ENERGY-REGULATED FUNCTIONAL TRANSITIONS OF CHLOROPLAST ATPASE

The functional transitions of the membrane-bound chloroplast ATPase (CF₁) as influenced by low ADP and uncoupler concentrations are investigated by measurements of initial and steady-state ATP hyrolysis and concomitant membrane energization. Following activation of latent ATP hydrolysis by light in the presence of dithioerythritol, the resulting steady-state ATP hydrolysis depends on the dark-period ( t _d) bteween light activation and ATP addition. ADP, added during t _d, inhibits this activity ( K _i about 2 μ M ) and induces a lag in the onset of ATP hydrolysis. The extent of membrane energization as monitored by an aminoacridine fluorescent probe is proportional to the ATPase activity.
An uncoupler amplifies the inhibitory effect of ADP if added during f _d, whereas it induces the normal stimulation of ATP hydrolysis in the absence of ADP. The ADP effect, which is different from product inhibition, is interpreted as a conformational interaction with CF₁ causing an increase of the energy threshold required for the inactive → active transition of the CF₁ molecules. These results are in harmony with currently proposed models of CF₁ regulation by adenine nucleotides based on binding studies.
The inactive → active transition of CF₁ conformation is investigated by analysis of the lag in the onset of ATP hydrolysis at different ADP concentrations and by means of varied light pulses and single-turnover flashes, using the electric potential indicating absorption change at 515 nm as a probe for the onset of ATP hydrolysis. The half-time of the process leading to fully (re)activated ATP hydrolysis is about 0.25 s. The ATP-dependent flash-induced inactive → active transition occurs within a few turnovers of electron flow.