| Dihydroorotate dehydrogenase, DHODH, is a flavin mononucleotide-dependent flavoprotein anchored to the outer surface of the inner mitochondrial membrane, where an N-terminal bipartite sequence combining a mitochondrial targeting signal and a membrane stop-transfer sequence governs its import and anchoring, while the catalytic core forms an alpha/beta-barrel domain housing the FMN cofactor and dihydroorotate binding site. DHODH catalyzes the fourth and rate-limiting step of de novo pyrimidine biosynthesis, oxidizing dihydroorotate to orotate while reducing its bound FMN to FMNH2, and this reduced flavin is then reoxidized by transferring electrons directly to ubiquinone within the inner mitochondrial membrane, physically coupling pyrimidine synthesis to the mitochondrial electron transport chain rather than operating as an isolated cytosolic reaction; the resulting electrons feed forward through ubiquinol-cytochrome c oxidoreductase and cytochrome c oxidase, contributing to the mitochondrial electrochemical gradient. Because ubiquinone serves as its direct electron acceptor, DHODH activity depends on a functional respiratory chain, and this same ubiquinone-coupling reaction positions DHODH as a source of regenerated reduced coenzyme Q10, a lipophilic antioxidant that scavenges lipid peroxyl radicals and suppresses mitochondrial lipid peroxidation, placing DHODH alongside GPX4 and FSP1 as a parallel, mitochondria-specific defense system against ferroptotic cell death; impairing DHODH activity removes this antioxidant regeneration function and sensitizes mitochondria to lipid peroxidation-driven death. In actively proliferating T and B lymphocytes, DHODH-dependent pyrimidine synthesis is essential to sustain nucleotide demand, and pharmacological inhibition of DHODH by teriflunomide interferes with oxidative phosphorylation and aerobic glycolysis in activated T cells specifically through functional inhibition of respiratory chain complex III, with this metabolic disruption preferentially suppressing proliferation of high-affinity T cell clones and altering T cell receptor repertoire diversity in patients with relapsing-remitting multiple sclerosis. DHODH contains a hydrophobic tunnel lined with residues including Gln47, His56, and Arg136, which allows ubiquinone to access the FMN cofactor, and this same tunnel is competitively occupied by teriflunomide, which inhibits DHODH with nanomolar potency by directly competing with ubiquinone. |
