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| photosystem II | |
|---|---|
| Name | Photosystem II |
| Organism | Plants; Cyanobacteria; Algae |
| Location | Thylakoid membrane; Grana |
| Discovered | 1960s |
| Function | Light-driven water oxidation; Oxygen evolution; Electron donation to plastoquinone |
photosystem II is a large, membrane-bound protein–pigment complex essential for oxygenic photosynthesis in Plant physiology-related organisms such as Arabidopsis thaliana, Synechocystis sp. PCC 6803, and marine Prochlorococcus. It catalyzes light-driven splitting of water, producing molecular oxygen and supplying electrons to downstream photosynthetic chains that power carbon fixation via the Calvin–Benson cycle and metabolic pathways in chloroplasts and cyanobacterial cells. Photosystem II operates within the stacked thylakoid regions of chloroplasts and the thylakoid membranes of cyanobacteria, coordinating redox chemistry, proton translocation, and protein turnover.
Photosystem II functions as the primary photo-oxidant in the light reactions of photosynthesis, absorbing photons and initiating charge separation that drives electron flow toward the Cytochrome b6f complex and on to Photosystem I. Embedded in the thylakoid membrane of chloroplasts and cyanobacteria, it interacts with mobile carriers such as Plastoquinone and is regulated in concert with complexes like the ATP synthase (mitochondrial)-related thylakoid ATP synthase. PSII activity underpins global processes including the Great Oxygenation Event, modern biogeochemical cycles, and ecosystems from Amazon rainforest canopies to open-ocean photic zones.
The core PSII supercomplex comprises multiple protein subunits organized around the D1 and D2 reaction center proteins, associated antenna proteins such as CP43 and CP47, and extrinsic proteins (e.g., PsbO, PsbP, PsbQ in plants) that stabilize the oxygen-evolving complex. The cofactors include the primary electron donor P680 (a chlorophyll a dimer), accessory chlorophylls, pheophytins, plastoquinone acceptors QA and QB, non-heme iron, and the Mn4CaO5 cluster that forms the catalytic heart. High-resolution structures have been resolved by techniques used in studies on Thermosynechococcus elongatus and model organisms, informed by methods developed in X-ray crystallography, Cryo-electron microscopy, and synchrotron facilities such as Diamond Light Source and European Synchrotron Radiation Facility.
Water oxidation occurs at the Mn4CaO5 cluster, progressing through a sequence of S-states described by the Kok cycle (S0→S4→S0), with four successive photo-induced charge separations driving extraction of four electrons from two water molecules to form O2. Photochemistry begins with excitation of P680 to P680*, leading to electron transfer to pheophytin and then QA and QB, while P680+ is a potent oxidant that abstracts electrons from the Mn cluster via a tyrosine residue (TyrZ, linked to PsbA/D1). Proton release and substrate water coordination involve contributions from nearby residues and the extrinsic protein environment, a mechanism elucidated by comparisons across Photosystem I research, enzymology of metalloenzymes, and spectroscopic investigations spanning EPR spectroscopy and time-resolved UV–visible spectroscopy.
Following charge separation, electrons reduce plastoquinone at the QB site to plastoquinol, which diffuses through the membrane to the Cytochrome b6f complex, coupling electron flow to proton translocation and generation of a proton motive force for ATP synthesis by the chloroplast ATP synthase. The redox potentials within PSII are tuned by protein environment and cofactors to enable efficient energy conversion while minimizing deleterious side reactions such as charge recombination and reactive oxygen species formation. Interplay with stromal processes including the Calvin–Benson cycle, cyclic electron flow pathways involving proteins like PGR5 and NDA2, and state transitions regulated by kinases such as STN7 coordinates the balance between ATP and NADPH production and photoprotection in fluctuating light environments like those experienced in the Mediterranean canopy or polar Antarctica.
PSII assembly and maintenance involve stepwise biogenesis pathways mediated by assembly factors (e.g., Ycf48, HCF136, OHPs) and chaperones conserved between Arabidopsis thaliana and cyanobacteria such as Synechococcus sp. Proteolytic turnover of the D1 subunit via the FtsH protease and Deg proteases enables repair following photodamage, a process studied in models ranging from Zea mays to marine cyanobacteria. Regulation integrates signaling networks including redox-regulated transcription factors, light-harvesting complex adjustments governed by proteins of the LHCB family, and nutrient-sensitive responses involving metal transporters for Mn and Ca essential to the Mn4CaO5 cluster, with physiological impacts observed in agricultural systems and climate-sensitive biomes like the Sahara fringe.
Oxygenic photosynthesis and PSII emerged in ancestral cyanobacteria prior to the Great Oxygenation Event, enabling the rise of aerobic metabolisms and complex eukaryotes. Comparative genomics across lineages such as Gloeobacter violaceus, Prochlorococcus marinus, and plastid-bearing eukaryotes reveals conserved core subunits and lineage-specific adaptations in antenna systems and extrinsic proteins. Horizontal gene transfer, endosymbiosis events exemplified by the origin of plastids in the Neoproterozoic era, and selective pressures from metal availability shaped PSII diversification across marine, freshwater, and terrestrial ecosystems including Boreal forest and coral reef habitats.
PSII is studied using structural methods like cryo-EM and X-ray crystallography, spectroscopies (EPR, FTIR, ultrafast pump–probe), genetic tools applied in Arabidopsis thaliana and cyanobacterial model systems, and biophysical assays at synchrotron and laser facilities. Applied research exploits PSII principles for artificial photosynthesis, solar fuel development supported by initiatives in institutions such as Massachusetts Institute of Technology and Lawrence Berkeley National Laboratory, and agricultural improvement strategies targeting photoprotection and repair pathways to boost crop yields in Wheat and Rice. Understanding PSII informs climate models, biogeochemical cycling studies by groups like the Intergovernmental Panel on Climate Change, and biotechnological approaches to sustainable energy.