Transmembrane Proton Transfer in Chloroplasts in Silico: pH Homeostasis of Lumen, Stroma, and Cytosol

The research described in this paper is based on a mathematical model of the light stages of photosynthesis, which considers the processes of electron and proton transport in chloroplasts. It examines the redox transformations of the photoreaction centers PS I (P), PS II (P) and other electron transport chain carriers–ferredoxin (Fd), plastoquinone (PQ) and plastocyanin (Pc)–as well as the transmembrane proton transport associated with electron transport and ATP synthesis by membrane ATP synthase. The simulation results are in good agreement with the literature data on pH measurements in the lumen (pH) and stroma (pH) of chloroplasts at different cytosol pH values (pH). The steady-state pH values established in these compartments when chloroplasts are illuminated under photophosphorylation conditions satisfy the following inequalities: pH ≈ 6.2–6.4 pH ≈ 7.2–7.4 pH ≈ 7.8–8.0. Variation of the pHcyt model parameter affects the electron flows carried by different electron transport pathways, such as non-cyclic electron transport from PS II to PS I and further into the Calvin–Benson cycle, cyclic electron transport around PS I, and pseudocyclic electron transport involving molecular oxygen (the “water – water” cycle). It has also been shown that sufficiently strong acidification of the cytosol (pH 7) reduces electron efflux from the acceptor side of PS I (non-cyclic and pseudocyclic electron transport) and stimulates cyclic electron transport around PS I, and also decreases the rate of pH-dependent ATP synthesis.