The First Step of the Dioxygenation Reaction Carried Out by Tryptophan Dioxygenase and Indoleamine 2,3-Dioxygenase as revealed by QM/MM Studies

L. CAPECE; A. LEWIS BALLESTER; D. BATABYAL; DI RUSSO; S.R. YEH; D.A. ESTRIN; M.A. MARTI

JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY

SPRINGER

Lugar: New York; Año: 2010 vol. 15 p. 811 - 823

0949-8257

Abstract Tryptophan dioxygenase (TDO) and indole-amine 2,3-dioxygenase (IDO) are two heme-containingenzymes which catalyze the conversion of L-tryptophan toN-formylkynurenine (NFK). In mammals, TDO is mostlyexpressed in liver and is involved in controlling homeo-static serum tryptophan concentrations, whereas IDO isubiquitous and is involved in modulating immuneresponses. Previous studies suggested that the first step ofthe dioxygenase reaction involves the deprotonation of theindoleamine group of the substrate by an evolutionarilyconserved distal histidine residue in TDO and the heme-bound dioxygen in IDO. Here, we used classical molec-ular dynamics and hybrid quantum mechanical/molecularmechanical methods to evaluate the base-catalyzedmechanism. Our data suggest that the deprotonation of theindoleamine group of the substrate by either histidine inTDO or heme-bound dioxygen in IDO is not energeticallyfavorable. Instead, the dioxygenase reaction can be initi-ated by a direct attack of heme-bound dioxygen on theC2=C3 bond of the indole ring, leading to a protein-sta-bilized 2,3-alkylperoxide transition state and a ferrylepoxide intermediate, which subsequently recombine togenerate NFK. The novel sequential two-step oxygenaddition mechanism is fully supported by our recentresonance Raman data that allowed identification of theferryl intermediate (Lewis-Ballester et al. in Proc NatlAcad Sci USA 106:17371–17376, 2009). The resultsreveal the subtle differences between the TDO and IDOreactions and highlight the importance of protein matrixin modulating stereoelectronic factors for oxygen activa-tion and the stabilization of both transition and interme-diate states.