Interconversion of two oxidized forms of taurine/α-ketoglutarate dioxygenase, a non-heme iron hydroxylase: Evidence for bicarbonate binding

Taurine/α-ketoglutarate (αKG) dioxygenase, or TauD, is a mono-nuclear non-heme iron hydroxylase that couples the oxidative decarboxylation of αKG to the decomposition of taurine, forming sulfite and aminoacetaldehyde. Prior studies revealed that taurine-free TauD catalyzes an O₂- and αKG-dependent self-hydroxylation reaction involving Tyr-73, yielding an Fe(III)-catecholate chromophore with a λ_max of 550 nm. Here, a chromophore (λ_max 720 nm) is described and shown to arise from O₂-dependent self-hydroxylation of TauD in the absence of αKG, but requiring the product succinate. A similar chromophore rapidly develops with the alternative oxidant H₂O₂. Resonance Raman spectra indicate that the ≈700-nm chromophore also arises from an Fe(III)-catecholate species, and site-directed mutagenesis studies again demonstrate Tyr-73 involvement. The ≈700-nm and 550-nm species are shown to interconvert by the addition or removal of bicarbonate, consistent with the αKG-derived CO₂ remaining tightly bound to the oxidized metal site as bicarbonate. The relevance of the metal-bound bicarbonate in TauD to reactions of other members of this enzyme family is discussed.