Cryoreduction of the NO-Adduct of Taurine: α-Ketoglutarate Dioxygenase (TauD) Yields an Elusive {FeNO}⁸ Species

The Fe(II)- and α-ketoglutarate (αKG)-dependent enzymes are a functionally and mechanistically diverse group of mononuclear nonheme-iron enzymes that activate dioxygen to couple the decarboxylation of αKG, which yields succinate and CO₂, to the oxidation of an aliphatic C-H bond of their substrates. Their mechanisms have been studied in detail by a combination of kinetic, spectroscopic, and computational methods. Two reaction intermediates have been trapped and characterized for several members of this enzyme family. The first intermediate is the C-H-cleaving Fe(IV)-oxo complex, which exhibits a large deuterium kinetic isotope effect on its decay. The second intermediate is a Fe(II):product complex. Reaction intermediates proposed to occur before the Fe(IV)-oxo intermediate do not accumulate and therefore cannot be characterized experimentally. One of these intermediates is the initial O₂ adduct, which is a {FeO₂}⁸ species in the notation introduced by Enemark and Feltham. Here, we report spectroscopic and computational studies on the stable NO-adduct of taurine:αKG dioxygenase (TauD), termed TauD-{FeNO}⁷, and its one-electron reduced form, TauD-{FeNO}⁸. The latter is isoelectronic with the proposed O ₂ adduct and was generated by low-temperature γ-irradiation of TauD-{FeNO}⁷. To our knowledge, TauD-{FeNO}⁸ is the first paramagnetic {FeNO}⁸ complex. The detailed analysis of experimental and computational results shows that TauD-{FeNO}⁸ has a triplet ground state. This has mechanistic implications that are discussed in this Article. Annealing of the triplet {FeNO}⁸ species presumably leads to an equally elusive {FeHNO}⁸ complex with a quintet ground state.