Abstract
We studied bicarbonate-induced stimulation of photophosphorylation in thylakoids isolated from leaves of Arabidopsis thaliana plants. This stimulation was not observed in thylakoids of wild-type in the presence of mafenide, a soluble carbonic anhydrase inhibitor, and was absent in thylakoids of two mutant lines lacking the gene encoding alpha carbonic anhydrase 5 (αCA5). Using mass spectrometry, we revealed the presence of αCA5 in stromal thylakoid membranes of wild-type plants. A possible mechanism of the photophosphorylation stimulation by bicarbonate that involves αCA5 is proposed.
Highlights
ATP production in chloroplasts under illumination [photophosphorylation (PP)] is required for the key step of photosynthesis—the inclusion of CO2 into organic compounds in the Calvin–Benson cycle, which proceeds in the chloroplast stroma
We have shown that PP is stimulated by bicarbonate in thylakoids isolated from leaves of Arabidopsis thaliana wildtype (WT) plants and that this stimulation was absent in thylakoids from the mutant lacking carbonic anhydrase (CA) αCA5
In the thylakoids from WT plants, the addition of ADP and inorganic phosphate to the thylakoid suspension noticeably increased the electron transfer rate as compared with the rate in the presence of methyl viologen (MV) only (Table 1)
Summary
ATP production in chloroplasts under illumination [photophosphorylation (PP)] is required for the key step of photosynthesis—the inclusion of CO2 into organic compounds in the Calvin–Benson cycle, which proceeds in the chloroplast stroma. ATP synthesis is catalyzed by the chloroplast coupling factor, CF1, the part of ATP-synthase complex exposed to the chloroplast stroma. In the early 1960s, the stimulation of PP in isolated thylakoids by adding bicarbonate (HCO3−) to thylakoid suspension was discovered (Punnett and Iyer, 1964). Later findings showed that HCO3− stimulates other types of pHdependent ATP synthesis in thylakoids, namely, ADP phosphorylation after turning off actinic light and phosphorylation initiated by acid–base transition in the dark (Cohen and Jagendorf, 1972). The PP rate increases along with the increase in bicarbonate concentration (Harris, 1978)
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