Dihydroorotate dehydrogenase (DHOD) and Dihydropyrimidine dehydrogenase (DHPD) FMN-binding domain. DHOD catalyzes the oxidation of (S)-dihydroorotate to orotate. This is the fourth step and the only redox reaction in the de novo biosynthesis of UMP, the precursor of all pyrimidine nucleotides. DHOD requires FMN as co-factor. DHOD divides into class 1 and class 2 based on their amino acid sequences and cellular location. Members of class 1 are cytosolic enzymes and multimers while class 2 enzymes are membrane associated and monomeric. The class 1 enzymes can be further divided into subtypes 1A and 1B which are homodimers and heterotetrameric proteins, respectively. DHPD catalyzes the first step in pyrimidine degradation: the NADPH-dependent reduction of uracil and thymine to the corresponding 5,6-dihydropyrimidines. DHPD contains two FAD, two FMN and eight [4Fe-4S] clusters, arranged in two electron transfer chains that pass its homodimeric interface twice. Two of the Fe-S clusters show a hitherto unobserved coordination involving a glutamine residue.
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