plastoquinone9

Plastoquinone-9, often abbreviated PQ-9, is a form of plastoquinone distinguished by a nine-unit polyprenyl (isoprenoid) side chain. It is the dominant plastoquinone species in the thylakoid membranes of many green plants and algae, where its hydrophobic tail anchors the molecule within the lipid bilayer and positions it for participation in photosynthetic electron transport.

PQ-9 serves as an electron carrier that shuttles electrons from photosystem II to the cytochrome b6f complex. In this role, the molecule cycles between oxidized (PQ) and reduced (PQH2) states. Two electrons and two protons are transferred to PQ at the QB site on the stromal side of PSII to form PQH2, which then diffuses within the membrane to the b6f complex. Oxidation of PQH2 at the b6f site releases protons into the thylakoid lumen and transfers electrons further along the chain.

Structurally, plastoquinone-9 consists of a benzoquinone head group linked to a long hydrophobic polyprenyl tail with nine isoprene units. The tail length influences membrane localization and mobility, affecting how efficiently PQ-9 can participate in electron transport under varying light conditions. In the thylakoid membrane, PQ-9 participates in the Q-cycle and contributes to the proton motive force that drives ATP synthesis.

Biosynthesis of plastoquinone-9 occurs in plastids through the integration of benzenoid precursors with isoprenoid side chains produced by the plastid isoprenoid pathway. The exact tail length is determined by specific polyprenyl synthases. PQ-9 is widespread in higher plants and many algae; related plastoquinones with different tail lengths occur in other species.

In research, PQ-9 is a key component of the photosynthetic electron transport system and serves as a useful indicator of thylakoid membrane redox state and photosynthetic efficiency.