Photosystem II
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BackgroundBackground
Photosystem II, crystallized from the bacteria, Thermosynechococcus elongatus, at 3.50 Å, is representative of the photosynthetic protein found in cyanobacteria and plants. The protein is associated with a variety of ligands. It is mainly composed of and is a symmetrical . Fifteen subunits are in each monomer, with multiple subunits associated with the oxygen evolving complex missing from this crystallization. Photosystem II is a membrane bound protein found in the thylakoid membrane of chloroplasts. Analysis of residues orients the protein within the membrane.
PhotosynthesisPhotosynthesis
surround Photosystem II and capture energy from sunlight, exciting electrons. Highly conjugated, these chlorophyll absorb visible light, along with as accessory light harvesting pigments such as that can absorb at other wavelengths.
Beta carotene also protects photosystem II by destroying reactive oxygen species that result from this photoexcitation. 
Electrons are passed from chlorophyllto . 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. These plastoquinones eventually move to a plastoquinone pool which goes to another large protein subunit, photosystem I, the next step in photosynthesis. 
Oxygen EvolutionOxygen Evolution
Another important facet of photosystem II is its ability to oxidize water to oxygen with its . These centers were shown to be cubane-like Mn3CaO4cluster linked to a fourth Mn by a mono-μ-oxo bridge. [1]
ReferencesReferences
[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.