studied. Kinetics of nucleotide binding to noncatalytic sites is described by the first-order equation that allows for two nucleotide binding sites that differ in kinetic features. Dependence of the nucleotide binding rate on nucleotide concentration suggests that tight nucleotide binding is preceded by rapid reversible binding of nucleotides. ADP binding is cooperative. The preincubation of CF 1 with Mg 2+ produces only slight effect on the rate of ADP binding and decreases the ATP binding rate. The ATP and ADP dissociation from noncatalytic sites is described by the first-order equation for similar sites with dissociation rate constants k −2(ADP)=1.5×10 −1 min −1 and k −2(ATP)≅10 −3 min −1, respectively. As follows from the study, the noncatalytic sites of CF 1 are not homogeneous. One of them retains the major part of endogenous ADP after CF 1 precipitation with ammonium sulfate. Its other two sites can bind both ADP and ATP but have different kinetic parameters and different affinity for nucleotides.