Photosystem II

Overview
Photosystem II is the first protein complex in Light Dependent Photosynthesis. It is located in the thylakoid membrane of plants, algea and cyanobacteria. The enzyme uses photons of light to energize electrons which are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By obtaining these electrons from water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP+ to NADPH or are used in cyclic photophosphorylation.

Structure
Photosystem II is composed of 16-18 subunits as well as other accessory, light harvesting proteins. There are several crystal structures of photosystem II. The PDB accession codes for this protein are, , , ,.

Oxygen Evolving Complex (OEC)
The oxygen evolving complex is the site of water oxidation. It is a metallo-oxo cluster comprising four manganese ions (in oxidation states ranging from +3 to +5) and one divalent calcium ion. When it oxidizes water, producing dioxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine (D1-T161) sidechain and thence to P680 itself. The structure of the oxygen evolving complex is still contentious. The structures obtained by X-ray crystallography are particularly controversial, since there is evidence that the manganese atoms are reduced by the high-intensity X-rays used, altering the observed OEC structure. However, crystallography in combination with a variety of other (less damaging) spectroscopic methods such as EXAFS and electron paramagnetic resonance have given a fairly clear idea of the structure of the cluster. One possibility is the cubane-like structure shown on the right.

Water Oxidation
Photosynthetic water oxidation (or oxygen evolution) is arguably one of the most important reactions on the planet, since it is the source of practically all the atmosphere's oxygen. The mechanism of water oxidation is still not fully elucidated but there is significant evidence that it occurs by the S-state mechanism.