How does photosystem 2 split water

This problem of radiation damage occurring at the Mn cluster had also been highlighted by Grabolle et al. Because of this, doubts were expressed about the cubane model of Ferreira et al.

However, a reinterpretation of the polarized EXAFS data by Sproviero and colleagues at Yale suggested that the cubane model could still be considered as the legitimate organization of the OEC metal cluster. The uncertainty of the cubane model was dismissed with the most recent 1. The resulting model overall was similar to that proposed by Ferreira et al. No other water molecules were directly associated with the metal cluster, suggesting that some of the four Ws may serve as the substrates for water oxidation, in line with the prediction of Ferreira et al.

Of these, D1Glu served as a monodentate ligand to Mn 1. In addition, D1His is coordinated to Mn 1. They also noted that imidazole e-nitrogen of D1His is hydrogen bonded to a bridging oxo, in agreement with the suggestion of Ferreira et al. Thus, these two residues may function to stabilize the cubane structure of the metal cluster as well as provide partial positive charges to compensate for the negative charges induced by the oxo bridges and carboxylate ligands of the metal cluster.

Ferreira et al. Furthermore, as postulated by Ferreira et al. Although the geometry of the Mn 4 Ca cluster and its ligand field characteristics are now known at a resolution of 1. Because free electrons are generated in the crystal during the collection of X-ray-diffraction data, it is likely that the Mn cluster is reduced to levels lower than that of the S 0 state.

Despite this, the Ferreira et al. One well-championed mechanism Messinger et al. This mechanism is dependent on Mn4 being converted to a high oxidation state possible Mn[V] during progression to the S 4 state just before O—O bond formation.

The other three Mn ions also progress to high valency states Mn[IV] e. An alternative mechanism proposed by Siegbahn , , , based on in-depth DFT calculations, suggests that an oxyl radical forms within the Mn 3 CaO 4 cubane and O—O bond formation therefore involves bridging oxo species see Fig. Two different mechanisms for the final step of the S-state cycle when the dioxygen bond of O 2 is formed. A Mechanism 1. The very high oxidation state of the Mn-cluster, particularly the Mn ion outside the Mn 3 CaO 5 cubane, leads to a high electron deficient oxo after deprotonation of water molecules during the S-state cycle.

B Mechanism 2. The formation of an oxyl radical on one of the bridging oxygen atoms of the cubane leads to a radical attack of an adjacent oxo-ligand within the Mn 3 CaO 4 cubane. The fifth oxo linking the Mn outside the cubane to an Mn of the cubane is not shown.

Although some progress has been made in mimicking photosynthesis in artificial systems, researchers have not yet developed components that are both efficient and robust for incorporation into a working system for capturing and storing solar energy in chemical bonds on a large scale, as does natural photosynthesis.

To date, the main focus of research has been on designing and synthesizing molecular catalysts that can be linked to a light-driven charge-separation system Tran et al. Dyes have been used for the latter, but inorganic semiconductors offer a more realistic and robust approach for providing the oxidizing and reducing potentials necessary to split water and power reductive chemistry.

Insights gleaned from the recent structural determination of PSII have initiated considerable efforts to identify artificial catalytic systems for water oxidation and hydrogen production using solar energy Eisenberg and Gray The hydrogen produced could be used directly as a source of energy but could also be used, as it is in natural photosynthesis, to reduce carbon dioxide to other types of fuels such as methane and methanol.

The challenge is to have a molecular arrangement such that the artificial catalysts efficiently use light energy to split water and concomitantly provide reducing potential for hydrogen gas production or CO 2 reduction.

It has been demonstrated that catalysts based on Mn or Mn doped with Ca are capable of water splitting and generating dioxygen Limberg et al. Frei and coworkers Jiao and Frei reported that nanostructured manganese oxide clusters supported on mesoporous silica efficiently evolved oxygen in aqueous solution under mild conditions. But perhaps the most practical catalysts for water splitting are based on Co, a relatively abundant element.

Kanan and Nocera have described a self-assembling catalyst composed of Co and phosphate ions, which can efficiently produce molecular oxygen from water at neutral pH with a low overpotential akin to that which operates in the OEC of PSII.

Dau and colleagues Risch et al. More recently, Yin et al. Indeed, hematite has been theoretically predicted to achieve a water oxidation efficiency of Another fundamental limitation of the hematite system is the need for externally applied bias because the conduction band of hematite is lower than the potential required to reduce protons to hydrogen. Nevertheless, it is a system that is receiving considerable attention at the present time Tran et al.

The next step will be to couple these oxygen-producing systems to another catalyst that will use the protons and high-energy electrons derived from the water-splitting reaction to produce hydrogen gas or reduce carbon dioxide. In the case of the former, considerable progress is being made Wang et al. In addition, a number of inorganic catalysts have been identified with activities that are almost as efficient as platinum.

One such class of catalysts are based on sulfides of Mo and W Zong et al. Identifying catalysts for the reduction of carbon dioxide, however, is more difficult because multielectron reactions are required and the emergence of catalysts for generating useful carbon fuels will require considerable effort Fujita ; Arakawa et al. The most successful coupling of catalysts using a semiconductor for light capture and charge separation was reported by Nocera and colleagues Reece et al.

They used a triple-junction amorphous Si wafer as the semiconductor, the CoPi catalyst for water splitting, and the NiMoZn alloy for the cathodic hydrogen-producing catalyst as shown in Figure 6. This latest discovery is a major step toward the development of an efficient, robust, low-cost, and scalable photocatalytic device for water splitting to generate molecular hydrogen using solar energy.

Diagramatic representation of Nocera's Reece et al. The determination of the structure of PSII has provided strong hints of how nature conducts the remarkable chemistry of water splitting. After all, there is no shortage of water for this reaction, and the energy content of sunlight falling on our planet well exceeds our needs. Previous Section Next Section. View larger version: In this window In a new window.

Figure 1. Figure 2. Proteins The first X-ray-derived structure of PSII used a preparation isolated from the cyanobacterium Thermosynechococcus elongatus and was elucidated by Zouni et al. Figure 3. View this table: In this window In a new window. Table 1. Gene nomenclature. Cofactors Chlorophylls and Carotenoids The Ferreira et al. Lipids and Detergents The first attempts to assign lipid and detergent molecules in the crystal structure of PSII was made by Loll et al.

Mn 4 -Ca Cluster The 3. Figure 4. Figure 5. Figure 6. Previous Section. Catalysis research of relevance to carbon management: Progress, challenges, and opportunities. Chem Rev : — Barber J Barber J. Photosynthetic reaction centres: A common link. Trends Biochem Sci 12 : — Photosystem II: The engine of life. Q Rev Biophys 36 : 71 — Too much of a good thing: Light can be bad for photosynthesis.

Trends Biochem Sci 17 : 61 — P, the primary electron donor of PSII. J Photochem Photobiol A : 97 — CrossRef Google Scholar. Computational studies of the O 2 -evolving complex of photosystem II and biomimetic oxo manganese complexes. Coord Chem Rev : — CrossRef Medline Google Scholar. Water oxidation chemistry of photosystem II. Amino acid residues that modulate the properties of tyrosine Y-Z and the manganese cluster in the water oxidizing complex of photosystem II.

Biochim Biophys Acta : — Medline Google Scholar. Protein ligation of the photosynthetic oxygen-evolving center. Evolution of oxygenic photosynthesis: Genome-wide analysis of the OEC extrinsic proteins. Trends Plant Sci 9 : 18 — Amino acid residues involved in the coordination and assembly of the manganese cluster of photosystem II. Proton-coupled electron transport of the redox-active tyrosines and its relationship to water oxidation. Site-directed mutagenesis of photosynthetic reaction centers.

Curr Opin Struct Biol 1 : — A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II. Nat Chem 4 : — A multimer model for P, the primary electron donor of photosystem II. Proc Natl Acad Sci 92 : — Preface to making oxygen. Inorg Chem 47 : — Rapid formation of the stable tyrosyl radical in photosystem II. Proc Natl Acad Sci 98 : — Architecture of the photosynthetic oxygen-evolving center. Science : — Carbon dioxide reduction.

Licker MD , pp. Google Scholar. Inorg Chem 51 : — Catalytic oxidation of water by an oxo-bridged ruthenium dimer. J Amer Chem Soc : — Photosystem II reaction centers and bicarbonate.

In The Photosynthetic Reaction Center ed. The upper half of the reaction center has the job of replacing this electron with a low-energy electron from water. The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino acid, which then delivers it to the chlorophyll, making it ready to absorb another photon.

Antenna proteins small triangular proteins at top and bottom associated with photosystem II. The central chlorophyll molecule of the reaction center is shown with an arrow.

Of course, this whole process wouldn't be very efficient if plants had to wait for photons to hit that one special chlorophyll in the reaction center. Fortunately, the energy from a light-excited electron is easily transferred through the process of resonance energy transfer. Thanks to the mysteries of quantum mechanics, the energy can jump from molecule to molecule, as long they are close enough to each other. To take advantage of this property, photosystems have large antennas of light-absorbing molecules that harvest light and transfer their energy inwards to the reaction center.

Plants even build special light-harvesting proteins that sit next to the photosystems and assist with light collection. The picture shows a top view of photosystem II PDB entry 1s5l , showing all of the light-absorbing molecules inside. The central chlorophyll molecule of the reaction center is shown with the arrow notice the second reaction center in the bottom half--photosystem II is composed of two identical halves. The little triangular molecules at top and bottom, stuffed full of chlorophyll and carotenoids, are light-harvesting proteins PDB entry 1rwt.

The oxygen-evolving center of photosystem II is a complicated cluster of manganese ions magenta , calcium blue green and oxygen atoms red. It grips two water molecules and removes four electrons, forming oxygen gas and four hydrogen ions. The actual binding site of the two water molecules is not known for certain, but in the PDB structure 1s5l a bicarbonate ion is bound to the cluster, providing a clue for location of the active site.

The picture shows two oxygen atoms from this ion colored blue : one is bound to a manganese ion, the other is bound to the calcium ion. Notice that the oxygen-evolving center is surrounded by histidines, aspartates and glutamates, which hold it in place.

The tyrosine shown in the middle forms a perfect bridge between the water site and the light-capturing chlorophyll. This picture was created with RasMol. You can create similar pictures by clicking on the accession codes here, and picking one of the options for 3D viewing.

When you go to explore this fascinating molecule, be prepared for a challenge. It is very complex and you will need to spend some time to make sense of it. If you want to look at just the reaction center, try displaying non-protein residue numbers , 40, and 41, along with tyrosine of chain A.

References J. The original experiment was performed by Pierre Joliot as early as Joliot et al. B Water oxidation cycle Kok cycle Kok et al. The reaction times for the single electron oxidation steps are also indicated Klauss et al. The oxidizing equivalents for the water oxidation reaction are stored transiently by the tetranuclear manganese cluster, an insight that came from Kok et al.

His so-called Kok cycle S-state cycle of water oxidation, shown in Fig. The kinetics of the proton release, which follow a pattern or have also been determined, see Dau and Haumann ; Klauss et al. The alternating electron and proton release led to an extended catalytic cycle for the water oxidation reaction in PS II with nine states that differ in their net electron and proton count Klauss et al. In this sequence, Y Z promotes both electron and proton transfer in the catalytic cycle displaying a dual function Bovi et al.

This is illustrated by the scheme in Fig. The Mn cluster thus acts as an interface and storage device between the very fast light reaction ps time scale and the slow catalytic reaction ms time scale of the 4-electron water oxidation chemistry, bridging a kinetic gap of nine orders of magnitude.

Thus, in the OEC, water is not oxidized by subsequent single electron removal from substrate water. Instead, it is the Mn cluster that is oxidized by four successive oxidation events; the two attached substrate water molecules release the protons for charge neutrality , and O 2 is released only in the last step after O—O bond formation in a concerted reaction.

Thereby, high-energy steps are avoided and the redox process is leveled. Figure adapted from Messinger and Renger A detailed understanding of the catalytic process of water oxidation in PS II requires knowledge of the electronic structure, i.

The oxidation and spin states of the Mn ions, representing the total number and configuration of electrons in the Mn valence orbitals, give a basic description thereof. These together with the magnetic interactions between the spin-bearing Mn ions, depending to a large part on the metal ligands, provide a comprehensive picture of the respective electronic state, which governs the chemical and catalytic properties of each S state.

Thus, the spin states provide information about how the structure of the cofactor evolves during the S -state cycle, for a recent review see Krewald et al. An experimental method to investigate the electronic structure of transition metal complexes is electron paramagnetic resonance EPR spectroscopy Goldfarb and Stoll ; Schweiger and Jeschke ; Weil and Bolton It exploits a fundamental property of matter, which is that unpaired electrons have an intrinsic angular momentum spin , which can be excited by microwave radiation in a magnetic field.

The unpaired electron spin also interacts with other electron and nuclear spins as well as with local electric field gradients, making it a sensitive reporter of its chemical environment. It is thus analogous to other magnetic spectroscopies such as nuclear magnetic resonance NMR.

Since the Mn ions are open-shell species, i. It has also been shown that all S states in the Kok cycle can be trapped except for the elusive S 4 state and that all exhibit paramagnetism Haddy The spin state depends on the oxidation state of the Mn ions, their geometry and in particular on the bridging ligands which connect the metal ions.

These mediate antiferromagnetic or ferromagnetic exchange interactions between the Mn ions leading to either a low-spin state, minimizing the number of unpaired electrons, or a high-spin state, maximizing the number of unpaired electrons. These data together with results collected on the S 0 state, which also resolves a multiline signal Ahrling et al. The oxidation state assignment for the S 2 state, which comes from this analysis, is shown in Fig.

The reader interested in details on how the electronic configuration of a polynuclear Mn cluster, especially the Mn 4 CaO 5 , can be probed by EPR spectroscopy and double resonance techniques in relation with calculations of magnetic properties by quantum mechanical methods is referred to the following review Lohmiller et al.

Britt Britt et al. X-ray spectroscopies also agree with this assignment Chatterjee et al. These studies rule out the alternative lower oxidation state models for the manganese cluster Kolling et al. Figure modified from Krewald et al. Mn1, respectively and different total effective spin ground states.

The bottom trace shows simulations of the two EPR absorption signals. Pantazis et al. In these two structures one of the oxygen bridges, O5, is occupying different positions. In turn this also led to a change of the Mn III position Jahn—Teller ion and thus of the open coordination site of the cluster resulting in a change of the precise electronic properties of the cofactor.

This shows that the effective spin S eff is a crucial parameter for describing a particular state of the cluster and assigning it a spatial structure. It is postulated that O5 has not a fixed position but toggles between two structures in the dynamic S 2 state, which is important for the water binding and the catalytic mechanism see below.

Instead, it might serve as stage for the delivery of water molecules to the reaction site Nakamura et al. In addition, both of these molecules reduce turnover efficiency; these results implicate that at least one or possibly both of these channels are involved in substrate delivery. The EPR measurements have been extended to the S 3 state. These results are of particular importance since this is the last metastable state prior to O—O bond formation and O 2 release. Thus, no ligand oxidation takes place as previously proposed Kawashima et al.

The results also show that a sixth ligand is binding to the open coordination site of the Mn III present in the S 2 state when it is oxidized. Mn oxidation in the S 2 to S 3 transition has been proposed by Dekker Dekker et al. It has been included in mechanistic models Pecoraro et al. Very recently, SFX data have shown additional electron density acquired during the S 2 to S 3 transition, consistent with the binding of a light atom e.

Figure modified from Cox et al. The magnetic resonance experiments described above together with theoretical calculations allowed a reliable characterization of the S 0 , S 2 and S 3 states with respect to oxidation and spin states of individual ions and their spin coupling in the tetranuclear Mn cluster summarized by Krewald et al.

Together with information about binding of the two substrate water molecules described below spatial models of the S states could be obtained that form the basis for developing a catalytic mechanism for the OEC Fig.

Knowledge of the binding and dynamics of the substrate water molecules is crucial for formulating the reaction mechanism of photosynthetic water oxidation.

Apart from X-ray diffraction spectroscopic techniques are used to investigate water binding, in particular EPR Cox et al. These are used in conjunction with time-resolved mass spectrometry MIMS which detects the uptake of H 18 2 O labeled water into the product O 2 ; for a comprehensive review on the identification of possible water substrates by MIMS see Cox and Messinger MIMS experiments showed that the two substrate water molecules exchange at different rates in all of the S states Hillier and Wydrzynski , , The more slowly exchanging water W s has an exchange rate of the order of seconds Messinger et al.

The observation of two rates implies that the two substrates bind at two chemically distinct sites. They also demonstrate that the O—O bond is not formed until the S 4 state is reached. For the detection of substrate water molecules, EPR spectroscopy makes use of interactions with nuclear spins, e. Methodological and instrumental developments in our laboratory Cox et al. O5 represents the most probable candidate for the two-fold deprotonated substrate W s for the following reasons:.

For further details see Rapatskiy et al. For structurally similar model compounds an exchange has only been observed on much longer timescales Rapatskiy et al. Figure changed from Krewald et al. The boxes show the determined oxidation states of the Mn ions in the respective S state. S 3 may also exist in an open S A 3 and a closed S B 3 cubane form not shown. A switching of the preferred total spin ground state configuration of the cluster is thought to take place in S 2 from low to high spin and between S 4 and S 0 back from high to low spin; this is indicated by the diagonal line dividing the green and blue boxes.

This switching of the spin state may be necessary for the formation of triplet dioxygen 3 O 2 in the final step of the cycle see also Fig. Its close proximity to O5 suggests it is the second substrate W f Fig.

How it is exactly inserted remains an open question. Recent SFX data Kern et al. Within this sequence the water that binds during the S 2 to S 3 transition is not one of the substrates of the current cycle, but a substrate of the next cycle Cox and Messinger There is significant theoretical support for this sequence, which explains how redox tuning of the cofactor can precede water binding Retegan et al.

This hypothesis also better agrees with MIMS data which shows both substrates are bound to the cofactor most likely to Mn ions in all S -states and that the rate of exchange does not dramatically change upon moving from S 2 to S 3 Cox and Messinger ; Hendry and Wydrzynski , As a consequence of the above results, it is assumed in the following discussion of the catalytic cycle of the OEC that the O5 bridge derives from one of the substrate waters W s.

A possible cycle is shown in Fig. A further oxidation and deprotonation step leads to S 4. Experimental evidence shows that formation of S 4 is triggered by proton release Haumann et al. Current theoretical models suggest that a proton is transferred from W1 H 2 O to Asp61, as in the preceding S 2 to S 3 transition Siegbahn This is mainly based on the SFX crystal structure of Suga et al.

Furthermore, earlier biophysical measurements categorically rule out O—O bond formation in the S 3 state. The MIMS data described above shows that both substrates still rapidly exchange with bulk water in the S 3 state Messinger et al.

How the O—O bond is precisely formed in the S 4 state is still unknown, owing to a lack of experimental data. There are two popular chemical mechanisms for this transition that differ with regard to what component of the cofactor is oxidized to form S 4 : i a ligand one of the substrate waters ; or ii a metal one of the Mn ions.

In the case of a ligand oxidation event, forming a Mn IV -oxyl moiety, O—O bond formation is proposed to proceed by a radical coupling mechanism described below.

In the case of a metal oxidation event, forming a Mn V -oxo type moiety, O—O bond formation is instead proposed to proceed by nucleophilic attack of the electrophilic oxygen O5 bound to the Mn V by a nearby water, e. Unfortunately the nature of the last oxidation ligand vs. XAS data which monitors an oxidation state change of the Mn ions during the S 3 to S 0 transition appears to rule out a Mn V intermediate Haumann et al.

We note that while the nucleophilic attack mechanism initially envisaged the involvement of a Mn V intermediate, it was later suggested to have substantial Mn IV oxyl character in the S 4 state based on theoretical studies Vinyard and Brudvig Only a nucleophilic attack mechanism has been observed for first row molecular transition metal water oxidation catalysts, for which there is mechanistic data, Codola et al.

Nucleophilic attack mechanisms have thus been proposed for the OEC by a number of groups Ferreira et al. In this sequence, a nearby water e. Upon re-reduction of the four Mn ion and loss of the product O 2 , the newly inserted O6 bridge presumably fills the site vacated by O5 leading to rapid recovery of the S 0 state. In contrast the oxo-oxyl radical coupling mechanism involving two Mn bound oxygens, is most compatible with the assignment of the two substrates sites described in the previous section O5 and O6.

Such a mechanism was first proposed ten years ago by Per Siegbahn Siegbahn To illustrate the current situation, in Fig.

In panel A and B the open and closed cubane structures are shown with all Mn IV and O5 and O6 in close contact that could react to form dioxygen via an oxyl-oxo coupling mechanism. In this context it should be mentioned that in the recent SFX structure Kern et al. Figure adopted from Pantazis Three representative model structures for the final step in the Kok cycle with O5 and O6 bound see Figs. The putative reacting oxygens are indicated by dotted red circles; for details see text. There are also other alternative mechanisms.

Similarly, there are proposed mechanisms that do not fall into either of the two categories, such as a recent proposal from the Sun group. In this novel mechanism, the dangler Mn acts as the site of catalysis, as in a nucleophilic attack like mechanism, forming a Mn VII -dioxo intermediate following charge rearrangement of the Mn cluster in the S 4 state Zhang and Sun As above for the nucleophilic attack mechanism, it is unclear if this mechanism is compatible with XAS data Haumann et al.

For a more detailed comparison of alternative mechanisms the reader is directed to a recent in-depth review Pantazis Owing to the increasing experimental support for the oxo-oxyl coupling mechanism first proposed by Siegbahn, a brief description of its key steps is given below Fig.

The first stage of the mechanism involves oxidation of O6 to form an oxyl radical, in concert with deprotonation. In the Siegbahn mechanism, the proton associated with O6 in S 3 migrates to W1, which is already deprotonated during S 3 formation. S 3 and S 0 have been characterized experimentally; [ S 4 ] has not been observed.

The spin ground state S G is indicated. The numbering of the Mn ions 1—4 is given for the S 3 state; the oxidation state of each Mn ion is given in light purple III or dark purple IV ; antiferromagnetic interaction between adjacent Mn ions is represented by yellow shading whereas ferromagnetic interaction by green shading.

For details of the mechanism see text. Cleavage of the two remaining Mn—O bonds leads to formation of the final O 2 product. The displacement of O 2 , via binding of the first substrate water of the next S cycle coupled to its deprotonation , leads to formation of the observed metastable S 0 state Fig. New experimental data is needed to probe these key stages in O—O bond formation and release.

SFX crystallography Kern et al. Specifically, as described above, the Siegbahn mechanism starts with internal proton transfer from O6 to W1. Glu is proximal to Mn1 and could act as an internal base, accepting a proton from O 6 , similar to old ideas put forward by Gerry Babcock Hoganson and Babcock Within this revised scheme W1 does not need to undergo deprotonation—with proton transfer to Asp61 switched off due to a conformational change gating Kern et al.

In biology, there is only one catalytic site that is able to split water—the OEC—and its structure and function is identical Su et al. The work compiled above shows that this unique metal cluster requires the presence and concerted action of all four manganese ions in the Mn 4 O x Ca complex.

The possible presence of a Mn V in the last step S 4 has, however, not yet been finally clarified. The importance of the total electron spin state of the cluster has been highlighted—and is understandable considering the necessary restriction to form and release triplet dioxygen 3 O 2.

Water delivery is optimized through highly efficient water channels leading to the Mn cluster. In the favored model the two substrate waters bind to neighboring redox-active Mn ions, and deprotonation of these waters is coupled to the oxidation events of the cluster.

Close proximity and proper alignment of the two active oxygens are assured to efficiently form the O—O bond and finally release the 3 O 2 molecule with high efficiency. One pertinent open question is the structure of the elusive S 4 state. A possible stabilization and characterization of this state would finally complete our knowledge of the S states of the Kok cycle. Furthermore the determination of the water binding kinetics should be followed for all state transitions by rapid freeze quench RFQ techniques combined with advanced EPR techniques.

The exchange of certain amino acids in the vicinity of the OEC could shed light on the fine-tuning of its electronic structure in the S state cycle and would be important for explaining its high turnover frequency. Furthermore, a great challenge for chemists working in the field of water oxidation is the understanding and modeling of the sophisticated mechanism by which PS II repairs photo-damage of its protein, of the pigments and of the manganese cluster.