Light-dependent reactions of photosynthesis at the thylakoid membrane

Cyanobacteria photosystem II, Dimer, PDB 2AXT.

Photosystem II is the first protein complex in Light Dependent Photosynthesis. It is located in the thylakoid membrane of plants, algae 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. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Coenzymes are small organic non- Protein Molecules that carry chemical groups between Enzymes Coenzymes are sometimes referred to as cosubstrates Plastoquinone, (often abbreviated PQ) is a Quinone molecule involved in the electron transport chain in the Light-dependent reactions of photosynthesis The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state In Chemistry, hydronium is the obsolete name for the Cation H 3 O + derived from Protonation of Water By obtaining these electrons from water, photosystem II provides the electrons for all of photosynthesis to occur. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. 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. In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane An ATP synthase ( is a general term for an Enzyme that can synthesize Adenosine triphosphate (ATP from Adenosine diphosphate (ADP and inorganic Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy The energized electrons transferred to plastoquinone are ultimately used to reduce NADP⁺ to NADPH or are used in cyclic photophosphorylation. Nicotinamide adenine dinucleotide phosphate ( NADP+, in older notation triphosphopyridine nucleotide TPN) is used in anabolic reactions such as Lipid

Structure

Cyanobacteria photosystem II, Monomer, PDB 2AXT.

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 2AXT, 1W5C, 1ILX, 1FE1, 1IZL. The Protein Data Bank ( PDB) is a repository for 3-D structural data of Proteins and Nucleic acids These data typically obtained by X-ray crystallography

Electron transfer

Oxygen Evolving Complex (OEC)

Proposed structure of Manganese Center

The oxygen evolving complex is the site of water oxidation. Cytochrome b559 is an important component of Photosystem II. PSII is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic Chlorophyll is a green Pigment found in most Plants Algae and Cyanobacteria. The term carotene is used for several related substances having the formula C40H56 A heme ( American English) or haem ( British English) is a Prosthetic group that consists of an Iron atom contained in the center of Pheophytin or Pheo is a Chlorophyll molecule lacking a central Mg2+ ion which serves as the first electron carrier intermediate in Plastoquinone, (often abbreviated PQ) is a Quinone molecule involved in the electron transport chain in the Light-dependent reactions of photosynthesis 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. X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters X-radiation (composed of X-rays) is a form of Electromagnetic radiation. 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. Extended X-Ray Absorption Fine Structure (EXAFS or more simply X-ray Absorption Spectroscopy (XAS is an experimental method in Physics and Chemistry of determining Electron paramagnetic resonance (EPR or electron spin resonance (ESR Spectroscopy is a technique for studying Chemical species that have one or more unpaired One possibility is the cubane-like structure shown on the right. ^[1]

Water Oxidation

Oxygen evolution by water oxidation during photosynthesis. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. The jagged lines represent four photons oxidizing the central cluster of the oxygen evolving complex by exciting and removing four electrons through a cycle of S-states. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena The oxygen evolving complex, (OEC also known as the water-splitting complex is a water oxidizing enzyme involved in the photooxidation of water during the Light reactions

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. Oxygen evolution is the process of generating molecular Oxygen through Chemical reaction. The mechanism of water oxidation is still not fully elucidated but there is significant evidence that it occurs by the S-state mechanism.

See Also

• Photosynthesis
• Oxygen Evolution
• Photosystem
• Reaction Centre
• P680

References

External Links

http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.html