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

Abstract

Tryptophan oxidation in biology has been recently implicated in a vast array of paramount pathogenic conditions in humans, including multiple sclerosis, rheumatoid arthritis, type-I diabetes, and cancer. This 2,3dioxygenative cleavage of the indole ring of tryptophan with dioxygen is mediated by two heme enzymes, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), during its conversion to Nformylkynurenine in the first and rate-limiting step of kynurenine pathway. Despite the pivotal significance of this enzymatic transformation, a vivid viewpoint of the precise mechanistic events is far from complete. A heme superoxide adduct is thought to be the active oxidant in both TDO and IDO, which, following O−O bond cleavage, presumably generates a key ferryl (Fe IV =O) reaction intermediate. This study, for the first time in model chemistry, demonstrates the potential of synthetic heme superoxide adducts to mimic the bioinorganic chemistry of indole dioxygenation by TDO and IDO, challenging the widely accepted categorization of these metal adducts as weak oxidants. Herein, an electronically divergent series of ferric heme superoxo oxidants mediates the facile conversion of an array of indole substrates into their corresponding 2,3-dioxygenated products, while shedding light on an unequivocally occurring, putative ferryl intermediate. The oxygenated indole products have been isolated in ∼31% yield, and characterized by LC-MS, 1 H and 13 C NMR, and FT-IR methodologies, as well as by 18 O 2(g) labeling experiments. Distinctly, the most electron-deficient superoxo adduct is observed to react the fastest, specifically with the most electron-rich indole substrate, underscoring the cruciality of electrophilicity of the heme superoxide moiety in facilitating the initial indole activation step. Comprehensive understanding of such mechanistic subtleties will benefit future attempts in the rational design of salient therapeutic agents, including next generation anticancer drug targets with amplified effectivity.

Synthetic heme dioxygen adducts: electronic structure and reactivity

“…very similar reactivity of dioxygen adduct of FeTPP and additionally observed the accumulation of the resulting ferryl-oxo species in the steady-state during the reaction, which resembled the nature of reaction intermediates proposed in the catalytic cycle of TDO [85]. They proposed that the electron-rich indole ring attacked the electrophilic ferric superoxide complex initially to form an intermediate epoxide species along with a ferryl-oxo species, which then decomposed to either porphyrin -μ-oxo dimer or bis imidazole porphyrin along with 2,3-oxygenated product of indole (Figure 11).…”

“…An erstwhile effort focused on understanding the electronic structure of these species, factors that affect Trends Trends in in Chemistry Chemistry Figure 11. Schematic representation of dioxygenase reaction by heme models for heme dioxygenase [85].…”

Proton-coupled electron transfer reactivities of electronically divergent heme superoxide intermediates: a kinetic, thermodynamic, and theoretical study

“…Indeed, our recent work marks the rst report where synthetic heme superoxide intermediates were shown to react with exogenously added indole substrates in the efficient modelling of tryptophan dioxygenation chemistry of indoleamine and tryptophan 2,3-dioxygenases. 22 Similarly, heme superoxide adducts that efficiently react with added acids (i.e., protons (H + )), reductants (i.e., electrons(e À )), and/or Hc donors are only a handful. 23 Intriguingly, Naruta, 24 Dey, 25 and their coworkers have presented unique examples of heme superoxide intermediates that react with intramolecular H + or Hc donors, ultimately giving rise to the corresponding heme hydroperoxo (Fe III -OOH) adduct.…”

“…Indeed, our recent work marks the first report where synthetic heme superoxide intermediates were shown to react with exogenously added indole substrates in the efficient modelling of tryptophan dioxygenation chemistry of indoleamine and tryptophan 2,3-dioxygenases. 22 Similarly, heme superoxide adducts that efficiently react with added acids ( i.e. , protons (H + )), reductants ( i.e.…”

Rejigging Electron and Proton Transfer to Transition between Dioxygenase, Monooxygenase, Peroxygenase, and Oxygen Reduction Activity: Insights from Bioinspired Constructs of Heme Enzymes

“…A series of ferric heme superoxo oxidants was reported to oxidize an array of indole substrates into their corresponding 2,3-dioxygenated products in solution ( Figure 13 B). 147 For the reasons described above, these transformations were not catalytic.…”