Donor function of phosphate ions in photosystem 2

Abstract

Evidence was obtained for the interaction between the photosystem 2 (PS2) reaction centre (RC) chlorophyll (Chl) P680 and inorganic phosphate, Pi. The light-induced endogenous basal electron transport to ferricyanide in PS2 depended on endogenous Pi. The electron transport in phosphate deficient was absent, and could be resumed upon the addition of exogenous Pi or of the exogenous electron donor, diphenylcarbazide. Some chloroplast Chl molecules were apparently bound with Pi to a complex via the magnesium atom that was detected by the increase in absorbance in the Chl a absorption maximum at 435 nm observed after the consumption of endogenous Pi in the photophosphorylation reactions. The electron paramagnetic resonance (EPR) Signal I, found in the spectra at 77 K after irradiation of frozen samples in poor in endogenous Pi, was the sum of P700+ and P680+ signals. The P680+ signal disappeared after addition of Pi, diphenylcarbazide or to the before freezing. In addition, the EPR doublet signal of the phosphate anion radicals was recorded at 77 K after irradiation in the ethanol solutions of Chl a containing potassium phosphate. The same doublet signal was discovered in the difference EPR spectrum chloroplasts minus with diuron at 77 K after irradation. The results are a possible evidence of the participation of phosphate ions in the primary light reactions of PS2.