The Thermal Stability of Electron Transfer in Membrane Preparations of Photosystem II with a Ca 2+ -Depleted Oxygen-Evolving Complex

Abstract

The temperature stability of electron transfer to the artificial electron acceptor 2,6-dichlorophenolindophenol in preparations of native photosystem II and photosystem II without the calcium cation in an oxygen-evolving complex was studied. The thermal stability of the processes of oxygen evolution and electron transfer from the oxygen-evolving complex to 2,6-dichlorophenolindophenol in photosystem II were significantly different: the reduction of 2,6-dichlorophenolindophenol was more resistant to temperature than oxygen evolution. The reaction of 2,6-dichlorophenolindophenol reduction in the Ca2+-depleted preparations of photosystem II was less resistant to heating than in the preparations of native photosystem II. The thermal inactivation of the photosystem II in the Ca2+-depleted membrane preparations was inhibited by cytochrome c at a concentration of 50 cytochrome c molecules per a photosystem II reaction center. The activity of this preparation (the rate of the 2,6-dichlorophenolindophenol reduction) increased by 19%, approaching the activity of the native photosystem II. The protective effect of cytochrome c appears to be determined by its protein nature, rather than its redox activity, since an equal protective effect was observed upon the addition of albumin at a similar concentration. Almost complete inactivation of the 2,6-dichlorophenolindophenol reduction reaction in the native and Ca2+-depleted preparations of photosystem II was observed at the same temperature (50°C). According to the EPR data, photosystem II in the Ca2+-depleted preparation after incubation at this temperature lacked a manganese cluster, while a peripheral protein of 33 kDa was present.