Plastoquinones

Plastoquinones are a class of lipid-soluble quinones that function as mobile electron carriers in the photosynthetic electron transport chain of chloroplasts in plants, as well as in algae and cyanobacteria. They reside in the thylakoid membranes and participate in shuttling electrons between photosystem II and the cytochrome b6f complex as part of the plastoquinone pool. The molecule consists of a benzoquinone headgroup linked to a long hydrophobic polyisoprenoid tail that anchors it in the lipid membrane. The tail length varies among species, with plastoquinone-9 (PQ-9) commonly reported in higher plants; other organisms may contain plastoquinones with longer or shorter tails (e.g., PQ-10).

In the photosynthetic electron transport chain, plastoquinone cycles between oxidized (PQ) and reduced (plastoquinol, PQH2) forms. Photosystem II reduces PQ to PQH2, which then diffuses within the membrane and is reoxidized to PQ by the cytochrome b6f complex in the Q-cycle. This oxidation is coupled to proton translocation across the thylakoid membrane, contributing to the proton motive force that drives ATP synthesis.

Beyond its role as an electron carrier, the plastoquinone pool can participate in photoprotection and antioxidant defense, helping to stabilize thylakoid membranes under stress by scavenging reactive oxygen species. Plastoquinones are specific to photosynthetic organisms and are not found in non-photosynthetic cells. They are related to ubiquinones in broader quinone biology but are distinct in their plant/algal/cyanobacterial context.