Substrate Inhibition in Human Indoleamine 2,3-Dioxygenase

Human indoleamine 2,3-dioxygenase (hIDO) catalyzes the oxidative cleavage of the L-tryptophan (l-Trp) pyrrole ring. Catalysis is inhibited at high substrate concentrations; mechanistic details of this observation are, however, still under debate. Using time-resolved optical spectroscopy, we have analyzed the dynamics of ternary complex formation between hIDO, l-Trp, and a diatomic ligand. The physiological ligand dioxygen (O₂) was replaced by carbon monoxide to exclude enzymatic turnover. Quantitative analysis of the kinetics reveals that the ternary complex forms whenever O₂ binds first, whereas an l-Trp substrate molecule arriving prior to O₂ in the active site causes self-inhibition. Bound l-Trp prevents the ligand from approaching the heme iron and, therefore, impedes formation of the catalytically active ternary complex