The effect of viscosity-increasing agents on ATP synthesis in chloroplast thylakoids

Abstract

The effect of an increase in the viscosity of the medium on cyclic photophosphorylation in chloroplast thylakoids and the Ca²⁺-dependent ATP hydrolysis by the chloroplast coupling factor CF₁ was studied. Using 0.1–0.2 mM ADP, it was found that the rate of ATP synthesis decreases after the addition of various agents that increase the viscosity of the medium (sucrose, dextran 40, or polyethylene glycol 6000) provided that these agents cause neither uncoupling nor electron-transport inhibition in the absence of ADP. Dextran and polyethylene glycol inhibited ATP synthesis by 50% when their concentrations were much lower (6–10%) than that of sucrose (30–40%), while 50% inhibition of the Ca²⁺-dependent ATP hydrolysis by CF₁-ATPase was observed at higher concentrations of dextran and polyethylene glycol (9–13%) and lower concentrations of sucrose (about 20%). For ADP, the effective Michaelis constant (K _M) was shown to increase by factors of 2–3 with increasing viscosity, while the maximum rate of cyclic photophosphorylation remained virtually unchanged. The dependence of K _M on the medium viscosity can serve as a criterion for the operation of diffusion-controlled photophosphorylation. Possible mechanisms of the diffusion of ADP and ATP are discussed.