UV Resonance Raman Characterization of a Substrate Bound to Human Indoleamine 2,3-Dioxygenase 1

Abstract

Human indoleamine 2,3-dioxygenase 1 (IDO) is a heme enzyme that catalyzes the first reaction of the main metabolic pathway of L-tryptophan (Trp) to produce N-formylkynurenin. The reaction involves cleavage of the C 2 ¼C 3 bond in the Trp indole ring and insertion of two atomic oxygens from the iron-bound O 2 into the indole 2 and 3 position. For establishment of the chemical mechanism of this unique enzymatic reaction, it is necessary to determine the conformation and electronic state of the substrate Trp bound to IDO. In this study, we measured the ultraviolet resonance Raman spectra of IDO in the presence of Trp to detect the vibrational modes of the substrate Trp. We compared the ultraviolet resonace Raman spectra of Trp in a ternary complex (Trp-bound cyanide enzyme) and a binary complex (Trp-bound reduced enzyme) of IDO with that of free Trp in solution and found that binding to IDO influences the conformation of Trp, resulting in similar changes in the two complexes, especially around the C 3 -C b bond. However, the presence of the diatomic ligand at the heme sixth coordination site in the ternary complex significantly alters the mobility and electronic structure of Trp, most likely resulting in the C 2 ¼C 3 bond cleavage in the enzymatic reaction.

Modeling Tryptophan/Indoleamine 2,3-Dioxygenase with Heme Superoxide Mimics: Is Ferryl the Key Intermediate?

“…Accordingly, the mechanistic viewpoint has rapidly evolved, leading to proposals that differentiate the initial heme superoxide attack on Trp between (1) an electrophilic or (2) a radical addition (Figure C). ,− Intriguingly, both of these proposed mechanisms converge at a common intermediate step: the formation of a ferryl intermediate and the indole epoxide (Figure C). Lending credence, a ferryl intermediate has been spectroscopically observed during human IDO turnover, where the formation and decay of a ferryl-based Fe=O resonance Raman feature has been evidenced at 799 cm –1 . − A handful of synthetic TDO/IDO models exist, − out of which, only a few have utilized synthetic heme mimics. − Nevertheless, insights into the identity and properties of the active metal oxidant, key reaction intermediates, and/or pivotal mechanistic events are severely lacking. − …”