Which photosystem splits water

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Water oxidation mechanism in photosystem II, including oxidations, proton release pathways, O—O bond formation and O2 release. Winkler, J. Mukherjee, S. Synthetic model of the asymmetric [Mn3CaO4] cubane core of the oxygen-evolving complex of photosystem II. Kanady, S. A synthetic model of the Mn3Ca subsite of the oxygen-evolving complex in photosystem II.

Fesseler, J. Download references. I want to dedicate this paper to the late G. Babcock, who influenced much of my thinking about the mechanism of water oxidation in PSII over the years. I would also like to thank R. Finally, I most sincerely thank J. Murray, who helped me with the construction of Fig. Malkin, who encouraged me to write this paper after discussing my ideas with him in the quiet and peaceful ambience of Lago d'Orta, Italy in September You can also search for this author in PubMed Google Scholar.

Correspondence to James Barber. Reprints and Permissions. A mechanism for water splitting and oxygen production in photosynthesis. Nature Plants 3, Download citation. Received : 25 November Accepted : 03 March Published : 03 April Anyone you share the following link with will be able to read this content:.

Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Nature Reviews Materials Photosynthesis Research Nano Convergence Nature Advanced search. Skip to main content Thank you for visiting nature. Subjects Photosynthesis Photosystem II. Abstract Sunlight is absorbed and converted to chemical energy by photosynthetic organisms.

Access through your institution. Buy or subscribe. Rent or Buy article Get time limited or full article access on ReadCube. Figure 1: The S-state cycle showing how the absorption of four photons of light hv by P drives the splitting of two water molecules and formation of O 2 through a consecutive series of five intermediates S 0 , S 1 , S 2 , S 3 and S 4. Figure 3: A structurally based diagrammatic comparison of a base nucleophilic attack of a hydroxyl on to an electrophile leading to oxygen atom transfer.

Figure 4: Diagrammatic representation of a mechanistic scheme for water splitting and dioxygen formation in PSII based on the arguments and discussion presented in this communication. References 1 Fontana, F. Google Scholar 2 Byron Smith, R. Google Scholar 4 Gong, W. Google Scholar 5 Kok, B. Google Scholar 6 Cox, N.

Google Scholar 7 Barber, J. Google Scholar 8 Renger, G. Google Scholar 9 Siegbahn, P. Google Scholar 10 Haumann, M. Google Scholar 11 McEvoy, J. Google Scholar 12 Yamanaka, S. Somewhere in the region of 2. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing, it provided biology with an unlimited supply of hydrogen equivalents needed to convert carbon dioxide into the organic molecules of life.

The by-product of the water-splitting reaction is molecular oxygen. The release of this gas also had dramatic consequences for the development of life: It created an oxygenic atmosphere and at the same time allowed the ozone layer to form. With oxygen available, the efficiency of cellular metabolism increased dramatically because for a given amount of substrate, aerobic respiration provides in the region of 15 times more energy than anaerobic respiration. It was probably this improved efficiency of cellular bioenergetics that paved the way for the subsequent evolution of eukaryotic cells and multicellular organisms.

The establishment of the ozone layer provided a shield against harmful ultraviolet radiation, allowing organisms to prosper and diversify on an enormous scale, as testified by fossil records and the extent and variety of the living organisms on our planet today.

PSII is a multisubunit protein complex located in the thylakoid membranes of all types of plants, algae, and cyanobacteria Fig. At its heart is the reaction center RC core, where light energy is converted to electrochemical potential energy and where the water-splitting reaction occurs.

There are also a number of other low-molecular-mass transmembrane subunits that are rather featureless except for two that bind the high-potential heme of cytochrome b Cyt b Finally, the PSIIRC core complex contains several extrinsic proteins attached to its lumenal surface that form a protein shield over the catalytic site of water splitting.

Although one of these proteins, PsbO, is ubiquitous to all oxygenic photosynthetic organisms, the others vary among different types of organisms. Other extrinsic proteins are transiently bound during the life cycle of PSII.

This core complex is serviced by an outer light-harvesting system that, in the case of plants and green algae, is composed of intrinsic Lhcb proteins that bind Chl a and Chl b as shown in Figure 1. In red algae and cyanobacteria, this outer antenna is replaced by phycobiliproteins comprising the phycobilisome bound extrinsically to the stromal surface of the PSII core.

Illustration of the structure and subunit composition of PSII of higher plants and green algae. Intrinsic and extrinsic proteins are labeled according to the gene nomenclature in Table 1 e. Arrows Electron transfer pathway from water oxidation to plastoquinone reduction see text. The way in which PSII uses light energy to generate an electrochemical potential in the form of a charge transfer state is essentially the same as that for other types of photosynthetic systems.

Chl molecules and other pigments e. All of the redox-active cofactors involved in the energy conversion process are bound to D1 and D2 proteins and the following sequence of reactions occurs see Fig.

Each oxidation state generated within the oxygen-evolving complex OEC is represented as an intermediate of the S-state cycle Kok et al. These side reactions occur on a tens of milliseconds time scale and therefore do not compete with the electron transfer pathway leading to water oxidation. As with studies of other enzymes, the reactions of PSII cannot be fully understood in molecular terms without a detailed knowledge of its structure. Many techniques give structural information at different degrees of complexity, but none match the overall level of detail gained from X-ray-diffraction analysis.

PSII is a membrane protein complex, making it more difficult to crystallize. Nevertheless, progress during the past 10 years has been substantial and is the focus of this report.

The structural information obtained has improved our understanding of the many reactions of PSII and most importantly, the water-splitting reaction itself, knowledge that has far-reaching implications in the development of new technologies for the production of fuel. The first X-ray-derived structure of PSII used a preparation isolated from the cyanobacterium Thermosynechococcus elongatus and was elucidated by Zouni et al.

Their structure was limited to a resolution of 3. Although no side-chain positioning was deduced, their model confirmed the dimeric organization of the isolated complex see Hankamer et al. It also provided information on the positioning of cofactors involved in excitation transfer and charge separation. Most importantly, the analysis of the diffraction data gave the first direct structural hints of the Mn cluster of the OEC.

Two years later, Kamiya and Shen reported a 3. However, it was not until the work of Ferreira et al. In total, there are 35 transmembrane helices per monomer, and these are depicted as cylinders in Figure 1 A. Details of the 19 different subunits are highlighted in Table 1 , with one subunit, Ycf12 originally suggested to possibly be PsbN , included based on more recent biochemical analysis of PSII Kashino et al.

Note that the other nonhighlighted proteins in Table 1 are exclusive to the PSII core complex of higher plants. A Side view of the structure of photosystem II, the water-splitting enzyme of photosynthesis. Structure was determined using X-ray crystallography Ferreira et al. The complex, embedded in the thylakoid membrane spanning their lumenal and stromal surfaces, is composed of two monomers related to each other by a twofold axis.

Each monomer contains 19 different protein subunits. In total, there are 35 transmembrane helices per monomer. D1 and D2 proteins that comprise the reaction center are shown in yellow and orange, respectively.

B Organization of the electron transfer cofactors that comprise the reaction center of photosystem II as revealed by X-ray crystallography Ferreira et al.

This electron transfer is aided by the presence of a nonheme iron located midway between them. These electron transfer processes occur mainly on the D1 side of the reaction center, red arrow. Some of the symmetrically related cofactors located on the D2 side Pheo D2 are nonfunctional.

A , Reprinted, with permission, from Ferreira et al. The three extrinsic proteins PsbO, PsbU, and PsbV form a cap over the catalytic site where oxygen evolution occurs, preventing access by reductants other than water. The exceptions to this are PsbE and PsbF proteins, which provide histidine ligands for the heme of Cyt b , and the PsbL, PsbM, and PsbT proteins located at the monomer—monomer interface where they possibly have a role in stabilizing the dimeric nature of the PSII complex.

All of the small subunits have a single transmembrane helix except for PsbZ, which has two see Table 1. The 2. All of the crystal structures of PSII confirmed that the transmembrane helices of D1 and D2 proteins are arranged in an almost identical way to those of the L and M subunits of the purple bacterial RC as anticipated by homology Barber ; Michel and Deisenhofer The six transmembrane helices of CP43 and CP47 are arranged in three pairs around a pseudo-threefold axis, as first shown by electron crystallography Rhee et al.

Both are characterized as having a very large extrinsic loop joining the lumenal ends of helices V and VI. The Ferreira et al. The Chls were arranged in layers toward the lumenal and stromal surfaces, with one Chl in each case located midway between the layers. The more recent crystal structures of cyanobacterial PSII at 2. Most of these water molecules form two layers at the stromal and lumenal surfaces. However, a few molecules are located in the membrane-spanning region of the complex, with seven acting as ligands to the Mg of Chls, which are not ligated by amino acids these are histidines in all cases except one, where CP43Asn39 is a ligand.

Umena et al. For Chl triplet quenching, they must be located close to the porphyrin head group of Chl, and this is borne out in the crystal structures. Similarly, the two pheophytins are referred to as Pheo D1 and Pheo D2. Q A and Q B are positioned equally on each side of the nonheme Fe and are bound to sites located within the D2 and D1 proteins, respectively. The remaining two Chls Chlz D1 and Chlz D2 are also symmetrically related, being ligated to histidines located in the B-transmembrane helices of D1 and D2 proteins.

Moreover, the recent high-resolution structure Umena et al. Indeed, according to the 1. The electron on Pheo D1 then proceeds down the thermodynamic gradient to the terminal PQ electron acceptor bound to the Q B site within the D1 protein. A channel leading from the Q B site has been identified Loll et al. A second potential quinone channel has been postulated that is based on the assignment of a third PQ molecule in the 2.

However, this additional PQ molecule was not identified in the 1. This electron transfer process is aided by a redox-active Try Z D1Tyr This tyrosine has a symmetrically related counterpart within the D2 protein, Tyr D see Fig. It may, however, because of its long lifetime help to direct primary charge separation to the D1 side of the RC by electrostatic biasing Faller et al.

This symmetrical relationship ends here because the Mn 4 Ca cluster is only located on the D1 side and the heme of Cyt b is located on the D2 side. Therefore, the D1 side of the RC functions directly in energy conversion and water splitting, whereas the D2 side is, in part, involved in protection against photoinduced damage.

Despite the symmetric arrangement of cofactors on the reducing side, electron transport from P D1 to Q B involves only Pheo D1 , whereas the other possible route via Pheo D2 does not occur—a situation also found for electron transfer in the reaction centers of purple photosynthetic bacteria Rutherford In fact, the arrangement of cofactors on the reducing side of PSII is essentially identical to that of their bacterial counterparts.

The only clear exceptions are that one of the ligands for the nonheme Fe of PSII is a bicarbonate ion and not a glutamate, as in bacteria, and the Q B site is a little larger and in closer contact with the stromal surface than in the bacterial RC. That bicarbonate provides a bidentate ligand for the nonheme Fe that has been known for some time Hienerwadel and Berthomieu , and the assignment by Ferreira et al.

The first attempts to assign lipid and detergent molecules in the crystal structure of PSII was made by Loll et al. Science News. Story Source: Materials provided by Uppsala University.

Young, Franklin D. Moriarty, James M. Holton, Holger Dobbek, Paul D. Adams, Uwe Bergmann, Nicholas K. Nature , ; DOI: ScienceDaily, 7 November Uppsala University.

Images of photosynthetic protein complex splitting water.