<p>Dihydroorotate dehydrogenase (DHOD), also known as dihydroorotate oxidase, catalyses the fourth step in de novo pyrimidine biosynthesis, the stereospecific oxidation of (S)-dihydroorotate to orotate, which is the only redox reaction in this pathway. DHODs can be divided into two mains classes: class 1 cytosolic enzymes found primarily in Gram-positive bacteria, and class 2 membrane-associated enzymes found primarily in eukaryotic mitochondria and Gram-negative bacteria [<cite idref="PUB00033267"/>].</p><p>The class 1 DHODs can be further divided into subclasses 1A and 1B, which differ in their structural organisation and use of electron acceptors. The 1A enzyme is a homodimer of two PyrD subunits where each subunit forms a TIM barrel fold with a bound FMN cofactor located near the top of the barrel [<cite idref="PUB00005052"/>]. Fumarate is the natural electron acceptor for this enzyme. The 1B enzyme, in contrast is a heterotetramer composed of a central, FMN-containing, PyrD homodimer resembling the 1A homodimer, and two additional PyrK subunits which contain FAD and a 2Fe-2S cluster [<cite idref="PUB00024613"/>]. These additional groups allow the enzyme to use NAD(+) as its natural electron acceptor.</p> <p>The class 2 membrane-associated enzymes are monomers which have the FMN-containing TIM barrel domain found in the class 1 PyrD subunit, and an additional N-terminal alpha helical domain [<cite idref="PUB00024104"/>, <cite idref="PUB00033268"/>]. These enzymes use respiratory quinones as the physiological electron acceptor.</p><p>This entry represents the FMN-binding subunit common to all classes of dihydroorotate dehydrogenase.</p>