Photosystem II

Background

This structure of Photosystem II was crystallized from the cyanobacteria, Thermosynechococcus elongatus, at 3.0Å ^[1] and at 3.50 Å ^[2]. PDB codes are 2axt and 1s5l, respectively. Cyanobacteria and plants both contain Photosystem II while photosynthetic bacteria contain the bacterial reaction center. This photosynthetic protein complex is associated with a variety of functional ligands. It is a dimer composed mainly of alpha-helices. Nineteen subunits 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 complex that in plants is associated with the thylakoid membrane of chloroplasts. Polar and hydrophobic regions correlate with membrane associated nature of the protein. Hydrophobic helices make up the transmembranal portion, while polar residues are concentrated externally on either side of the membrane.

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 energy, glucose is synthesized from carbon dioxide and water. See also Photosynthesis.

Electron Transfer

structure of chlorophyll a

Chlorophyll surround Photosystem II and capture energy from sunlight, exciting electrons. Chlorophyll are highly conjugated and absorb visible light, along with accessory light harvesting pigments such as beta carotene. Beta carotene absorbs visible light of other wavelengths and also protects Photosystem II by destroying reactive oxygen species that result from this photoexcitation. Electrons are passed from chlorophyll to pheophytin. Pheophytin are very similar to chlorophyll except they contain 2 H⁺ instead of a Mg²⁺ ion. From the pheophytin, electrons transferred to plastoquinones, which are reduced. Located between each pair of quinones, an iron helps to transfer the electron. These plastoquinones eventually move to a plastoquinone pool which travels to another large protein subunit, cytochrome b ₆/ f. Eventually these electrons reduce NADP⁺ to NADPH. The electron pathway through Photosystem II 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 oxygen evolving centers. These centers are cubane-like structures with 3 manganese, 4 oxygen and a calcium linked to a fourth manganese^[2]. Oxidation of water 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.

structure of beta carotene

reduced plastoquinone

3D structures of photosystem II

Photosystem II 3D structures