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Pheophytin or phaeophytin is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
that serves as the first electron carrier intermediate in the electron transfer pathway of Photosystem II (PS II) in
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, and the type II photosynthetic reaction center (RC P870) found in purple bacteria. In both PS II and RC P870, light drives electrons from the reaction center through pheophytin, which then passes the electrons to a
quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with ...
(QA) in RC P870 and RC P680. The overall mechanisms, roles, and purposes of the pheophytin molecules in the two transport chains are analogous to each other.


Structure

In biochemical terms, pheophytin is a
chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
molecule lacking a central Mg2+ ion. It can be produced from chlorophyll by treatment with a weak acid, producing a dark bluish waxy pigment. The probable
etymology Etymology ( ) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. ...
comes from this description, with ''pheo'' meaning ''dusky'' and ''phyt'' meaning ''vegetation''.phyt
CollinsDictionary.com. Collins English Dictionary - Complete & Unabridged 11th Edition. Retrieved October 19, 2012.


History and discovery

In 1977, scientists Klevanik, Klimov, Shuvalov performed a series of experiments to demonstrate that it is pheophytin and not
plastoquinone Plastoquinone (PQ) is a terpenoid-quinone ( meroterpenoid) molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4- ...
that serves as the primary electron acceptor in photosystem II. Using several experiments, including
electron paramagnetic resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spin ...
(EPR), they were able to show that pheophytin was reducible and, therefore, the primary electron acceptor between P680 and plastoquinone (Klimov, Allakhverdiev, Klevanik, Shuvalov). This discovery was met with fierce opposition, since many believed pheophytin to only be a byproduct of chlorophyll degradation. Therefore, more experiments ensued to prove that pheophytin is indeed the primary electron acceptor of PSII, occurring between P680 and plastoquinone (Klimov, Allakhverdiev, Shuvalov). The data that was obtained is as follows: # Photo-reduction of pheophytin has been observed in various mixtures containing PSII reaction centers. # The quantity of pheophytin is in direct proportion to the number of PSII reaction centers. # Photo-reduction of pheophytin occurs at temperatures as low as 100K, and is observed after the reduction of plastoquinone. These observations are all characteristic of photo-conversions of reaction center components.


Reaction in purple bacteria

Pheophytin is the first electron carrier intermediate in the photoreaction center (RC P870) of purple bacteria. Its involvement in this system can be broken down into 5 basic steps. The first step is excitation of the bacteriochlorophylls (Chl)2 or the special pair of chlorophylls. This can be seen in the following reaction. *(Chl)2 + 1 photon → (Chl)2* (excitation) The second step involves the (Chl)2 passing an electron to pheophytin, producing a negatively charged radical (the pheophytin) and a positively charged radical (the special pair of chlorophylls), which results in a charge separation. *(Chl)2* + Pheo → ·(Chl)2+ + ·Pheo (charge separation) The third step is the rapid electron movement to the tightly bound menaquinone, QA, which immediately donates the electrons to a second, loosely bound quinone (QB). Two electron transfers convert QB to its reduced form (QBH2). *2·Pheo + 2H+ + QB → 2Pheo + QBH2 (quinone reduction) The fifth and final step involves the filling of the “hole” in the special pair by an electron from a heme in cytochrome ''c''. This regenerates the substrates and completes the cycle, allowing for subsequent reactions to take place.


Involvement in photosystem II

In photosystem II, pheophytin plays a very similar role. It again acts as the first electron carrier intermediate in the photosystem. After P680 becomes excited to P680*, it transfers an electron to pheophytin, which converts the molecule into a negatively charged radical. Two negatively charged pheophytin radicals quickly pass their extra electrons to two consecutive plastoquinone molecules. Eventually, the electrons pass through the
cytochrome Cytochromes are redox-active proteins containing a heme, with a central iron (Fe) atom at its core, as a cofactor. They are involved in the electron transport chain and redox catalysis. They are classified according to the type of heme and its ...
''b6f'' molecule and leaves photosystem II. The reactions outlined above in the section concerning purple bacteria give a general illustration of the actual movement of the electrons through pheophytin and the photosystem. The overall scheme is: # Excitation # Charge separation # Plastoquinone reduction # Regeneration of substrates


See also

*
Photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
* Photosystem *
Chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
* Reaction center * P680 * Chlorophyllide


References

* * *
Photosynthetic Molecules Section.
''Library of 3-D Molecular Structures''. 22 April 2007 *Xiong, Ling, and Richard Sayre. "The Identification of Potential Pheophytin Binding Sites in the Photosystem II Reaction Center of Chlamydomondas by Site-Directed Mutagenesis." (2000). ''America Society of Plant Biologists''. 22 Apr. 2007.


References

{{tetrapyrroles Photosynthetic pigments Tetrapyrroles