FIGURE 3.

CoQ is a cofactor for mitochondrial dihydroorotate dehydrogenase (DHODH). DHODH catalyzes the oxidation of dihydroorotate to orotate during the fourth step of the de novo biosynthesis of pyrimidine. The reaction is coupled to the reduction/oxidation of flavin mononucleotide (FMN) and CoQ. Orotate diffuses back to the cytosol to be converted by uridine monophosphate synthase (UPMS) to uridine 5-monophosphate (UMP), the precursor of all pyrimidine nucleotides. Preexisting uridine can be phosphorylated to UMP by uridine kinase (UCK), thereby bypassing the need for the DHODH-catalyzed step. See glossary for other abbreviations.