Photosystem II

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Contents 1 Background 2 Photosynthesis 3 Electron Transfer 4 Oxygen Evolution 5 References

Background

This structure of photosystem II was crystallized from the bacteria, Thermosynechococcus elongatus, at 3.50 Å. Cyanobacteria and plants both contain Photosystem II with a similar structure. This photosynthetic protein is associated with a variety of functional ligands. It is a composed mainly of alpha-helices. Nineteen are in each monomer, with multiple extrinsic subunits associated with the oxygen evolving complex missing from this crystallization. Photosystem II is a membrane bound protein associated with the thylakoid membrane of chloroplasts. residues show how the protein is situated within the membrane. The hydrophobic transmembranal residues are gray, while polar residues, concentrated externally, are purple.

Photosynthesis

Photosystem II is an integral part of photosynthesis, the conversion of light energy into chemical energy by living organisms. Photosystem II is linked to a variety of other proteins, including Photosytem I. These proteins ultimately produce NADPH and ATP that power the Calvin cycle. Using this enery, glucose is synthesized from carbon dioxide and water.

Electron Transfer

surround Photosystem II and capture energy from sunlight, exciting electrons. Highly conjugated, these chlorophyll absorb visible light, along with accessory light harvesting pigments such as that can absorb light of other wavelengths.

Beta carotene also protects Photosystem II by destroying reactive oxygen species that result from this photoexcitation.

Electrons are passed from chlorophyll to . Pheophytin are very similar to chlorophyll except they contain 2 H⁺ instead of a Mg²⁺ ion. From the pheophytin, electrons go to , which are reduced. Between each pair of quinones, an iron, in red, helps to transfer the electron between them. These plastoquinones eventually move to a plastoquinone pool which goes to another large protein subunit, cytochrome b ₆/ f. Eventually these electrons reduce NADP⁺ to NADPH. Here the is shown, with beta-carotenes, pheophytins, iron and plasotoquinones.

Oxygen Evolution

Another important facet of photosystem II is its ability to oxidize water to oxygen with its . These were shown to be cubane-like Mn₃CaO₄cluster linked to a fourth Mn by a mono-μ-oxo bridge. [1] Purple represents manganese, red represents oxygen and green is for calcium. Oxidation of water to leaves 2 H ⁺ on the lumenal side of the membrane, helping to establish the proton gradient essential for ATP synthesis in the CF₁CF₀-ATP sythase protein.

References

[1] Ferreira, K.N., Iverson, T.M., Maghlaoui, K., Barber, J., Iwata, S. "Architecture of the photosynthetic oxygen-evolving center." Science, March 19, 2004, 303 (5665), 1831-8.

[2] Garrett, R.H., Grisham, C.M. Biochemistry, 3rd Edition. Belmont, CA: Thomson Brooks/ Cole, 2005.