Dihydroorotate dehydrogenase

DHODH can vary in cofactor content, oligomeric state, subcellular localization, and membrane association. An overall sequence alignment of these DHODH variants presents two classes of DHODHs: the cytosolic Class 1 and the membrane-bound Class 2. In Class 1 DHODH, a basic cysteine residue catalyzes the oxidation reaction, whereas in Class 2, the serine serves this catalytic function. Structurally, Class 1 DHODHs can also be divided into two subclasses, one of which forms homodimers and uses fumarate as its electron acceptor, and the other which forms heterotetramers and uses NAD+ as its electron acceptor. This second subclass contains an addition subunit (PyrK) containing an iron-sulfur cluster and a flavin adenine dinucleotide (FAD). Meanwhile, Class 2 DHODHs use coenzyme Q/ubiquinones for their oxidant. This sequence is adjacent to a pair of α-helices, α1 and α2, which are connected by a short loop. Together, this pair forms a hydrophobic funnel that is suggested to serve as the insertion site for ubiquinone, in conjunction with the FMN binding cavity at the C-terminal. == Function ==

Human DHODH is a ubiquitous FMN flavoprotein. In bacteria (gene pyrD), it is located on the inner side of the cytosolic membrane. In some yeasts, such as in Saccharomyces cerevisiae (gene URA1), it is a cytosolic protein, whereas, in other eukaryotes, it is found in the mitochondria. It is also the only enzyme in the pyrimidine biosynthesis pathway located in the mitochondria rather than the cytosol. Mechanism In mammalian species, DHODH catalyzes the fourth step in de novo pyrimidine biosynthesis, which involves the ubiquinone-mediated oxidation of dihydroorotate to orotate and the reduction of FMN to dihydroflavin mononucleotide (FMNH2): : (S)-dihydroorotate + O2 \rightleftharpoons orotate + H2O2 Image:4,5-Dihydroorotic acid.svg|4,5-Dihydroorotic acid Image:Orotic acid.svg|Orotic acid. Note the double bond in the ring. The particular mechanism for the dehydrogenation of dihydroorotic acid by DHODH differs between the two classes of DHODH. Class 1 DHODHs follow a concerted mechanism, in which the two C–H bonds of dihydroorotic acid break in concert. Class 2 DHODHs follow a stepwise mechanism, in which the breaking of the C–H bonds precedes the equilibration of iminium into orotic acid. == Inhibitors ==

• Brequinar • Teriflunomide • Leflunomide • S416 • Vidofludimus Calcium • Olorofim • Orludodstat ==Clinical significance==

Brequinar, a potent and selective inhibitor of the enzyme dihydroorotate dehydrogenase, has been shown to inhibit completely vaccinia virus in human cells. The immunomodulatory drugs teriflunomide and leflunomide have been shown to inhibit DHODH. Human DHODH has two domains: an alpha/beta-barrel domain containing the active site and an alpha-helical domain that forms the opening of a tunnel leading to the active site. Leflunomide has been shown to bind in this tunnel. Leflunomide is being used for treatment of rheumatoid and psoriatic arthritis, as well as multiple sclerosis. Mutations in this gene have been shown to cause Miller syndrome, also known as Genee-Wiedemann syndrome, Wildervanck-Smith syndrome or post-axial acrofacial dystosis. == Interactions ==

DHODH binds to its FMN cofactor in conjunction with ubiquinone to catalyze the oxidation of dihydroorotate to orotate. == References ==