Ubiquinone and Plastoquinone Metabolism in Plants

Abstract

The purpose of this chapter is to provide the tool for explanation of the localization of the plastoquinone (PQ) and Ubiquinone (UQ) biosynthetic machinery in the spinach cell. Identification of the ER/Golgi apparatus as the important locus of this process does not exclude involvement of other cellular compartments. The organization of the isoprenoid biosynthesis in plants might be far more complex (e.g., several enzymes have recently been assigned to the liverwort oil bodies). UQ and plastoquinone PQ function as carriers of electrons in the mitochondrial inner membrane and chloroplast thylacoids, respectively. In the reduced form, both UQ and PQ are shown to act as antioxidants in plant cells. Plastoquinone is also involved in the chlororespiratory pathway and serves as the effector of carotenoid biosynthesis. In addition, activation of specific kinases is the key factor to the redox status of the PQ pool. On the other hand, UQ is the obligatory cofactor of uncoupling proteins (responsible for the dissipation of the mitochondrial chemiosmotic gradient), producing heat rather than ATP; this observation has been made for bovine heart mitochondria, nevertheless, could be possibly generalized for plant uncoupling proteins described recently. Biosynthesis of PQ and UQ is studied in detail, and sequences of reactions leading from the isoprenoid precursors, namely prenyl diphosphate (solanesyl diphosphate for nine-isoprene-unit long side chain) and aromatic “head” (4-hydroxybenzoate and homogentizate for UQ and PQ, respectively) are also described in the chapter.