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-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, where the hydrophobic tails are usually found.

Plastoquinol, the reduced form, also functions as an antioxidant by reducing reactive oxygen species, some produced from the photosynthetic reactions, that could harm the cell membrane. One example of how it does this is by reacting with superoxides to form hydrogen peroxide and plastosemiquinone.

Role in photosynthesis

The structure of photosystem II is shown above, with the flow of electrons detailed by the red arrows. Plastoquinone binding sites Q<sub>A</sub> and Q<sub>B</sub> are included in this flow of electrons, with plastoquinol leaving Q<sub>B</sub> to participate in the next step of the light-dependent reactions.|thumb

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. Q<sub>A</sub> only transfers a single electron, so it has to transfer an electron to Q<sub>B</sub> twice before Q<sub>B</sub> is able to pick up two protons from the stroma and be replaced by another plastoquinone molecule. The protonated Q<sub>B</sub> then joins a pool of free plastoquinone molecules in the membrane of the thylakoid. 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). 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.

References

  • Plastoquinones History, absorption spectra, and analogs.