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

Abstract

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 (Δe) 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/ΔH⁺=0.5 and ΔH⁺/Δe=1, where ΔH⁺ 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.