Biosynthesis of Ubiquinone

Abstract

Publisher Summary This chapter discusses the biosynthesis of ubiquinone. Ubiquinone serves as a lipid-soluble electron carrier in the membrane-bound electron transport chains of both prokaryotes and eukaryotes. Eighty-five percent of cell ubiquinon is located in the mitochondria or equivalent membrane system. The structure of ubiquinone and the dimensions of various ubiquinone homologs with respect to a typical membrane bilayer are diagrammatically represented in the chapter. During respiration, reducing equivalents entering the electron transport chain are transferred through a series of redox components from pyridine nucleotides and flavoproteins to ubiquinone, and then by the cytochromes to oxygen. Ubiquinone provides a linkage between the two-electron transport enzymes and the one-electron cytochrome system through semiquinone formation. Acetone extraction of the endogenous ubiquinone disrupts electron transport. The readdition of ubiquinone-9 to ubiquinone-depleted mitochondria restores both cyanide and antimycin sensitivity to levels approaching the unextracted systems. The first step in ubiquinone biosynthesis is the alkylation of 4-hydroxybenzoate by polyprenyl pyrophosphate. The initial stages of the biosynthesis of polyprenyl pyrophosphate are accomplished in the cytosol and the product is transported through the external mitochondrial membrane to the internal membrane where the alkylation reaction occurs.