Plastoquinone
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Plastoquinone (PQ) is a
terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...
-
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 ...
( meroterpenoid) molecule involved in the electron transport chain in the light-dependent reactions of
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 ...
. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4- benzoquinone molecule with a side chain of nine isoprenyl units. There are other forms of plastoquinone, such as ones with shorter side chains like PQ-3 (which has 3 isoprenyl side units instead of 9) as well as analogs such as PQ-B, PQ-C, and PQ-D, which differ in their side chains. The benzoquinone and isoprenyl units are both nonpolar, anchoring the molecule within the inner section of a
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organisms and many viruses a ...
, where the
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
tails are usually found. Plastoquinones are very structurally similar to ubiquinone, or coenzyme Q10, differing by the length of the isoprenyl side chain, replacement of the
methoxy group In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula . On a benzene ring, the Hammett equation classifies a methoxy substituent at the ''para'' position a ...
s with methyl groups, and removal of the methyl group in the 2 position on the quinone. Like ubiquinone, it can come in several oxidation states: plastoquinone, plastosemiquinone (unstable), and plastoquinol, which differs from plastoquinone by having two hydroxyl groups instead of two
carbonyl group In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...
s. Plastoquinol, the reduced form, also functions as an antioxidant by reducing
reactive oxygen species In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
, some produced from the photosynthetic reactions, that could harm the cell membrane. One example of how it does this is by reacting with
superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
s to form
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
and plastosemiquinone. The prefix ''plasto-'' means either
plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
or
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
, alluding to its location within the cell.


Role in photosynthesis

The role that plastoquinone plays in photosynthesis, more specifically in the light-dependent reactions of photosynthesis, is that of a mobile electron carrier through the membrane of the
thylakoid Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a #Membrane, thylakoid membrane surrounding a #Lumen, ...
. Plastoquinone is reduced when it accepts two electrons from photosystem II and two hydrogen cations (H+) from the stroma of the chloroplast, thereby forming plastoquinol (PQH2). It transfers the electrons further down the
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...
to plastocyanin, a mobile, water-soluble electron carrier, through the cytochrome ''b''6''f'' protein complex. The cytochrome ''b''6''f'' protein complex catalyzes the electron transfer between plastoquinone and plastocyanin, but also transports the two protons into the lumen of thylakoid discs. This proton transfer forms an electrochemical gradient, which is used by ATP synthase at the end of the light dependent reactions in order to form ATP from ADP and Pi.


Within photosystem II

Plastoquinone is found within photosystem II in two specific binding sites, known as QA and QB. The plastoquinone at QA, the primary binding site, is very tightly bound, compared to the plastoquinone at QB, the secondary binding site, which is much more easily removed. QA is only transferred a single electron, so it has to transfer an electron to QB twice before QB is able to pick up two
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
s from the stroma and be replaced by another plastoquinone molecule. The protonated QB then joins a pool of free plastoquinone molecules in the membrane of the thylakoid. The free plastoquinone molecules eventually transfer electrons to the water-soluble plastocyanin so as to continue the light-dependent reactions. There are additional plastoquinone binding sites within photosystem II (QC and possibly QD), but their function and/or existence have not been fully elucidated.


Biosynthesis

The p-hydroxyphenylpyruvate is synthesized from
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
, while the solanesyl diphosphate is synthesized through the MEP/DOXP pathway. Homogentisate is formed from p-hydroxyphenylpyruvate and is then combined with solanesyl diphosphate through a
condensation reaction In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
. The resulting intermediate, 2-methyl-6-solanesyl-1,4-benzoquinol is then methylated to form the final product, plastoquinol-9. This pathway is used in most photosynthetic organisms, like
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
and plants. However,
cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
appear to not use homogentisate for synthesizing plastoquinol, possibly resulting in a pathway different from the one shown below.


Derivatives

Some derivatives that were designed to penetrate mitochondrial cell membranes ( SkQ1 (plastoquinonyl-decyl-triphenylphosphonium), SkQR1 (the rhodamine-containing analog of SkQ1), SkQ3) have anti-oxidant and protonophore activity. SkQ1 has been proposed as an anti-aging treatment, with the possible reduction of age-related vision issues due to its antioxidant ability. This antioxidant ability results from both its antioxidant ability to reduce reactive oxygen species (derived from the part of the molecule containing plastoquinonol), which are often formed within mitochondria, as well as its ability to increase ion exchange across membranes (derived from the part of the molecule containing cations that can dissolve within membranes). Specifically, like plastoquinol, SkQ1 has been shown to scavenge superoxides both within cells (in vivo) and outside of cells (in vitro). SkQR1 and SkQ1 have also been proposed as a possible way to treat brain issues like Alzheimer's due to their ability to potentially fix damages caused by
amyloid beta Amyloid beta (Aβ, Abeta or beta-amyloid) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid-beta precursor prot ...
. Additionally, SkQR1 has been shown as a way to reduce the issues caused by brain trauma through its antioxidant abilities, which help prevent cell death signals by reducing the amounts of reactive oxygen species coming from mitochondria.


References

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External links


Plastoquinones
History, absorption spectra, and analogs. Photosynthesis Light reactions 1,4-Benzoquinones Meroterpenoids