Dihydroorotate dehydrogenase of Escherichia coli.

Abstract

Dihydroorotate dehydrogenase (DHOD) catalyzes the fourth reaction in the pathway for de novo synthesis of UMP and forms the 5,6-double bond of the pyrimidine base. In this reaction, two electrons and two protons are transferred from dihydroorotate to an electron acceptor that varies between different types of the enzyme. Sequence alignments have shown that all DHODs contain a polypeptide chain that is encoded by a pyrD gene. This polypeptide forms the catalytic core structure, folding into an ( / )8-barrel. The active site, which contains a tightly bound molecule of flavin mononucleotide (FMN), is formed by loops that protrude from the top of the barrel (e.g., ref. 1). The first half reaction, in which the enzyme is reduced and dihydroorotate is oxidized to orotate, is initiated by binding of dihydroorotate at the si-side of the isoalloxazine ring of FMN (2) and, after abstraction of a proton from the 5'-position of dihydroorotate by a cysteine or a serine residue in the enzyme, a hydride ion is transferred to FMN from the 6-position of the substrate (3,4). The first half reaction is common to all DHODs, but different types of DHODs deviate from each other in quaternary structure, subcellular location, and use of electron acceptors to reoxidize the reduced enzyme in a second half reaction (5).