HYSTERESIS OF PROTONATION DEPROTONATION OF CHLOROPLASTS DURING ACID-ALKALINE TITRATION WITHIN 5-8-PH RANGE

The interaction of pea chloroplasts with protons during titration in a medium maintaining the packing of grana in unswollen thylakoids was s tudied Under near-equilibrium conditions, the curves for acid-alkaline titration of non-energized chloroplasts make a closed hysteresis loop with pK values of 5.7 and 6.6 for the acid and alkaline branches, res pectively. The chloroplasts have a buffer system which binds and relea ses protons by a cooperative processes similar to phase transitions. T he rates of the protonation/deprotonation at pH values near the pK val ues is 60-80 neq H+/min per 1 mg chlorophyll. However, with pH shifted by 0.6 units to the acid or alkaline region, the rates increase by a factor of 6-8. The light-induced formation of a pH gradient eliminates much of the hysteresis; thus, in the light the chloroplast titration curves approach the alkaline branch of the hysteresis curve of the non -energized chloroplasts. The buffer system exhibiting hysteresis appea rs to be located within the thylakoid membrane and to include an ensem ble of components interacting with protons of both the reaction medium and the intrathylakoid space. A model for the functioning of the vect orial proton translocation system during energization is proposed.