-!- This Class 2 dihydroorotate dehydrogenase enzyme contains FMN. -!- The enzyme is found in eukaryotes in the mitochondrial membrane, in cyanobacteria, and in some Gram-negative and Gram-positive bacteria associated with the cytoplasmic membrane. -!- The reaction is the only redox reaction in the de-novo biosynthesis of pyrimidine nucleotides. -!- The best quinone electron acceptors for the enzyme from bovine liver are ubiquinone-6 and ubiquinone-7, although simple quinones, such as benzoquinone, can also act as acceptor at lower rates. -!- Methyl-, ethyl-, tert-butyl and benzyl-(S)-dihydroorotates are also substrates, but methyl esters of (S)-1-methyl and (S)-3-methyl and (S)-1,3-dimethyldihydroorotates are not. -!- Class 1 dihydroorotate dehydrogenases use either fumarate (EC 1.3.98.1), NAD(+) (EC 1.3.1.14) or NADP(+) (EC 1.3.1.15) as electron acceptor. -!- Formerly EC 1.3.99.11.