Meanwhile, water molecule, the authentic electron donor, is oxidized to molecular oxygen and P is eventually reduced. After the reaction, the electrons are ultimately transferred to the thylakoid-embedded cytochrome b 6 f, which oxidizes plastoquinols to plastoquinones and reduces plastocyanins Cramer et al. The core complex is largely conserved from cyanobacteria to plants with nine membrane-embedded subunits, whereas the LHCI complexes are variable in subunit composition, binding pigments and sizes due to the different habitats of cyanobacteria, algae, and plants Ben-Shem et al.
However, although PSI and PSII evolved from the same ancestor, belonging to the same superfamily, their structures are largely different. The structures of D1 and D2 are similar to each other, both containing five helices all tilted against the membrane planes Zouni et al. Afterward, Loll et al. They displayed the positions of 20 protein subunits and their interactions with 77 cofactors Figure 1A. The overall structures of the supercomplex and protein subunits are similar to those previously reported Zouni et al.
Lipids have long been thought to play a role in the assembly and function of PSII, and for the first time the authors showed the lipid integrally bound to PSII. Eleven lipids surrounding the RC form a belt to separate it from the antenna and small protein subunits, while the remaining lipids are mostly located at the monomer-monomer interface.
The lipid-rich property renders PSII both structural flexibility for local mobility and convenience in subunit-subunit recognition Guskov et al. In the study by Guskov et al. A summary of the subunit composition information, including the subunit-cofactor interactions in PSII from T. In , Broser et al. The assembly of the protein subunits, tetrapyrrole cofactors and the non-heme iron in the monomeric PSIIcc are all identical to those in the dimer structure. The monomer-monomer interface is indicated by a black dashed line.
Only the four large subunits and the intrinsic subunits of PSIIs are shown. The subunits D1 blue , D2 lime , CP43 yellow , and CP47 cyan and the other small subunits are labeled in the monomer on the left. Cofactors are shown in sticks, including Chl green , Car orange , heme light magenta and lipids red. The Mn cluster is shown in spheres. TABLE 1. During photosynthesis, water oxidation happens in the oxygen-evolving complex OEC , which comprises the Mn 4 CaO 5 cluster as the catalytic center.
Water splitting is a process fulfilled in five consecutive stages named S 0 to S 4. It has been a model system for synthesizing catalysts for inorganic water oxidation and dioxygen evolution Kanady and Agapie, ; Mukherjee et al. In the work of Ferreira et al. However, neither water nor hydroxide could be observed to find the water oxidation site accurately in their study.
Then a 1. Subsequently, a simultaneous femtosecond X-ray spectroscopy and diffraction of the PSII system showed that the electron density maps of the dark and illuminated states are similar with an overall correlation coefficient CC of 0.
However, with a serial time-resolved crystallography, the authors acquired PSII structures in the dark S 1 and putative S 3 states, in which they found that the distance between the Mn 3 O x Ca cubane and the distant protruding Mn dangler Mn increased in the putative S 3 state, allowing the binding of the second water molecule during the S 2 to S 3 state transition Kupitz et al.
In addition, the position of O5 is also unusual. The results showed that it functions more as a hydroxide ion instead of a normal oxygen dianion to serve as one of the substrate oxygen atoms Suga et al. Furthermore, it was reported that the chloride ion is essential for oxygen evolution, and there are two anion binding sites positioned on the two sides of the MnCa cluster with the same distance from the cluster to stabilize its structure Kawakami et al.
Compared to the structure of the spinach C 2 S 2 -type supercomplex, the structure reported by Su et al. The plant PSII indeed exhibits the same composition and organizations of the subunits and cofactors as their cyanobacterial counterparts Table 1.
Specifically, D1 and D2 form the photochemical RC, which is responsible for the charge separation and electron transfer, and CP47 and CP43 act as internal antenna proteins involved in light harvesting and energy transportation from peripheral antenna to the RC.
In the core complex, there are also 12 low molecular-mass MM membrane-spanning subunits surrounding the reaction center, forming a belt-like structure. Most of these subunits are structurally conserved with a single transmembrane helix except PsbZ with two helices. These subunits are essential for both the dimerization and stabilization of the core complex and the association between the core complex and the peripheral antenna complex. In addition, they bind cytochrome b to protect the PSII complex from photo-damage.
Among them, PsbO stabilizes the Mn complex while PsbP and PsbQ are involved in optimizing the oxygen evolution at physical concentration of calcium and chloride ions.
Structure comparison also revealed that the flexible regions of these subunits experience significant conformational changes when they bind to the core complex Wei et al. Each polypeptide spans the thylakoid membrane three times with its C terminus positioned on the luminal side Kuhlbrandt et al. For the second function, the stromal surface of the LHCII trimer is negatively charged whereas its N-terminal first 15 residues contain 4 positively charged residues.
It has been reported that constitutively expression of Lhcb1 robustly increased grana stacks in the transgenic tobacco plants Labate et al. This study provided new insights into how adjacent thylakoids might be linked to mediate the stacking of grana membranes by interactions between pairs of PSII-LHCII supercomplexes. Similar to those in cyanobacteria, these cofactors mainly include chlorophylls, carotenoids, lipids etc. Table 1. Interestingly, the LHCII monomer shows both amino acid sequence and structure similarities to those of CP29, however, the type, quantity and location of the chlorophylls they bind are significantly different Liu et al.
Photosynthesis plays very important roles in molecular oxygen production, atmospheric carbon dioxide control and global food supply. Structural information of the photosystems is invaluable for our understanding of photosynthesis, probably the most important process on earth.
The information will also help design artificial photosynthetic system for the improvement of bioenergy production and the enhancement of agricultural productivity.
Most recently, the structure of the largest light-harvesting complex, the phycobilisome PBS from Griffithsia pacifica was also reported Zhang et al. As the main light-harvesting antenna in cyanobacteria and red algae, it exhibits a very fast energy transfer rate with a high quantum yield Glazer, The structural information of the PBS will provide a firm basis for understanding its energy transfer pathways and further applications in the designs of artificial light-harvesting machineries.
Recent advances in single-particle cryo-EM have provided unprecedented structural information about these huge membrane complexes. However, there are also several open questions to be answered. First, the exact reaction mechanism underlying water oxidation and possible structural rearrangements during the S-state transitions still await the structures of PSII in more intermediate S states.
Since static structures solved thus far has provided no conclusive clues in this respect, new studies investigating the dynamic nature of PSII might shed more light on this, which is very relevant to make PSII not only a proton pump but also the site of O 2 evolution. Third, more structural information is needed to figure out the localizations and functions of PsbR and PsbS, PSII subunits that are essential for oxygen-evolving activity Allahverdiyeva et al. Last, new high-resolution structures of the photosystems from cyanobacteria, algae, and plants will provide more insights into the evolution of oxygenic photosynthesis, based on which better artificial photosynthetic machineries could be developed.
YF reviewed and revised the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Albanese, P. Albertsson, P. A quantitative model of the domain structure of the photosynthetic membrane. Trends Plant Sci. Allahverdiyeva, Y. Insights into the function of PsbR protein in Arabidopsis thaliana.
Acta , — Andersson, J. Absence of the Lhcb1 and Lhcb2 proteins of the light-harvesting complex of photosystem II-effects on photosynthesis, grana stacking and fitness. Plant J. Cell Mol. Chapter Ecosystems. Chapter Population and Community Ecology. Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. Chapter Natural Selection. Chapter Population Genetics. Chapter Evolutionary History.
Chapter Plant Structure, Growth, and Nutrition. Chapter Plant Reproduction. Chapter Plant Responses to the Environment. Full Table of Contents. This is a sample clip. Sign in or start your free trial. JoVE Core Biology. Previous Video Next Video. Next Video 9. Embed Share. Similar to photosystem two, in photosystem one, photons are also absorbed by pigment molecules in the light-harvesting complex, to relay energy to its reaction center, and following oxidation, a high-energy electron is passed to the primary electronic scepter.
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Please enjoy a free hour trial. Photosystem 2: Photolysis of water occurs in photosystem 2. Photosystem 2: The main function of the photosystem 2 is ATP synthesis and hydrolysis of water. Photosystem 1: Released high energy electrons are replaced by the releasing energy of photolysis.
Photosystem 2: Released high energy electrons are replaced by the electrons released from photosystem II. Photosystem 2: PS 2 contains chlorophyll B, chlorophyll A, chlorophyll A, chlorophyll A, chlorophyll A, chlorophyll A, phycobilins and xanthophylls. Each of the two photosystems are made up of a collection of proteins and pigments. Chlorophylls are the major pigments found in photosystems. Other than chlorophylls, carotenoids are also present in photosystems. Water molecules are hydrolyzed at PS II in order to replace the releasing electrons of each of the two photosystems.
However, the main difference between Photosystem 1 and 2 is the wavelengths of sunlight, which are absorbed by each of the reaction centers of photosystems.
Reference: 1. Jennings, and Stefano Santabarbara. Bentham Science Publishers, June Image Courtesy: 1. Figure 2: Photosystem 2. View all posts.
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