Optimizing oxygenic photosynthesis: pH-regulation of electron transport in chloroplasts in silico

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In this work, we carried out mathematical modeling of the electron and proton transport regulation in the thylakoid membranes of chloroplasts under different operating conditions of the electron transport chain (ETC) of chloroplasts. The study is based on the kinetic model of the functioning of the chloroplasts ETC proposed by us earlier, which describes the redox transformations of the reaction center of photosystem 1 (PS1), ferredoxin molecules, several forms of plastoquinone (PS2-related molecules PQA, PQB, and the pool of plastoquinones PQ/PQH2), as well as plastocyanin molecules. The model also simulates the induction curve of chlorophyll a fluorescence in the leaves of higher plants adapted to darkness. The multiphase kinetic curves obtained by varying the model parameters reflecting the rate of functioning of the Calvin–Benson cycle and the cyclic electron transport path around PS1 are in reasonable agreement with the published experimental data. The main result of our work is that it mathematically describes how pH-dependent regulatory processes occurring in various parts of the chloroplast ETC (non-cyclic, cyclic, and pseudocyclic electron transport) affect the kinetics of induction processes (slow induction of fluorescence and redox transformations of the PS1 photoreaction center) in dark-adapted chloroplasts of plants.

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Sobre autores

A. Vershubskii

Moscow Lomonosov State University

Email: an_tikhonov@mail.ru

Faculty of Physics

Rússia, Moscow

A. Tikhonov

Moscow Lomonosov State University

Autor responsável pela correspondência
Email: an_tikhonov@mail.ru

Faculty of Physics

Rússia, Moscow

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2. Fig. 1. Processes of electron and proton transport considered in the model. Abbreviations: PS1 and PS2 are photosystems 1 and 2, respectively, CBC is the Calvin–Benson cycle, NET is non-cyclic electron transport, CET is cyclic electron transport, PCET is pseudo-cyclic electron transport (the water–water cycle), NPT is non-photochemical quenching (weakening of the photochemical activity of PS2). Fd is ferredoxin, PQ and PQH2 are plastoquinone and plastoquinol, PQA and PQB are molecules of plastoquinone bound to PS2; CF0-CF1 is the ATP synthase complex. The diagram shows the designations of the effective rate constants of electron transport, which we varied when modeling the induction processes shown in Figs. 2–9 (see the text for details). L1 and L2 are the fluxes of light quanta exciting PS1 and PS2; the rate constants kP700 and kP680 characterize the efficiency of photochemical processes in the reaction centers of PS1 and PS2, respectively. The numerical values ​​of all rate constants are given in our previous work [34].

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3. Fig. 2. Kinetic curves of changes in the relative concentration of the oxidized form of the P700 + reaction center, calculated for different ratios of the kFQ and kFN constants, which determine the rates of electron outflow from Fd along the ETC and NET chains (see definitions in Fig. 1). The fluxes of light absorbed by the pigments PS1 and PS2 are 150 quanta/s (PS1) and 250 quanta/s (PS2). The intrathylakoid buffer capacity Bi = 200. a – Kinetic curves were obtained for different values ​​of the kFQ parameter, which determines the electron flow along the ETC chain (at a constant value of the kFN coefficient = 10–2 s–1 (see more details [34]), which determines the electron transfer from Fd to NADP+). The ratios of rate constants kFQ/kFN are: 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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4. Fig. 3. Kinetic curves of photoinduced changes in relative concentrations of the oxidized form of ferredoxin, calculated for the same parameter values ​​as shown in Fig. 2. a – The ratios of rate constants kFQ/kFN are: kFQ/kFN = 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The kCBC coefficient determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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5. Fig. 4. Kinetic curves of photoinduced changes in the relative concentrations of the reduced form of plastocyanin ([Pc–]), calculated for the same values ​​of the model parameters as indicated in the caption to Fig. 2. a – The ratios of the rate constants kFQ/kFN are: kFQ/kFN = 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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6. Fig. 5. Kinetic curves of light-induced changes in the relative concentrations of the reduced form of plastoquinone (PQH2), calculated for the same values ​​of the model parameters as indicated in the caption to Fig. 2. a – The ratios of the rate constants kFQ/kFN are: kFQ/kFN = 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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7. Fig. 6. Kinetic curves of photoinduced changes in intrathylakoid pHin and pHout of the stroma, calculated for the same values ​​of the model parameters as indicated in the caption to Fig. 2. a – The ratios of the rate constants kFQ/kFN are: kFQ/kFN = 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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8. Fig. 7. Kinetic curves of photoinduced changes in the relative ATP concentration (variable [ATP]), calculated for the same values ​​of the model parameters as indicated in the caption to Fig. 2. a – The ratios of the rate constants kFQ/kFN are: 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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9. Fig. 8. Kinetic curves of photoinduced changes in intrathylakoid pHin and pHout of the stroma (a), relative concentrations of the oxidized form of the reaction center P700 + (b), chlorophyll fluorescence induction curves (dependence of fluorescence intensity on the duration of actinic light action) (c), calculated at different values ​​of the buffer capacity: Bi = 100 (1), 200 (2), 400 (3). The fluxes of light absorbed by the pigments PS1 and PS2 are equal to 150 quanta/s (PS1) and 250 quanta/s (PS2). kFQ/kFN = 0.15, kCBC = 10–2 s–1. The remaining values ​​of the model parameters are given in the Appendix.

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10. Fig. 9. Chlorophyll fluorescence induction curves (dependence of fluorescence intensity on duration of actinic light action), calculated for the same values ​​of the model parameters as indicated in the caption to Fig. 2. a – The ratios of rate constants kFQ/kFN are: 0 (1), 0.15 (2), 0.3 (3), 0.5 (4). b – Kinetic curves calculated for different values ​​of the constant kCBC, simulating the intensity of the CBC work: kCBC = 10–2 s–1 (1), 2 ∙ 10–2 s–1 (2), 5 ∙ 10–2 s–1 (3). The coefficient kCBC determines the rate of electron transfer from NADPH to the CBC (for more details, see [34]).

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