Inhibition and uncoupling of the ADP-regulated electron transport in isolated chloroplasts

Effects of purine nucleotides on photosynthetic electron transport in isolated chloroplasts

The effects of guanylates and inosinates (and adenylates) on phosphorylation, ferricyanide reduction, and light-induced H+ uptake in spinach chloroplasts were studied. GDP, GTP, IDP, and ITP (but not GMP and IMP) stimulated the light-induced H+ uptake and partially inhibited ferricyanide reduction. Phosphate, arsenate, and phlorizin increased the extent of inhibition by these nucleotides and decreased the values of their apparent dissociation constants for the inhibition process. In the presence of phosphate (or arsenate), restoration of ferricyanide reduction from the level inhibited by guanylates and inosinates was observed as phosphorylation (or arsenylation) proceeded. These results suggest that phosphorylation of GDP and IDP as well as ADP takes place after two steps of nucleotide binding to the chloroplast coupling factor 1. The apparent dissociation constants of GDP and IDP for these two binding steps were estimated to be about 34 and 38 muM for the first and 110 and 160 muM for the second step, respectively (at pH 8.3, 15 degrees C). Above pH9, the ratio (P/delta-e) of the extent of phosphorylation to the increment of electron transport from the basal level measured in the presence of [ATP + Pi] or [ADP + Pi+ phlorizin], became increasingly large. When the electron transport level inhibited by dicyclohexyl-carbodiimide was taken to be the basal activity, the P/delta-e ratio remained almost constant (approximately 1) from pH 7.0 up to 10.

Adenylate regulation of photosynthetic electron transport and the coupling sites of phosphorylation in spinach chloroplasts

The regulation by adenylates of activities of various partial electron transport systems in spinach chloroplasts was studied using systems from H2O to 2,5-dimethyl-p-benzoquinone, H2O to 2,6-dichlorophenolindophenol, reduced 2,6-dichlorophenolindophenol to methyl viologen, and H2O to methyl viologen or ferricyanide. Adenylates regulated all of them. The ratio of the amount of esterified Pi (P) to that of electrons transported (Deltae) in coupling with phosphorylation manifested that there are two phosphorylation sites: one between H2O and 2,5-dimethyl-p-benzoquinone or 2,6-dichlorophenolindophenol and another between reduced 2,6-dichlorophenolindophenol and methyl viologen, under the proposed stoichiometries, i.e., P/DeltaH+ = 0.5 and DeltaH+/Deltae = 1, where DeltaH+ is the amount of protons pumped by electron transport (= those translocated during phosphorylation), when the basal electron transport (the part not regulated by adenylates) was excluded. The effects of pH, phlorizin, and methylamine on the adenylate regulation of electron transport, and the stimulation profile of electron transport coupled with quasiarsenylation suggested no distinction between the two phosphorylation sites.