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.
Benjamin
B
Weber
Elena
E
Nickel
Michael
M
Horn
Karin
K
Nienhaus
§Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
G Ulrich
GU
Nienhaus
L-tryptophan; flash photolysis; heme enzyme; ligand binding; substrate inhibition