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

摘要

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