Inhibitor binding in a class 2 dihydroorotate dehydrogenase causes variations in the membrane‐associated N‐terminal domain

Abstract

The flavin enzyme dihydroorotate dehydrogenase (DHOD; EC 1.3.99.11) catalyzes the oxidation of dihydroorotate to orotate, the fourth step in the de novo pyrimidine biosynthesis of UMP. The enzyme is a promising target for drug design in different biological and clinical applications for cancer and arthritis. The first crystal structure of the class 2 dihydroorotate dehydrogenase from rat has been determined in complex with its two inhibitors brequinar and atovaquone. These inhibitors have shown promising results as anti‐proliferative, immunosuppressive, and antiparasitic agents. A unique feature of the class 2 DHODs is their N‐terminal extension, which folds into a separate domain comprising two α‐helices. This domain serves as the binding site for the two inhibitors and the respiratory quinones acting as the second substrate for the class 2 DHODs. The orientation of the first N‐terminal helix is very different in the two complexes of rat DHOD (DHODR). Binding of atovaquone causes a 12 Å movement of the first residue in the first α‐helix. Based on the information from the two structures of DHODR, a model for binding of the quinone and the residues important for the interactions could be defined. His 56 and Arg 136, which are fully conserved in all class 2 DHODs, seem to play a key role in the interaction with the electron acceptor. The differences between the membrane‐bound rat DHOD and membrane‐associated class 2 DHODs exemplified by the Escherichia coli DHOD has been investigated by GRID computations of the hydrophobic probes predicted to interact with the membrane.