Skip to main content
Biology LibreTexts

20.3: Plant Electron Transport and ATP Synthesis

  • Page ID
    15048
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)

    Introduction

    In the previous sections we studied light absorption by chlorophylls, transfer of energy to the reaction center of photosystem II, the oxidation of H2O by the oxygen evolving complex (OEC) and the transfer of electrons from these events to the lipophilic carrier of electrons, plastoquinone. Now we are ready to see how the process continues as electrons are passed on from reduced plastoquinone to the cytochrome b6f complex, through photosystem I (which has no OEC) and on to the terminal electron acceptor NADP+, which forms NADPH. This is used for reductive biosynthesis of glucose after fixation of atmospheric CO2 by ribulose bisphosphate carboxylase (RuBisCo). As we saw in mitochondrial electron transport, this passage of electrons is accompanied by the movement of protons from the lumen to the stroma with the ultimate collapse back into the lumen through an rotatory ATP synthase to form the ATP required for reductive biosynthesis. Figure \(\PageIndex{1}\) reviews again the light reactions of photosynthesis.

    photosyntheisOverall_Kegg.png
    Figure \(\PageIndex{1}\): Light reactions of photosynthesis

    Cytochrome b6f

    This complex moves electrons from the mobile lipophilic electron carrier reduced plastoquinol (PQH2), an isoprenoid quinone, to the mobile Cu-containing protein plastocyanin, which plays an analogous role to the mobile protein carrier in mitochondrial electron transport, cytochrome C. It catalyzes the rate limiting step in electron transport in the light reactions. Figure \(\PageIndex{2}\) below shows an interactive iCn3D model of the spinach plastocyanin (1AG6)

    spinachplastocyanin1AG6.png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{2}\): Spinach plastocyanin (1AG6). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...DwSDDrYMUb9Xn7

    The complex that mediates electron transfer between reduced plastoquinone and the protein plastocyanin, cytochrome b6f, is centrally positioned between the two photosystems. In addition it moves 2 H+s from the stroma into the lumen. These are joined by 2 H+s from the oxidation of water by the OEC (on the luminal side of PSII) to create a transmembrane proton gradient, which will power ATP synthesis.

    Electrons transfer with cytochrome b6f takes place through the quinol (Q) cycle in a similar fashion to Complex III in mitochondrial electron transport so we won't go into much detail here. Figure \(\PageIndex{3}\) shows a summary diagram with electron and proton flow.

    QcyclecytoB6f.png
    Figure \(\PageIndex{3}\): Q cycle in the cytochrome b6f complex of photosyntheis. https://twitter.com/BiologyNowadays/...232768/photo/1

    Figure \(\PageIndex{4}\) show another version of the Q cycle as an alternative representation.

    Photosynthetic growth despite a broken Q-cycleFig1.svg
    Figure \(\PageIndex{4}\): Alternative version of the photosynthetic cytochrome b6f Q cycle

    Left, box schematic; the b6f complex transfers two protons (green arrows) per electron transferred (blue arrows) along high (Fe2S2 cluster, cytochrome f) and low potential chains (bl, bh, ci hemes). Quinol (QH2) oxidation at Qo site, Quinone (Q) reduction at Qi site. Right, structure depicting haems b (purple), ci and f (red), Fe2S2 cluster (yellow and orange ball-and-stick model), cytochrome b6 (cyan), subunit IV (blue), Rieske subunit (yellow), cytochrome f (red), PetG, L, M and N subunits (green). Magnification of Qi site comprising bh and ci haems. Malnoë et al. Nat Commun 2, 301 (2011). https://doi.org/10.1038/ncomms1299. Creative Commons Attribution-NonCommercialShare Alike 3.0 Unported License. http:// creativecommons.org/licenses/by-nc-sa/3.0/

    Both cytochrome bc1 and cytb6f are dimeric complex, with 2 Fe2S2 clusters, two cytochrome bs and a cytochrome c. The cytb6f complex also has 9-cis β-carotene and additional c1 heme. The electrons move from PQH2 through complex in a similar fashion as in the bc1 complex.

    PQH2 is oxidized at the Qp site with a bifurcation of electrons:

    • one electron moves through the high potential Fe2S2 center and cyt f pathway (bottom left in pathway diagram of Figure 3), with the electron moving to the soluble peripheral protein plastocyanin and one to photosystem I.
    • the other moves through the low potential bl, bh, ci hemes pathway (top left in pathway diagram of Figure 3), with the electron moving to a plastoquinone at the Qn site near the stroma. A second round of oxidation of PQH2 at the Qp site eventually donates another electron to a plastoquinone-. which regenerates PQH2 after addition of 2 H+ from the stroma.

    Having two successive oxidations of PQH2 leads to twice the number of protons moving into the lumen. In the process 2 H+ move into the lumen. Cytochrome b6f also is a redox sensor of the status of the plastoquinol/plastiquinone pool.

    Figure \(\PageIndex{5}\) below shows an interactive iCn3D model of the spinach cytochrome b6f complex (6RQF)

    spinach cytochrome b6f complex (6RQF).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{5}\): Spinach cytochrome b6f complex (6RQF). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...tgUhjekB4Tndw6

    The region away from the red leaflet represents the lumen side of the complex. The FeS cluster and heme C are in the luminal domains. The coloring scheme is as follows:

    • cytochrome bcf subunits 4, 5, 6, 7, and 8: light gray
    • cytochrome b6: dark gray
    • cytochrome b6 FeS subunit: plum
    • cytochrome F: cyan
    • chlorphylls: orange spacefill, labeled
    • FeS clusters: CPK spacefill, labeled
    • HEC - Heme C: CPK spacefill, labeled
    • heme - porphoryin IX containing Fe: yellow spacefill

    Cytochrome b6f is the rate limiting step for electron flow but what it's role in regulation the entire photosynthetic pathway (light reaction plus the dark reaction of carbon metabolism? Data suggest that the complex regulates electron transport in low light condition but effects a switch to carbon metabolism under saturating light. Johnson and Berry have analyzed electron flow with carbon metabolism in a fashion analagous to transistors in a circuit board, as illustrated in Figure \(\PageIndex{6}\) below.

    Cytochrome b6f in the control of steady-state photosynthesis_Fig1.svg
    Figure \(\PageIndex{6}\): Electron transport system as an electrical circuit (and associated text below). Johnson and Berry. Photosynthesis Research (2021) 148:101–136. Creative Commons Attribution 4.0 International License. http:// creativecommons.org/ licenses/by/4.0/.

    They define a transistor as a regulated circuit element that uses variable conductance to control current flow. The linear flow of electrons from water to reductant is viewed as a light-driven current that is under the control of a many regulatory feedbacks stemming from carbon metabolism. In limiting light, Cyt b6f presents maximal conductance to flow, and feedback from carbon metabolism adjusts the excitation of PS I and PS II in such a way as to balance the relative rates of linear and cyclic electron flow to the NADPH, Fd, and ATP requirements of the sinks. When light becomes saturating, feedback from carbon metabolism also decreases the apparent conductance of Cyt b6f, controlling the linear flow of electrons through the plastoquinone pool and the associated flow of protons into the thylakoid lumen. In this way, the regulation of Cyt b6f simultaneously permits efficient photosynthesis and protects the system from photodamage.

    This model is organized around the idea that the distribution of excitation between PS II and PS I is regulated in such a way as to minimize losses of absorbed light and maximize potential electron transport through Cyt b6f. The expression for the potential electron transport rate has the form of a Michaelis-Menten expression for a single substrate (i.e., light), but describes the kinetic behavior of the entire electron transport chain (i.e., including both photochemical and biochemical reactions). It predicts that electron transport has a hyperbolic dependence on irradiance, with the maximum efficiency realized at the limit where absorbed irradiance goes to zero and the maximum speed realized at the limit where absorbed irradiance is infinite.

    The trade-off between the speed and efficiency of potential electron transport is driven by the need for the supplies of reduced plastoquinone and oxidized plastocyanin to be balanced in order to sustain Cyt b6f turnover at the maximum catalytic rate. This causes progressive closure of the PS II and PS I reaction centers, with PS II accumulating in a reduced state and PS I in an oxidized state. As the excitation pressure on PS II and PS I increases, the closure of the reaction centers causes the photochemical yields of PS II and PS I as well as the absorbed quantum yield to decrease as the potential electron flow through Cyt b6f and the potential photosynthetic rate increase"

    The key prediction of the expression for the potential electron transport rate is that the maximum activity of Cyt b6f defines the upper limit for the theoretical maximum speed of electron transport. The expressions describing feedback control over Cyt b6f activity are based on the idea that Cyt b6f functions like a transistor, i.e., a component of an electrical circuit that uses variable conductance to control current.

    "The fact that Cyt b6f can modulate its conductance to linear electron flow within milliseconds of a perturbation in light suggests that photosynthetic control is the first line of defense against overexcitation, protecting the acceptor side of PS I from being flooded with highly reduced intermediates.

    I"n response to a sustained increase in light, the induction of photosynthetic control is followed by induction of PQN. As NPQ alleviates the electron overpressure in the PQ pool, photosynthetic control progressively relaxes. The two forms of regulation gradually settle to a steady-state at the new light intensity. This interaction seems to allow electron transport to proceed at the Cyt b6f-limited rate under low light intensities, and then smoothly switch to the Rubisco-limited rate once the light intensity is high enough to become saturating. It also seems to allow photosynthesis to operate safely and efficiently in a wide range of biochemical milieus, from those characteristic of natural variation in photosynthetic capacity (with balanced electron transport and carbon metabolism) to those characteristic of genetic manipulations (with imbalances in electron transport and carbon metabolism).

    In this framework, the excitation balance of PS II and PS I and the maximum activities of Cyt b6f and Rubisco emerge as key limits on system dynamics. For example, the trade-off between the speed and efficiency of electron transport is shown to be controlled by the excitation balance of PS II and PS I and the maximum activity of Cyt b6f. The development of PQN is shown to be controlled by the excitation balance of PS II and PS I and the demand for linear electron flow (LEF) through the light reactions and circular electron flow (CEF) around PSI. The onset of photosynthetic control is shown to be dependent on the maximum activities of Cyt b6f and Rubisco.

    Plant photosystem I-LHCI super-complex

    If the goal of the photosystem complexes is to transduce light energy delivered by photons into electrons that can be used for reductive biosynthesis of glucose from atmospheric carbon capture of CO2, then PSI is quite amazing. It has a "quantum efficiency" close to 1 which implies that one absorbed photon produces 1 electron that can be used to reduce NADP+. This happens since the transfer of photon energy to other molecules in PSI is so quick compared to nonradiative decay processes for the excited state chlorophylls.

    The same process for excitation and electron (charge) transfer that we saw in PSII occurs in PSI, with chlorophyll in the reaction center involved in charge transfers. The light absorbing molecules of PSI enable light at the far red of the spectra to be absorbed. The complex has 16 proteins, 155 chlorophylls and 35 carotene derivatives. As with PSII there is a core complex that is similar to cyanobacterial PSI. The supercomplex has in addition light harvesting complex I proteins around one side of the complex (looking down on it) that have four LHCI proteins (Lhca 1-4) which allow for more absorption of life. They have 57 chlorophylls and 13 carotene derivatives.

    The apoproteins that bind the chlorophylls, incljding the LHCI proteins, have similar but sliglthly different topologies, allows for tuning of the absorbance spectra of the bound chromophores. Seven of the chlorophylls when bound have local environments that allow absorption of far red light. These are not found in PSII. the far red light would be more abundant in the low parts of the plant canopies as photons of lower wavelength would be more filtered out by upper leaves in the canopy.

    Because of the complexity of PSI, we will offer several different iCn3D models to illustrate different features of the photosystem I-LHCI super-complex. As with PSII, PSI has a core with a surrounding light-harvesting comple (LHCI). Figure \(\PageIndex{7}\) below shows an interactive iCn3D model of the Plant (pea) photosystem I-LHCI super-complex (4XK8)

    Plant photosystem I-LHCI super-complex (4XK8).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{7}\): Plant photosystem I-LHCI super-complex (4XK8). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...uWBzFrVBUDHHQ6

    Again the red leaflet represents the lumen side of the complex. Gray represents the chlorophyll and other lipids molecules in the complex. Here is the same complex but just the nonprotein components

    Figure \(\PageIndex{8}\) below shows an interactive iCn3D model of the lipid and FeS components of Plant photosystem I-LHCI super-complex (4XK8)

    Lipid and FeS comp plant photosystem I-LHCI super-complex (4XK8).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{8}\): Lipid and FeS components of plant photosystem I-LHCI super-complex (4XK8). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...uWBzFrVBUDHHQ6

    Again the red leaflet represents the lumen side of the complex. Gray represents the chlorophyll and other lipids molecules in the complex. Rotate the image to see how the chlorophyll and other lipids encircle the membrane protein components.

    Figure \(\PageIndex{9}\) below shows an interactive iCn3D model of the ApoA1 protein with surrounding chlorophyll and other components from plant photosystem I-LHCI super-complex (4XK8)

    apoA1 and surround  comp plant photosystem I-LHCI super-complex (4XK8).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{9}\): ApoA1 protein with surrounding chlorophyll and other components from plant photosystem I-LHCI super-complex (4XK8). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...L6tsJ3oMDZ2YQA

    ApoA1 is shown in cartoon and colored according to hydrophobicity. The chlorophylls, carotenes and other components within 5 Å are shown in red. Rotate the image to see how the chlorophyll and other lipids encircle the membrane protein components.

    Figure \(\PageIndex{10}\) below shows an interactive iCn3D model of the Plant (pea) photosystem I-LHCI super-complex highlighting LHCIs (4XK8)

    Plant (pea) photosystem I-LHCI super-complex highlighting LHCIs  (4XK8).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{10}\): Plant (pea) photosystem I-LHCI super-complex highlighting LHCIs (4XK8). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...9vHynkRTLxAct7

    The LHCI subunits (Lhca 1-4) are shown in alternating magenta and cyan cartoon structure surround one side of the core complex. The view in the model above is a top-down view.

    We discussed in Chapter 20.1 that PSI absorbance is tilted toward the red end of the spectra, with the effective absorbance ratio over a broad wavelength range for the two photosystems, PSI/(PSI+PSII) deviating from around 0.5 at the red/far red end of the spectrum (670-730 nm), where the ratio is close to 1. When exposed to far red light, the systems move to state I. In this state, the major mobile antenna proteins (LHCIIs) move to PSII to restore a "photoabsorption" balance. When exposed to light depleted in the high end of visual spectra, the system move to state II, in which mobile LHCIIs move to PSI.

    The location of LHCs is regulated by phosphorylation of LHCs controlled by the levels of plastoquinone, which makes great biological sense. When the concentration of plastoquinones in the reduced state, PQH2 (plastoquinol), is high, it would be optimal to increase the activity of PSI to relieve the high concentration of the substrate for cyto b6f and shift the system to higher PSI activity and continue eletron flow. This regulation is mediated by phosphorylation of LHCs by LHCII kinase (a Ser/Thr kinase), which is activated by high reduced PQH2 concentrations. Phosphorylation of LHCs lead to their movement from PSII to PSI. When the oxidized form for plastoquinone is high, LHCII kinase is inactivated by dephosphorylation, causing the mobile LHC to move back to PSII to increase output (PQH2) from PSII. These event occur in low light. In high light conditions, when the system is functioning at a high level, the LHCII kinase is inhibited by stromal thioredoxin. Specifically, the phosphorylation status of Lhcb1 and Lhcb2 in LHCII homo- or heterotrimers determine the movement between PSII and PSI. The chloroplastic serine/threonine-protein kinase (STT7 also known as STN7) is another LHCII kinase.

    Chloroplast ATP synthase - CF1FOATPase

    Finally, let's take a quick look at the ATP synthase in chloroplasts. It is similar in structure and function to mitochondrial FoF1ATPase so we won't spend much time on the mechanism. Like its mitochondrial counterpart, it is a rotary enzyme that transduces the free energy of a proton gradient collapse into chemical energy in the form of ATP. One difference is the rotary enzyme should be regulated by light levels with its activity decreased in the night. This is accomplished in higher plants by a conversion between a reduced and oxidized state, an effective redox switch, in one subunit (γ) of the ATP synthase. The deactivation is important at night since if the rotary enzyme runs in reverse, ATP hydrolysis would ensue.

    Figure \(\PageIndex{11}\) below shows an interactive iCn3D model of the reduced R1 state of chloroplast ATP synthase (R1, CF1FO) (6VON). It should look familiar to you given its similarity to mitochondrial ATP synthase.

    chloroplast ATP synthase reduced (6VON).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{11}\): Reduced R1 state of chloroplast ATP synthase (R1, CF1FO) (6VON) (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/icn3d/share.html?hbnDCm1Kgqgu2k1NA (long load)

    • alpha - light green
    • beta - dark green
    • gamma - yellow
    • gamma - crimson
    • epsilon - indigo
    • b - blue
    • b' - light blue
    • a - light pink
    • c ring - purple
    • ATP - spacefill, cyan
    • TENTOXIN (TTX) - spacfill gray
    • ADP - spacefill magenta

    In the oxidized state there is a disulfide in the the γ subunit which inhibits torsion by stabilizing two β hairpins. This constraint is relieved on reduction and rotation is enhanced. In the reduced structure, tentoxin, an uncompetitive inhibit is present which allowed the structure to be determined.

    below Yang, JH., Williams, D., Kandiah, E. et al. Structural basis of redox modulation on chloroplast ATP synthase. Commun Biol 3, 482 (2020). https://doi.org/10.1038/s42003-020-01221-8. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/.

    At night, the enzyme is in the low activity form. When light becomes available in the day, the pH gradient collapse leads to release of tightly bound ATP from the oxidized state of the enzyme. PSI can engage in photon-induced electrons transfer to ferrodoxin. This binds to NADP reductase and leads to NADPH formation. These process are illustrated in Figure \(\PageIndex{12}\) below.

    photosyntheisOverall_KeggPS1Ferrodoxin.png
    Figure \(\PageIndex{12}\) : Fd bound to PSI is ferrodoxin and FNP is ferrodoxin NADP reductase.

    Increased reduced ferredoxin leads to formation of reduced thioredoxin, a small redox proteins involving in redox signaling and protection of cells from oxidative stress. A similar process activates the enzyme sedoheptulose-1,7-bisphosphatase (SBPase) found in the dark reaction Calvin cycle of photosynthesis. Its activation is illustrated in Figure \(\PageIndex{13}\) below.

    SBPase_regulation_by_ferredoxin-thioredoxin_system.png
    Figure \(\PageIndex{13}\): SBPase regulation by ferredoxin-thioredoxin system. https://commons.wikimedia.org/wiki/F...xin_system.png. Creative Commons Attribution-Share Alike 3.0 Unported l

    Reduced thioredoxin also reduces and activates chloroplastic ATP synthase.

    Figure \(\PageIndex{14}\) below the activities and structures of the ATP synthase (CF1FO)

    redox modulation on chloroplastATPSynthaseFig1.svg
    Figure \(\PageIndex{14}\) below the activities and structures of the ATP synthase (CF1FO). Yang, JH., Williams, D., Kandiah, E. et al. Structural basis of redox modulation on chloroplast ATP synthase. Commun Biol 3, 482 (2020). https://doi.org/10.1038/s42003-020-01221-8. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/.

    Panel a: For the experiments, purified CF1FO was reconstituted into a liposome (orange) mixed with lipids of phosphatidylcholine and phosphatidic acid. The generated pH gradient across the membrane drove the reconstituted CF1FO to synthesize ATP molecules, which were detected using a luciferin/luciferase assay (green). Val indicates valinomycin.

    Pane b: The activity of the enzyme is shown in the reduced state (in the presence of dithiothreitol - DTT, blue curve), oxidized state (in the presence of iodosobenzoate - IBZ, orange curve, and with no additive (green curve). Note that the activity is reduced about 80% in the oxidized state.

    Panel c: The images are from Cryo-EM density maps of the oxidized and reduced forms of the CF1FO. The color codes are as follows: α (light green), β (dark green), δ (yellow), bb' (blue and light blue), γ (crimson), ε (indigo), a (light pink), and c ring (purple). R indicates a reduced state, and O indicates an oxidized state. The three-dimensional (3D) reconstructions are categorized into three different rotary states (states 1, 2, and 3). Upper insets are the density maps of the F1 domains.

    Figure \(\PageIndex{15}\) below shows the details of the conformation changes in the γ subunit between the oxidized and reduce forms.

    redox modulation on chloroplastATPSynthaseFig3.svg
    Figure \(\PageIndex{15}\): Conformation changes in the γ subunit between the oxidized and reduce forms

    Panel a: Structures of the reduced (light blue) and oxidized (orange) γ subunits. Two β hairpin structures (from γGlu238 to γLeu282) are shown in light green, and the two cysteines of the redox switch are shown in yellow in circular enlarged views. Diagram on right shows the topology of the two β hairpin structures.

    Panel b Superposition of the reduced and oxidized γ subunits (RMSD 1.016 Å). The two β hairpins are shown in light blue and orange for the reduced and oxidized forms, respectively. Other regions are shown in white.

    Panel c Interaction networks of the β hairpin 2 and βDELSEED motif. Left and right panels are the reduced (γ subunit in light blue) and oxidized (γ subunit in orange) forms. Light green represents the β hairpin 2, dark green for the β subunit, and yellow for the βDELSEED motif. The distances connecting the residues of the γ coiled coil (γArg73, γGln76, and γGlu77) with the βGlu412 are labeled.

    Panel d Interaction of the EDE motif with the γ subunit. The EDE motif (yellow) does not interact with any part of the reduced γ subunit but forms an extensive interaction network with its neighborhood when the γ subunit is oxidized.

    Figure \(\PageIndex{16}\) below shows a cartoon model illustrating the structures of the overall oxidized and reduced states of ATP synthase

    redox modulation on chloroplastATPSynthaseFig5.svg
    Figure \(\PageIndex{16}\): cartoon model illustrating the structures of the overall oxidized and reduced states of ATP synthase

    Panel a: This shows a cartoon schematic of the redox modulation. Upper and lower models are the reduced and oxidized states, respectively. Color codes are the same as in Fig. 1c and the β hairpin structures of the γ subunit are shown in light green. The two redox states are aligned in the same view.

    Pane b: At night, no energy input from light is available for the photosynthetic electron transport chain, and thus, no electrochemical potential (ΔΨ) and proton gradient (ΔpH) are generated. The oxidized γ subunit prevents CF1FO from hydrolyzing ATP. During the day, light induces charge separation to generate an electrochemical potential across the membrane. Although the CF1FO begins to synthesize ATP molecules, the γ subunit is still oxidized while ΔΨ is small. The rate of ATP synthesis is not at its maximum. At sunrise, thioredoxin subsequently reduces the γ subunit, fully activating CF1FO. The molecular motor, consisting of the γ-ε central shaft and the c14-ring, is free to rotate at full speed to maximize its ATP synthesis activity. Three ATP molecules per rotation of the c14 ring are produced. At sunset, the membrane becomes de-energized, leading to small ΔΨ and ΔpH, and the ATP hydrolysis starts to take place. To prevent ATP loss from excess ATP hydrolysis, the γ subunit is then oxidized again. This process of light regulation and redox modulation on the CF1FO will cyclize daily.


    20.3: Plant Electron Transport and ATP Synthesis is shared under a not declared license and was authored, remixed, and/or curated by Henry Jakubowski.

    • Was this article helpful?