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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/06 19:08:37」(JST)

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Light-dependent reactions of photosynthesis at the thylakoid membrane

Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplasts), or in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: II and I, respectively.

Reaction centers

At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce molecules. In a photosystem, this reaction center is surrounded by light-harvesting complexes that enhance the absorption of light.

Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulphur purple bacteria).

Each photosystem can be identified by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used.

Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.

Structure

A reaction center comprises several (>10 or >11) protein subunits, providing a scaffold for a series of cofactors. The latter can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters.^[1]

Relationship between Photosystems I and II

For oxygenic photosynthesis, both photosystems I and II are required. Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are believed to be the progenitors of the photosystem-containing chloroplasts of eukaryotes. Photosynthetic bacteria that cannot produce oxygen have a single photosystem called BRC, bacterial reaction center.

The photosystem I was named "I" since it was discovered before photosystem II, but this does not represent the order of the electron flow.

When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. To replenish the deficit of electrons, electrons are extracted from water by a cluster of four Manganese ions in photosystem II and supplied to the chlorophyll via a redox-active tyrosine.

Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis), to transport hydrogen (H⁺) through the membrane, to the lumen, to provide a proton-motive force to generate ATP. The protons are transported by the plastoquinone. If electrons only pass through once, the process is termed noncyclic photophosphorylation.

When the electron reaches photosystem I, it fills the electron deficit of the reaction-center chlorophyll of photosystem I. The deficit is due to photo-excitation of electrons that are again trapped in an electron acceptor molecule, this time that of photosystem I.

ATP is generated when the ATP synthase transports the protons present in the lumen to the stroma, through the membrane. The electrons may either continue to go through cyclic electron transport around PS I or pass, via ferredoxin, to the enzyme NADP⁺ reductase. Electrons and hydrogen ions are added to NADP⁺ to form NADPH. This reducing agent is transported to the Calvin cycle to react with glycerate 3-phosphate, along with ATP to form glyceraldehyde 3-phosphate, the basic building-block from which plants can make a variety of substances.

External links

Photosystems I + II: Imperial College, Barber Group
Photosystem I: Molecule of the Month in the Protein Data Bank
Photosystem II: Molecule of the Month in the Protein Data Bank
Photosystem II: ANU
UMich Orientation of Proteins in Membranes families/superfamily-2 - Calculated spatial positions of photosynthetic reaction centers and photosystems in membrane
Rutherford AW, Faller P (January 2003). "Photosystem II: evolutionary perspectives". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 358 (1429): 245–53. doi:10.1098/rstb.2002.1186. PMC 1693113. PMID 12594932.

References

^ Jagannathan, B; Golbeck, JH (2009). "Photosynthesis:Microbial". Encyclopedia of Microbiology, 3rd Ed: 325–341. doi:10.1016/B978-012373944-5.00352-7.

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English Journal

The Rice TCM5 Gene Encoding a Novel Deg Protease Protein is Essential for Chloroplast Development under High Temperatures.

Zheng K1, Zhao J1,2, Lin D1, Chen J1,2, Xu J3, Zhou H1,4, Teng S2, Dong Y5.
Rice (New York, N.Y.).Rice (N Y).2016 Dec;9(1):13. doi: 10.1186/s12284-016-0086-5. Epub 2016 Mar 21.
BACKGROUND: High temperature affects a broad spectrum of cellular components and metabolism in plants. The Deg/HtrA family of ATP-independent serine endopeptidases is present in nearly all organisms. Deg proteases are required for the survival of Escherichia coli at high temperatures. However, it is
PMID 27000876

Data supporting the absence of FNR dynamic photosynthetic membrane recruitment in trol mutants.

Vojta L1, Fulgosi H1.
Data in brief.Data Brief.2016 Feb 26;7:393-6. doi: 10.1016/j.dib.2016.02.044. eCollection 2016.
In photosynthesis, the flavoenzyme ferredoxin:NADP(+) oxidoreductase (FNR) catalyses the final electron transfer from ferredoxin to NADP(+), which is considered as the main pathway of high-energy electron partitioning in chloroplasts (DOI: 10.1111/j.1365-313X.2009.03999.x[1], DOI: 10.1038/srep10085[
PMID 26977444

Effects of di-n-butyl phthalate and di (2-ethylhexyl) phthalate on the growth, photosynthesis, and chlorophyll fluorescence of wheat seedlings.

Gao M1, Qi Y2, Song W3, Xu H4.
Chemosphere.Chemosphere.2016 May;151:76-83. doi: 10.1016/j.chemosphere.2016.02.061. Epub 2016 Mar 15.
Phthalates are commonly used man-made chemicals that can be released into soil, water, and the atmosphere. The potential toxicity of phthalates on wheat seedlings has not been well studied. To better understand the deleterious effects of di-n-butyl phthalate (DBP) and di (2-ethylhexyl) phthalate (DE
PMID 26928333

Japanese Journal

Harvesting Far-Red Light by Chlorophyll f in Photosystems I and II of Unicellular Cyanobacterium strain KC1

Plant and cell physiology 56(10), 2024-2034, 2015-10
NAID 120005691342

21aAF-2 光合成における光化学系II第二キノン電子受容体Q_Bの酸化還元電位計測

日本物理学会講演概要集 70(1), 3235, 2015-03-21
NAID 110009992506

21aAF-1 光合成光化学系IIにおけるMnCaクラスターの歪んだ椅子型構造の起源

日本物理学会講演概要集 70(1), 3234, 2015-03-21
NAID 110009992505

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★リンクテーブル★

リンク元	「光化学系」「photochemical system」
拡張検索	「photosystem II protein complex」「photosystem I protein complex」「photosystem II」「photosystem I」