20.2: The Kok Cycle and Oxygen Evolving Complex of Photosystem II

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Learning Goals 

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

The Z Scheme and Overall Electron Flow in the Light Reactions

• Trace the complete Z scheme of the light reactions, describing the flow of electrons from water through PSII (P680 → pheophytin A → plastoquinone A → plastoquinone pool), through the cytochrome b₆f complex to plastocyanin, and through PSI to ferredoxin and ultimately NADP⁺, identifying the standard reduction potentials at each step and explaining why two sequential photon-driven uphill steps are thermodynamically required.
• Explain why the net reaction of PSII (2PQ + 2H₂O → 2PQH₂ + O₂) is thermodynamically unfavorable based on standard reduction potentials alone, and describe how photon energy provides the thermodynamic driving force to overcome the unfavorable electron transfer from water (E°′ = +0.816 V) to plastoquinone (E°′ ≈ +0.11 V).

Water Oxidation: The Kok Cycle and the Oxygen-Evolving Complex

• Describe the Kok cycle (S₀–S₄ states) of the oxygen-evolving complex (OEC), explaining how four sequential single-electron oxidation events driven by Tyr161 (Yz) radical cation formation after each photon absorption progressively oxidize the OEC until the S₄ state returns to S₀ with release of O₂, and explain the experimental evidence from flash-illumination experiments that established the four-photon periodicity of O₂ release.
• Describe the structure of the OEC Mn₄CaO₅ cluster from Thermosynechococcus vulcanus, explaining the distorted-cubane geometry of three Mn and one Ca at alternating corners bridged by oxides, the significance of the "dangling" fourth Mn ion, and why the O5 bridging oxygen — with its longer, weaker metal-oxygen bonds — may be the site of O–O bond formation during water oxidation.
• Explain how the multiple accessible oxidation states of manganese (Mn²⁺ through Mn⁴⁺) make Mn uniquely suited for the four-electron oxidation of water, using standard reduction potential data to identify which Mn oxidation state transitions are thermodynamically capable of oxidizing water, and describe the proposed sequence of Mn oxidation states from (III, IV, III, III) in S₀ to (IV, IV, IV, IV) in S₃/S₄.
• Describe the role of water channels (O1, O4, and Cl1) and the Yz network connecting Tyr161 to the thylakoid lumen, explaining how these structural features supply substrate water to the OEC, direct the four protons produced per water oxidation cycle into the lumen to establish the pH gradient, and how channel rigidity and bottleneck residues control water mobility differently in each channel.

Electron Transfer from PSII and Charge Recombination

• Trace the electron transfer pathway from pheophytin A (the primary electron acceptor after P680 photoexcitation) through plastoquinone A to the mobile lipophilic electron carrier plastoquinone in the thylakoid membrane, drawing explicit parallels to ubiquinone in mitochondrial electron transport, and explain how the proximity of pheophytin A and plastoquinone A in the PSII reaction center facilitates rapid, productive forward electron transfer.
• Explain the competing charge recombination pathways in PSII using the P⁺–Pheophytin–QA standard reduction potential diagram, distinguishing the safe blue pathway (direct P⁺ and QA⁻ recombination) from the damaging red pathway (reverse electron transfer to form triplet P, which sensitizes ground-state O₂ to singlet oxygen), and describe why stabilizing the P⁺PheQA⁻ state is essential for both efficient photosynthesis and photoprotection.

Introduction

We have just seen how photoexcitation of the non-reaction-center chlorophyll turns that molecule into a good reducing agent, which transfers its electron to the nearest excited-state level of the reaction-center chlorophyll. If you consider both steps together, the non-reaction-center chlorophyll gets "photooxidized" in the process, producing the "strong" oxidizing agent, a positively charged chlorophyll derivative. The extra electron transferred to the second molecule will eventually be transferred to NADP⁺ to produce NADPH.

These reactions occur in the presence of light and hence are called the light reactions. The light reactions of photosynthesis in green plants are shown in Figure \(\PageIndex{1}\), along with the standard reduction potentials of the participants, the Z scheme.

Figure \(\PageIndex{1}\): Light reaction of photosynthesis and associated standard reduction potentials

The combined processes of PSII and PSI resemble a "Z" scheme (rotate the standard reduction potential figure 90 degrees clockwise in your mind). In an organization reminiscent of electron transport in mitochondria, water is oxidized by photosystem II (PSII). Electrons from water are transferred through PSII to a mobile, hydrophobic molecule, plastoquinone (PQ), forming its reduced form, PQH₂. Another photosystem, photosystem I (PS1), is next in the electron transport pathway. It takes electrons from another reduced mobile carrier, plastocyanin (PCred), and transfers them to ferredoxin, which becomes a strong reducing agent. Ferredoxin is a protein with an Fe-S cluster (Fe-S-Fe-S in a 4-membered ring, with two additional cysteine residues coordinating each Fe). It ultimately passes its electrons along to NADP⁺ to form NADPH. Note the complexes that produce a transmembrane proton gradient. In contrast to mitochondria, the lumen (compared to the mitochondrial matrix) becomes more acidic than the stroma. Protons then can move down a concentration gradient through the C₀C₁ATPase to produce the ATP required for the reductive biosynthesis of glucose.

Figure \(\PageIndex{2}\) shows a more detailed view of the molecular players in the light reaction.

Figure \(\PageIndex{2}\): Detailed View of Light Reaction of Photosynthesis (reprinted with permission from Kanehisa Laboratories and the KEGG project. The boxed numbers represent the Enzyme Commission Number. Original KEGG Map with embedded links.

Photosystem II

PSII has a complex structure comprising multiple polypeptide chains, chlorophylls, and Mn, Ca, and Fe ions. A Mn cluster, called the oxygen-evolving complex (OEC, also called OEX), is directly involved in the oxidation of water. Two key homologous 32 kDa protein subunits, D1 and D2, in PSII are transmembrane proteins at the heart of the PSII complex. It has been said of PSII that "Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest". (Sessions, A. et al., Current Biology 19 (2009)

The net reaction of PSII is the oxidation of water and the reduction of plastoquinone.

2PQ + 2H₂O → 2PQH₂ + O₂ (g)

The oxidation number of oxygen in water is -2, and in O₂ it is 0, indicating that water loses electrons (oxidizes) in the reaction. Note that water is not converted to 2H₂ + O₂, as it is in the electrolysis of water. Instead, the Hs are removed from the water as protons in the lumen of the chloroplast since the part of PSII that oxidizes water is near the lumenal end of the transmembrane complex. Protons from the stroma are required to protonate the reduced (anionic) form of plastoquinone to form PQH₂, an activity of PSII. That being said, researchers are actively developing a photosynthetic scheme or mimic that produces H₂ for use as a clean, essentially boundless fuel source to replace climate-warming fossil fuels.

A quick look at standard reduction potentials (SRP) shows that the passing of electrons from water (dioxygen SRP = +0.816 V) to plastoquinone (approx SRP of 0.11 ) is not thermodynamically favored. The process is thermodynamically driven by the energy of absorbed photons.

The crystal structure of PSII from a photosynthetic cyanobacterium consists of 17 polypeptide subunits with metal and pigment cofactors and over 45,000 atoms. The P680 chlorophyll reaction center is of particular interest, consisting of four monomeric chlorophylls adjacent to a key Tyr 161 side chain. When H₂O oxidizes to form dioxygen, 4 electrons must be removed by photoactivated P680. In PSII, this process occurs in four single-electron steps, with the electrons first being transferred to the oxygen-evolving complex. The electrons that passed through the Mn complex are delivered to P680 by a photoactive Tyr 161 (Tyr Z or YZ) free radical.

Five discrete intermediates of the OEC, S₀-S₄, are suggested from the experimental data and are consistent with the Kok cycle, which we will discuss below. These were postulated from experiments in which spinach chloroplasts were illuminated with short light pulses. A pattern of dioxygen release was noted, and it was repeated after four flashes. Ultimately, light absorption by P680 forms the excited state P680*, which donates an electron to pheophytin, which passes it to quinones. Hence, P680 gets photooxidized as it forms the cationic P680⁺. This removes an electron from Tyr 161 (YZ), producing the tyrosine radical cation, Tyr 161^.+. Given its positive charge and its reactive nature as a free radical, as well as its proximity to Mn ions in the OEC, it pulls an electron from a Mn ion in the OEC. This process repeats 4 times to oxidize 2 H₂O molecules, injecting 4 electrons back into the OEC and returning it to the basal state.

The mechanism is very complicated and still not fully understood. It is perhaps easiest to imagine it involving a series of sequential electron and proton transfers, along with their accompanying changes in charge and redox states. Most biochemistry students have a limited understanding of transition-state complexes and chemical kinetics, and even experts struggle with the underlying mechanisms.

In summary, for PSII in plants:

1. a pair of chlorophylls (P680) in the D subunits absorb light (maximum absorbance around 680 nm) and reach an excited state
2. an electron transfer from P680 to a nearby chlorophyll with a lower energy level for the excited state electron occurs, which produces an anionic chlorophyll. This chlorophyll has 2 H⁺ ions instead of Mg²⁺ (again, note the charge balance). After the electron transfer, P680 now becomes the cation P680⁺.
3. This "anionic" chlorophyll transfers an electron to oxidized plastoquinone.
4. The P680⁺, a strong oxidizing agent, removes one electron from an adjacent Tyr 161 to reform P680 and the radical cation Tyr 161^.+. Its proximity to the OEC leads it to remove an electron from the OEC, making it a more potent oxidizing agent.
5. This process repeats four times to fully oxidize two water molecules, producing one O₂, with the four electrons removed from water being added back to the metal centers of the OEC.

This suggests that there are five states of the OEC: an initial state, which we will call S₀, and four other states (S₁, S₂, S_3, and S₄). S₁ forms after removing one electron from the OEC by the adjacent radical cation Tyr 161^.+ (formed after absorption of one photon). S₂, S_3, and S₄ are sequentially formed after removing one electron by a newly regenerated Tyr 161^.+ after another round of photoexcitation. S₄ then returns to its original state, S₀. This series of reactions is called the Kok cycle, which is shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): The Kok cycle

No structural information is provided in the figure above. What is shown instead are possible and consistent oxidation numbers of the four Mnⁿ⁺ ions in the OEC that are consistent with charge balance and the changes in the oxidation number (-2) of the oxygen atom in water as it progresses to O₂ with an oxidation number of 0. The Mn ion states in the Kok diagram denote different discrete oxidation states, where n is the number of oxidative “equivalents” stored in the OEC during cycle progression. Think of the OEC as the key catalyst interacting with substrate H₂O molecules. We start with the S₀ state and must return to it at the end of the full cycle.

Remember that when O₂ acts as an oxidizing agent in combustion reactions, it forms two H₂Os. That requires adding four electrons. If done sequentially, the oxygen intermediates include superoxide, peroxide, and oxide, the latter of which, when protonated, is water. Hence, two water cycles and four cycles are required to remove the four electrons needed to produce dioxygen. Intermediate but transient oxygen states are also likely to be important in this mechanism.

A similar mechanism is found in PSI, except that plastocyanin, rather than dioxygen, is oxidized, with electrons being transferred to ferredoxin. This process is also challenging, as the reduction potential of oxidized plastocyanin (the form that can act as a reducing agent) is +0.37, whereas that of ferredoxin is -0.75. This electron transfer is an uphill thermodynamic battle: the more positive the standard reduction potential, the better the oxidizing agent, and the more likely it is to be reduced. What drives this uphill flow of electrons? Of course, it is the energy input from photons. We won't go into more detail about PSI, as it is very similar to PSII but lacks the OEC.

The Oxygen Evolving Complex - OEC

Although this is not a bioinorganic textbook, we must move beyond the "simple" Kok cycle diagram and examine the actual structures of the minicatalyst (the OEC) and the protein and water (substrate) environments surrounding it to understand the mechanism. The mechanism of the OEC is still not fully understood. It's experimentally challenging to unravel, given its complexity, as the intermediates are highly labile and X-ray-induced transient alterations in the OEC structure further complicate matters. Paradoxically, it is quite simple overall. Here is the essential reaction:

2H₂O + 4 photons → 4 H⁺(lumen) + 4 e^- + O₂.

The crystal structure of PS2 from T. vulcanus has significantly improved our understanding of the OEC and electron flow on water oxidation. We will concentrate on developing an understanding of the amazing Photosystem II from Thermosynechococcus vulcanus, a cyanobacterium (19 subunits with 35 chlorophylls, two pheophytins, 11 beta carotenes, two plastoquinones, two heme irons, one non-heme iron, four Mn ions, 3-4 Ca ions, three Cl ions, one carbonate ion, and around 2800 water molecules).

Nature appears to have evolved a single gene for the central protein in PSII that binds the OEC. The Mn₄CaO₅ cluster appears identical in all photosynthetic organisms, as shown below. Researchers were surprised to find that the Ca ion was an integral part of the basic geometric “framework” of the OEC, rather than a Mn, which was found to be “dangling” from the framework. A detailed structure of the OEC from T. vulcanus is shown in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Structure of the OEC

Note that the basic structure is a distorted cube, with metal ions at every other corner separated by oxides. Again, it was a surprise that not all of the 4 Mn²⁺ ions were in the cubic structure. Note one "dangling" Mn²⁺ with the other last metal site in the distorted cube occupied by Ca²⁺. Four oxygens (presumably from water) interact with MN4 and CA1.

It isn't easy to visualize this structure correctly from a 2D figure. Figure \(\PageIndex{5}\) shows an interactive iCn3D model of the OEC with bound water of photosystem II from Thermostichus vulcanus (3WU2), which should help in visualizing this structure.

   

Figure \(\PageIndex{5}\): OEX with bound water of photosystem II from Thermostichus vulcanus (3WU2). (Copyright; author via source). Click the image for a popup (followed by Style, Background, White) or use this external link:  https://www.ncbi.nlm.nih.gov/Structu...abEp67sSM1y1J9 

The shape outlined by O5-CA-O1-MN1 and MN3-O2-MN2-O3 is similar to that of a cubane (shown above right).  Four oxygen atoms from bound H₂O,  two each for MN4 and CA1, are attached to the OEX.  This model does not show the bond between O5 and MN1, so draw it in your mind to see the full distorted cubane.

Now, let's zoom out and view some of the amino acid side chains that interact with the OEC from T. vulcanus. These are shown in Figure \(\PageIndex{6}\).

Figure \(\PageIndex{6}\): OEC and surrounding amino acids from T. vulcanus

​The coordination number for all the Mn ions (including those interacting with water) is identical. Note the proximity of Tyr 161, which is involved in electron removal from the OEC after it becomes the radical cation Tyr 161^.+ in the primary photooxidation event.

Figure \(\PageIndex{7}\) shows an interactive iCn3D model of the OEX with surrounding amino acids and water in photosystem II from Thermostichus vulcanus (3WU2), presented to once again help you better understand the 2D structure shown above.

Figure \(\PageIndex{7}\): OEX with surrounding amino acids and water in photosystem II from Thermostichus vulcanus (3WU2). (Copyright; author via source). Click the image for a popup or use this external link:  https://www.ncbi.nlm.nih.gov/Structu...YGtFkmyGL1NjC9.  Zoom in to see the labels on the amino acids.  (Again, this model does not show the bond between O5 and MN1.)

The OEC can be considered a distorted cubane with a MN3-O4-MN4-O5 back.  Bonds to O5 are longer than the other bonds, which suggests they are weaker than the other metal-oxygen bonds. This could indicate that it is not an oxo (O^2-) ligand, but rather another variant, such as OH^- (lower charge), which may imply involvement in the splitting of dioxygen in the reaction mechanism. From a mechanistic perspective, an O-O bond must form between two waters. Both sets of bound "waters" (purple spheres in Figure 4 and red spheres in Figure 5, shown without Hs attached if they are present) are close to O5.

It is essential to recall that electrons removed from the metal ions in the OEC by Tyr 161⁺ must be replenished to the OEC to enable the catalytic cycle to continue. These electrons come from the waters that get oxidized. Such reversible loss and gain of electrons most readily occur from the transition metal ion Mn, which you all remember from introductory chemistry, which has multiple oxidation states. To bring back introductory chemistry again, we present the standard reduction potentials of different Mn ions in Table \(\PageIndex{1}\) below.

Reduction reaction	Standard Reduction Potential
Mn^2^+ (aq) + 2 e^-→ Mn (s)	-1.185
MnO4^-(aq) + 2 H2O (l) + 3 e^- → MnO2 (s) + 4 OH^-	+0.595
MnO2(s) + 4H^+ + e^- → Mn^3^+ + 2H2O	+0.95
MnO2(s) + 4H^+ + 2e- → Mn^2^+ + 2H2O	+1.23
MnO4- (aq) + 8 H^+ (aq) + 5 e^- → Mn^2^+ (aq) + 4 H2O (l)	1.507
MnO4^- (aq) + 4 H^+ (aq) + 3 e^- → MnO2 (s) + 2 H2O (l)	1.679
HMnO4^- + 3H^+ + 2^e- → MnO2(s) + 2H2O	+2.09
O2(g) + 4H^+ + 4e^- → 2H2O +1.229	+1.229

Table \(\PageIndex{1}\): Standard reduction potentials for Mn ions compared to O₂.

You should be able to determine the oxidation number of the Mn ion in each compound. Based on standard reduction potentials, which oxidation states might be sufficient for the oxidation of H₂O in the OEC?

How does this translate into structural/chemical changes in the OEC? Figure \(\PageIndex{8}\) provides a recent mechanism consistent with each of the Kok states (S₀-S₄).

Figure \(\PageIndex{8}\): Mechanism for water oxidation and O₂ formation through the OEC consistent with the Kok cycle (adapter from Britt & Marchiori, 10.1126/science.aaz4522)

The proposed change in redox state for each Mn ion is illustrated by purple Mn(III) and yellow Mn(IV) ions. Note the change in the oxidation state of the 4 Mn ions from (III, IV, III, and III) in S₀ to (IV, IV, IV, and IV) for all of them in S₃ and S₄. A flip in the side chain of E180 in S₂ allows the binding of Mn1 through an oxy link to Ca. ​​ There are many possible forms of oxygen in the structure, including waters, oxides (bridging oxos and possibly terminal oxides), and hydroxides, and the exact form at some sites remains uncertain. Note also the elegance of having an Mn₄ cluster to catalyze the four-electron oxidation of 2 water molecules through losing 4 electrons. Also, the redox states of the Mn ions differ from those in the Kok diagram in Figure 3.

Additionally, the final O₂-producing step, which transitions from S₄ → S₀, remains uncertain. The above mechanism is based on X-ray structures of intermediates and quantum calculations. In it, S₄ has an Mn(IV)O^. that bonds with the bridging O5 to form O₂.

Waters

As water is a reactant in PSII, water channels leading to the OEC must provide a pathway for water to enter and for protons to be removed and directed to the lumen, thereby developing a proton gradient. Another rendering of the Kok cycle, the position of the OEC in PSII on the luminal side of the membrane, and the presence of water channels (Cl1, O4, O1) and the Yz network, which connects Tyr 161 (Yz) to the lumen, are shown in Figure \(\PageIndex{9}\).

Figure \(\PageIndex{9}\): An overview of Photosystem II and the main water channels and networks from the OEC to the lumenal side. Hussein, R., Ibrahim, M., Bhowmick, A., et al. Structural dynamics in the water and proton channels of photosystem II during the S₂ to S₃ transition. Nat Commun 12, 6531 (2021). https://doi.org/10.1038/s41467-021-26781-z. Creative Commons Attribution 4.0 International License. http://creativecommons.org/ licenses/by/4.0/.

The left part of the figure illustrates the structure of PS II, showing the membrane-embedded helices and the extrinsic subunits in beige. The OEC, water channels, and the Yz network are shown in color. The Kok cycle of the water-oxidation reaction, triggered by photon absorption, is shown on the right and highlighted in blue. F represents a photon.

A more detailed representation of water channels and the Yz networks is shown in Figure \(\PageIndex{10}\).

Figure \(\PageIndex{10}\): A detailed view of the water channels showing the waters within each channel (O1 Channel red dotted, O4 Channel blue dotted, and Cl1 Channel green dotted). The region highlighted with solid yellow represents the Yz network. Residues involved in forming bottlenecks in the channels are shown in black. Water molecules found in the crystal structures are labeled with blue numbers in the channel. Hussein, R., Ibrahim, M., Bhowmick, A., et al., ibid.

These structures show that there is no direct water pathway from across the OEC and that all channels restrict water movement to some degree. The O4 and Cl1 channels are narrower than the O1 channel, so water in those is less mobile. In the O4 channels, waters 50-53 are near charged groups and are close to a major bottleneck (residues D1-N338, D2-N350, and CP43-P334, -L334).

Based on the X-ray structures and molecular dynamics simulations, the O1 channels appear to allow water access to the OEC. The more rigid Cl1 channel A may be involved in H⁺ transfer during S₂ → S₃.

The Last Step: Electron Transfer to Plastoquinone

We are almost ready for the next section, in which we will present the flow of electrons away from PSII through mobile electron carriers, leading to the synthesis of NADPH for the reductive biosynthesis of carbohydrates. Before we leave PSII, let's look at what happens to the radical anion P680^-, also known as PheA^- (pheophytin A) or PheA_D1 (the reaction center chlorophyll without a central Mg²⁺ ion), which received an electron from photooxidation of the reaction center P680, as summarized again in Figure \(\PageIndex{11}\).

Figure \(\PageIndex{11}\): The reaction center electron transfer reactions in PSII.

Pheophytin A passes its electron to plastoquinone A (in PSII), which passes it on to the lipophilic mobile electron carrier in the thylakoid membrane, plastoquinone. This is similar to the mobile electron carrier in mitochondrial electron transport, ubiquinone. Ultimately, these are passed to NADP⁺ to form NADPH, which is used for reductive biosynthesis.

Figure \(\PageIndex{12}\) shows an interactive iCn3D model highlighting just the OEX, pheophytin A (PHO), and plastoquinone A (PL9) in photosystem II from Thermostichus vulcanus (3WU2). (long load time).

Figure \(\PageIndex{12}\): the OEX, pheophytin A (PHO), and plastoquinone A (PL9) in photosystem II from Thermostichus vulcanus (3WU2). (long load time). (Copyright; author via source). Click the image for a popup or use this external link: https://www.ncbi.nlm.nih.gov/Structu...eamDFQNb2ZLKh7

The OEX (OEC) is shown in spacefill, CPK colors, PHO in spacefill magenta, and PL9 in spacefill cyan. Note the proximity of PHO and PL9 for easy electron transfer to plastoquinone A.

Given the number and proximity of high-energy reactive species in the reaction center, it should not be surprising that side reactions can occur. These would decrease the efficiency of light energy transduction and also could damage molecular components of PSII (and similarly PSI)

Figure \(\PageIndex{13}\)s shows a standard reduction potential diagram for the P680 - Pheophytin A - PlastoQuinone A triad (abbreviated P Phe Q) in PSII.

Figure \(\PageIndex{13}\): Charge recombination pathways in PSII (The energy gaps are not drawn to scale). Cardona T, Shao S, Nixon PJ. Enhancing photosynthesis in plants: the light reactions. Essays Biochem. 2018;62(1):85-94. Published 2018 Apr 13. doi:10.1042/EBC20170015Creative Commons Attribution License 4.0 (CC BY).

Safe routes for charge recombination between P⁺ and Q_A⁻ are indicated in blue, the damaging route producing ¹O₂ in red, and the radiative pathway in green. Stabilizing the P⁺PheoQ_A⁻ state helps prevent reverse electron flow to form P⁺Pheo⁻Q_A and subsequent charge recombination to form PPheoQ_A. For clarity, the details of the additional electron-transfer steps, including water oxidation and PSII-mediated reduction of plastoquinone to plastoquinol, collectively referred to as photosynthesis, are omitted. Abbreviations: P, primary electron donor of PSII; Pheo, pheophytin electron acceptor; Q_A, primary plastoquinone electron acceptor; ¹O₂, singlet oxygen; ³O₂, triplet oxygen; ³P, triplet excited state of P.

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

This chapter describes the molecular mechanism by which Photosystem II (PSII) uses light energy to oxidize water to molecular oxygen and reduce plastoquinone, presenting the Kok cycle and the oxygen-evolving complex (OEC) in structural and mechanistic detail, as well as the electron transfer pathway from PSII to the mobile electron carriers that connect to Photosystem I.

The overall framework for understanding the light reactions is the Z scheme, named for the characteristic zigzag shape of the electron energy diagram when standard reduction potentials are plotted vertically, and the sequence of electron carriers is plotted horizontally. Water is oxidized at PSII at E°′ = +0.816 V, and NADP⁺ is reduced at the end of PSI at E°′ = −0.32 V. Because the net electron transfer is thermodynamically uphill (from a good oxidant, water, to a good reductant, NADPH), it requires the input of energy from two sequential photon absorption events — one at P680 in PSII and one at P700 in PSI — each driving a quantum mechanical "uphill" electron transfer that establishes a sufficiently strong reductant for the next downhill segment of the chain. The complexes that facilitate electron transfer (PSII, cytochrome b₆f, PSI) also pump protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis by the chloroplast CF₁F_o ATP synthase in direct analogy to mitochondrial oxidative phosphorylation — but with the direction of the gradient reversed, with the lumen becoming acidic relative to the stroma.

The net reaction of PSII (2PQ + 2H₂O → 2PQH₂ + O₂) is thermodynamically unfavorable under standard conditions based on the standard reduction potentials of the reactants and products alone (water E°′ = +0.816 V, plastoquinone E°′ ≈ +0.11 V). The thermodynamic driving force is provided entirely by the energy of absorbed photons. Photoexcitation of P680 generates the excited state P680*, which transfers an electron to pheophytin A (PheoA) — the reaction center chlorophyll lacking its central Mg²⁺ — producing the strongly oxidizing radical cation P680·⁺ (E°′ ≈ +1.2 V, the strongest biological oxidant known) and the reducing radical anion PheoA·⁻. P680·⁺ rapidly oxidizes the adjacent Tyr161 (Yz), generating a Yz·⁺ radical cation that is poised structurally and electrostatically to extract electrons from the Mn₄CaO₅ oxygen-evolving complex of the OEC.

The OEC catalyzes the four-electron oxidation of two water molecules via the Kok cycle, a series of five discrete states (S₀–S₄). Each state transition requires one photon (absorbed by P680), one electron removed from the OEC by Yz·⁺, and one proton released to the lumen. After four photons and four single-electron oxidation events, the S₄ state accumulates sufficient oxidizing power to form the O–O bond and release O₂, returning to S₀. The four-photon periodicity of oxygen release was established experimentally by flash-illumination studies of spinach chloroplasts, which showed a maximum in O₂ release on the third flash (and every fourth thereafter). The structure of the OEC from Thermosynechococcus vulcanus reveals a Mn₄CaO₅ inorganic cluster with a distorted-cubane geometry: three Mn ions and one Ca²⁺ occupy alternating corners of the cube, bridged by oxo groups, while the fourth Mn ion ("dangling Mn") projects outside the cubane framework. The Ca²⁺ occupies one corner of the cube rather than a Mn ion — a structural surprise that implies a specific functional role for Ca²⁺ in substrate water binding and O–O bond formation. The bridging O5 oxygen has longer and weaker bonds to its metal centers, suggesting it may be hydroxide rather than oxide, and its position — close to two water molecules bound to MN4 and CA1 — makes it the leading candidate for the site of O–O bond formation in the transition from S₃/S₄ to S₀.

The four Mn ions of the OEC cycle through multiple oxidation states during the Kok cycle. Proposed oxidation state sequences move from (III,IV,III,III) in S₀ to (IV,IV,IV,IV) in S₃ and S₄, with each electron removal by Yz·⁺ oxidizing one Mn(III) to Mn(IV). The multiple accessible oxidation states of manganese (Mn²⁺ through Mn⁴⁺) make it uniquely suited for multi-electron water oxidation chemistry, with the Mn³⁺/Mn⁴⁺ couple having E°′ ≈ +0.95–1.23 V — sufficient to drive the thermodynamically demanding oxidation of water. The surrounding protein environment — including Asp, Glu, and His residues coordinating the Mn ions, Tyr161 as the proximal electron relay, and conserved water molecules — is absolutely essential for the function of this inorganic catalyst.

Three water channels (O1, O4, Cl1) and the Yz hydrogen-bonding network connect the OEC to the thylakoid lumen. The O1 channel, the widest and most flexible, appears to be the primary route for water delivery to the OEC. The narrower, more rigid Cl1 channel A may serve preferentially as a proton-transfer conduit during the S₂ → S₃ transition. Bottleneck residues in each channel restrict water mobility and likely have gating functions. The directional removal of protons through these channels into the lumen is essential for generating the pH gradient that drives ATP synthesis.

The radical anion PheoA·⁻ passes its electron forward to plastoquinone A (a fixed quinone in the PSII reaction center), which transfers it to the mobile lipophilic plastoquinone pool in the thylakoid membrane — directly analogous to ubiquinone in mitochondrial electron transport. This forward electron transfer is facilitated by the close proximity of pheophytin A and plastoquinone A in the PSII reaction center. However, competing charge recombination reactions can occur when forward electron transfer fails: if the P·⁺PheQA·⁻ state reverts by reverse electron transfer to form P·⁺Pheo·⁻QA, subsequent charge recombination can generate the triplet excited state of P680 (³P), which sensitizes ground-state triplet O₂ to its reactive singlet form — a major source of photodamage in PSII. The safe recombination pathway involves direct electron transfer from QA·⁻ to P·⁺, bypassing pheophytin, and is a key target for photoprotective adaptations. Stabilizing the productive P·⁺PheQA·⁻ state through protein-mediated electrostatic interactions is therefore critical for both photosynthetic efficiency and the prevention of oxidative damage within the reaction center itself.