Function of Quinones in Photosynthesis

Abstract

Publisher Summary The quinones that are commonly found localized in chloroplasts are of three major types: the plastoquinones, tocopherylquinones, and vitamin K. In each of these groups, there is more than one type present. Plastoquinone A has a side chain with nine monounsaturated isopentenyl (isoprenoid) units. Plastoquinone B has the same long side chain apparently with an extra double bond in the second isoprenoid unit from the ring. Plastoquinone C and D have the same long side chain as plastoquinone A with a hydroxyl group in the second isoprenoid unit from the ring. Some indication has been obtained by extraction or spectrophotometric studies that α-tocopherylquinone, vitamin K, and possibly other plastoquinones may have a role in photosynthesis, but these studies are still at an early stage. Although the presence of plastoquinone A is essential for photophosphorylation, there is no evidence that a quinoid derivative is involved as an initial activated molecule or in phosphate transfer. The presence of chromanol forms of plastoquinone A and α-tocopherylquinone in chloroplasts may indicate some function for quinones other than as a proton carrier; but at present, there is no evidence for special chromanol functions.