20.2: The Kok Cycle and Oxygen Evolving Complex of Photosystem II (2024)

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 count both steps together, the non-reaction center chlorophyll gets "photooxidized", in the process producing the "strong" oxidizing agent which is the positively charged chlorophyll derivative. The extra electron passed onto the second molecule will eventually be passed on 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.

The combined processes of PSII and PSI resemble a "Z" scheme (rotate in your mind the standard reduction potential figure 90 degrees clockwise). In an organization reminiscent of electron transport in mitochondria, water is oxidized by photosystem II (PSII). Electrons from water are moved through PSII to a mobile, hydrophobic molecule, plastoquinone (PQ) to form its reduced form, PQH₂. Another photosystem, photosystem I (PS1), is next in the electron transport pathway. It takes electrons from another reduced mobile carrier of electrons, plastocyanin (PC_red) 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 2 additional cysteineresidues 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 (as compared to the mitochondrial matrix) becomes more acidic than the stroma. Protons then can move down a concentration gradient through the C₀C₁ATPase to produce ATP required for the reductive biosynthesis of glucose.

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

Photosystem II

PSII has a complicated structure with many polypeptide chains, lots of chlorophylls, and Mn, Ca, and Fe ions. A Mn cluster, called the oxygen-evolving complex, OEC (also called the OEX) is directly involved in the oxidation of water. Two key hom*ologous 32 KD protein subunits, D1 and D2, in PSII are transmembrane proteins and are 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 carried out by PSII is the oxidation of water and reduction of plastoquinone.

2PQ + 2H₂O → 2PQH₂ + O₂ (g)

The oxidation number of oxygen in water is -2 and 0 in O₂, so this is a loss of electrons or oxidation of the water. Note that water is not converted to 2H₂ + O₂, as in the electrolysis of water. Rather the Hs are removed from the water as protons in the lumen of the chloroplast, since the part of PSII that oxides water is near the lumenal end of the transmembrane complex. Protons are required to protonate the reduced (anionic) form of plastoquinone to form PQH₂, an activity of PSII found closer to the stroma, derive from the stroma. That being said, researchers actively trying to develop a photosynthetic scheme or mimic that does produce H₂ for use as a clean and 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 driven thermodynamically by the energy of the 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. Of particular interest is the P680 chlorophyll reaction center, which consists of four monomeric chlorophylls adjacent to a key Tyr 161 side chain. When H₂O gets oxidized to form dioxygen, 4 electrons must be removed by photoactivated P680. In PSII, this process occurs in 4, single electron steps, with the electrons first being transferred to the oxygen-evolving complex. The electrons 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 that repeated after 4 flashes. Ultimately, light absorption by P680 forms excited state P680*, which donates an electron to pheophytin, which passes them to quinones. Hence P680 gets photooxidized as it forms the cationic P680⁺. This then removes an electron from Tyr 161 (YZ) producing the tyrosine radical cation, Tyr 161^.+. Given its positive charge, its reactive nature as a free radical, and its proximity to Mn ions in the OEC, it pulls an electron from a Mn ion in the OEC. This processrepeats itself 4 times for the oxidation of two H₂Os, which injects 4 electrons back into the OEC to return to the basal state.

The mechanism is very complicated and still not fully understood. It is perhaps easiest to think about the mechanism involving a series of sequential electron and proton transfer and their accompanying change in charge and redox states. Most biochemistry students have a limited understanding of transition state complexes and chemistry but even the experts struggle with the mechanism.

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. 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 in the chlorophyll instead of Mg²⁺ (again note the charge balance). After 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 complex leads to it removing an electron from the OEC, making it a more potent oxidizing agent
5. This process repeats a total of 4 times to fully oxidize two water molecules to produce 1 O₂ with the 4 electrons removed from 2 glasses of water added back to the metal centers of the OEC.

This suggests that there are 5 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 the removal of 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 the removal of one electron by a newly regenerated Tyr 161^.+ after another round of photoexcitation. S₄ then returns to its original state, S₀. This series of reactionsiscalled the Kok cycle, which is shown in Figure \(\PageIndex{3}\).

There is no structural information given in the above figure. 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, which will interact with substrate H₂O molecules. We start with the S₀ state and must return to it in the full cycle.

Remember that when O₂ acts as an oxidizing agent in combustion reactions, it forms 2H₂O. That requires the addition of four electrons. If done sequentially, the oxygen intermediates include superoxide, peroxide, and oxide, the latterof which when protonated is water. Hence two waters and four cycles are required to remove the four electrons required to produce dioxygen. Intermediate but transient oxygen states are also presumably important in this mechanism.

A similar mechanism is found in PSI, except plastocyanin, not dioxygen is oxidized, with electrons moved to ferredoxin. This is likewise a difficult process since the reduction potential for oxidized plastocyanin (the form that can act as a reducing agent) is +0.37 while for ferredoxin it is -0.75. This transfer of electrons is an uphill thermodynamic battle since the more positive the standard reduction potential, the better the oxidizing agent and the more likely the agent becomes reduced. What drives this uphill flow of electrons. Of course, it is the energy input from photon. We won't go into any more detail about PSI since it is very similar to PSII but of course, does not have the OEC.

The Oxygen Evolving Complex - OEC

Even though this is not a bioinorganic textbook, we must move past the "simple" Kok cycle diagram and look at the actual structure of the minicatalyst, the OEC, and the protein and water (substrate) environment around it to understand the mechanism. The mechanism of the OEC is still not fully understood. It's experimentally difficult to unravel given its complexity as the intermediates are very labile and the x-ray-induced transient alterations in the structure of OEC complicate matters more. Paradoxically it is quite simple in overall terms. 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, 2 plastoquinones, 2 heme irons, 1 non-heme iron, 4 Mn ions, 3-4 Ca ions, 3 Cl ions, 1 carbonate ion, and around 2800 water molecules).

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

Note the basic structure is a distorted cube the 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 waters) are shown interacting with MN4 and CA1.

It is very difficult 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. (very long load time!)

Figure \(\PageIndex{5}\): OEX with bound water of photosystem II from Thermostichus vulcanus (3WU2). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...F8vwgkDXV6C1S6. (very long load time)

The shape outlined by O5-CA-O1-MN1 and MN3-O2-MN2-O3 is similar to cubane (shown above right)

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). (long load time) 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://structure.ncbi.nlm.nih.gov/i...bP6ttbaWUmQAg7

The OEC can be thought of as a distorted cubane with a MN3-O4-MN4-O5 back. Bonds to O5 are longer than the other bonds which suggestthey are weaker than the other metal-oxygen bonds. This could suggest that it may not be an oxo (O^2-) ligand but 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 important to remember that electrons removed from the metal ions in the OEC by Tyr 161^.+ must be restored to the OEC to allow the catalytic cycle to continue. These electrons come from the waters that get oxidized. Such a reversible loss and gain of electrons most readily occur from the transition state Mn ions, which you all remember from introductory chemistry have multiple oxidation states. To bring back introductory chemistry again, we present the standard reduction potentials of different Mn ions in Table \(\PageIndex{1}\) below.

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).

The proposed change in redox state for each Mn ion is illustrated with Mn (III) ions in purple and Mn (IV) ions in yellow. 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 different forms of oxygens in the structure including waters, oxides (bridging oxos and possibly terminal oxides), and hydroxides, and the exact form at some sites are still a bit uncertain. Note also the elegance of having a Mn₄ cluster to catalyze the 4 electron oxidation of 2 water through the loss of 4 electrons. Also, the redox state change in the Mn ions is different than the one shown in the Kok diagram in Figure 3. In addition, the final O2-producingstep going from S₄ → S₀ is still 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, there must be water channels leading to the OEC that provide a way for water to enter and for protons to be removed and directed to the lumen to develop 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 connect Tyr 161 (Yz) to the lumen, are shown in Figure \(\PageIndex{9}\).

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

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 dynamic simulations, it appears that the O1 channels allow access of water to the OEC. The Cl1 channel A, which is more rigid, 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 \(\PageIndex{11}\).

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. It is similar to the mobile electron carrier in mitochondrial electron transport, ubiquinone. Ultimately these are passed to NADP⁺ to form NADPH 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://structure.ncbi.nlm.nih.gov/i...H7Mycbx1PsFbt6

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

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.

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. Stabilization of 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 the oxidation of water and the reduction of plastoquinone to plastoquinol by PSII, collectively termed 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.