X-ray structure and reaction mechanism of human indoleamine 2,3-dioxygenase

The oxidative cleavage of the pyrrole ring of indoleamines by the insertion of molecular oxygen is catalyzed by indoleamine 2,3-dioxygenase (IDO). The reaction involves the addition of both atoms of a molecule of oxygen to break the C2–C3 double bond in the indole moiety of the substrate. We analyzed the X-ray crystal structure of human IDO in complex with the ligand inhibitor 4-phenylimidazole and cyanide. IDO folds into two alpha-helical domains with the heme between them. The conserved Ala of the flexible loop in the heme distal side is in close proximity to the iron. A mutant analysis suggests that, unlike the heme-containing monooxygenases or peroxidases, no protein side chain of IDO is essential in dioxygen activation or proton abstraction. The characteristics of the IDO structure provide support for a reaction mechanism involving the abstraction of a proton from the substrate by iron-bound dioxygen.