Plastoquinonides

Plastoquinonides are a class of quinone compounds primarily involved in the photosynthetic electron transport chain of plants, algae, and cyanobacteria. Structurally related to plastoquinone (PQ), plastoquinonides are reduced forms of plastoquinone, characterized by the presence of hydrogen atoms added to the quinone ring. Plastoquinone functions as a mobile electron carrier, shuttling electrons between photosystem II (PSII) and the cytochrome b₆f complex in the thylakoid membrane. Upon reduction, plastoquinone becomes plastoquinol (PQH₂), a fully reduced form that can donate electrons to the cytochrome b₆f complex, facilitating ATP synthesis via chemiosmosis.

The reduction of plastoquinone to plastoquinol occurs in the lumen of the thylakoid membrane, where it accepts electrons from plastoquinone pool intermediates. This process is coupled with proton uptake, contributing to the proton gradient essential for ATP production. Plastoquinonides play a critical role in cyclic and non-cyclic photophosphorylation, ensuring efficient energy conversion during photosynthesis. Their redox properties make them vital for balancing electron flow and maintaining membrane potential.

In addition to their role in photosynthesis, plastoquinonides have been studied for their potential applications in bioenergetics and synthetic biology. Their ability to mediate electron transfer has led to investigations into their use in artificial photosynthetic systems and biofuel production. However, their biological significance remains primarily confined to oxygenic phototrophs, where they are indispensable for sustaining photosynthetic electron transport.