Kinetics of nucleotide binding to chloroplast coupling factor (CF₁)

Studies of the kinetics of association and dissociation of the formycin nucleotides FTP and FDP with CF₁ were carried out using the enhancement of formycin fluorescence. The protein used, derived from lettuce chloroplasts by chloroform induced release, contains only 4 types of subunit and has a molecular weight of 280 000. In the presence of 1.25 mM MgCl₂, 1 mol of ATP or FTP is bound to the latent enzyme, with K_d = 10^-7 or 2 · 10^-7, respectively. The fluorescence emission (λ_max 340 nm) of FTP is enhanced 3-fold upon binding, and polarization of fluorescence is markedly increased. The fluorescence changes have been used to follow FTP binding, which behaves as a bimolecular process with K₁ = 2.4 · 10⁴ M^-1 · s^-1. FTP is displaced by ATP in a process apparently involving unimolecular dissociation of FTP with k_-1 = 3 · 10^-3 s^-1. The ratio of rates is comparable to the equilibrium constant and no additional steps have been observed. The protein has 3 sites for ADP binding. Rates of ADP binding are similar in magnitude to those for FTP. ADP and ATP sites are at least partly competitive with one another. The kinetics of nucleotide binding are strikingly altered upon activation of the protein as an ATPase. The rate of FTP binding increases to at least 10⁶ M^-1 · s^-1. This suggests that activation involves lowering of the kinetic barriers to substrate and product binding-dissociation and has implications for the mechanism of energy transduction in photophosphorylation.