Photosystem II

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the energy-dependent reactions of oxygenic photosynthesis.

Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol.

[2] Photosystem II (of cyanobacteria and green plants) is composed of around 20 subunits (depending on the organism) as well as other accessory, light-harvesting proteins.

It is composed of three protein subunits, OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ); a fourth PsbR peptide is associated nearby.

The first structural model of the oxygen-evolving complex was solved using X-ray crystallography from frozen protein crystals with a resolution of 3.8Å in 2001.

So now the race has started to solve the structure of Photosystem II at different stages in the mechanistic cycle (discussed below).

The experimental evidence that oxygen is released through cyclic reaction of oxygen evolving complex (OEC) within one PSII was provided by Pierre Joliot et al.[18] They have shown that, if dark-adapted photosynthetic material (higher plants, algae, and cyanobacteria) is exposed to a series of single turnover flashes, oxygen evolution is detected with typical period-four damped oscillation with maxima on the third and the seventh flash and with minima on the first and the fifth flash (for review, see[19]).

There are two main chemical families, the triazines derived from cyanuric chloride[24] of which atrazine and simazine are the most commonly used and the aryl ureas which include chlortoluron and diuron (DCMU).