Plastoquinones in photosynthesis

Abstract

Several plastoquinones with different or modified side chains have been characterized in plant material: they are localized in the inner thylakoid membrane of the chloroplast. So far only plastoquinone-45 (PQ-45) has been identified as an obligatory functional component of the photosynthetic electron transport chain in chloroplasts between photosystem II and photosystem I. A special form (semiquinone) of PQ-45 acts as primary acceptor Q of photosystem II, a large pool of PQ-45 as electron buffer, interconnecting several electron transport chains. The rôle of PQ, in energy conservation (ATP formation) is of particular current interest. Owing to vectorial electron flow across the thylakoid membrane, plastoquinone is thought to be reduced on the outside and plastohydroquinone to be oxidized on the inside of the membrane. This results in a proton translocation across the membrane and a build-up of a proton motive force which drives ATP formation. Old and new plastoquinone antagonists are described and the relevance of inhibitor studies on the rôle of plastoquinone in electron flow and photophosphorylation is discussed. Open questions and current problems of the mechanism of plastoquinone/plastoquinol transport across the membrane - and of proton translocation connected to it - relevant for the mechanism of energy conservation in photosynthesis, are pointed out.