Plastoquinonide

Plastoquinone, often referred to as plastoquinone or plastoquinonide in its reduced form, is a benzoquinone derivative that plays a critical role in the photosynthetic electron transport chain of plants, algae, and cyanobacteria. Chemically, plastoquinone consists of a quinone ring (a six-membered carbon ring with two carbonyl groups) linked to a hydrophobic isoprenoid tail, typically containing six to ten isoprene units. This structure allows plastoquinone to function as both a mobile electron carrier and a lipid-soluble antioxidant within the thylakoid membrane of chloroplasts.

During photosynthesis, plastoquinone cycles between its oxidized (plastoquinone, PQ) and reduced (plastoquinol, PQH₂) forms. In the light-dependent reactions, plastoquinone accepts electrons and protons from photosystem II (PSII), forming plastoquinol. Plastoquinol then diffuses within the thylakoid membrane to photosystem I (PSI) and the cytochrome b₆f complex, where it donates electrons back to the electron transport chain, regenerating plastoquinone in the process. This plastoquinone-plastoquinol pool is essential for establishing a proton gradient across the thylakoid membrane, driving ATP synthesis via ATP synthase.

Beyond its role in electron transport, plastoquinone contributes to photoprotection by scavenging reactive oxygen species (ROS) and participating in the non-photochemical quenching (NPQ) mechanism, which dissipates excess light energy as heat. Deficiencies in plastoquinone or its biosynthesis can impair photosynthetic efficiency and increase oxidative stress in photosynthetic organisms. Synthetic analogs of plastoquinone are also studied for potential applications in bioenergy and synthetic biology.