O2- and α-Ketoglutarate-Dependent Tyrosyl Radical Formation in TauD, an α-Keto Acid-Dependent Non-Heme Iron Dioxygenase

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Abstract
Taurine/α-ketoglutarate dioxygenase (TauD), a non-heme mononuclear Fe(II) oxygenase, liberates sulfite from taurine in a reaction that requires the oxidative decarboxylation of α-ketoglutarate (αKG). The lilac-colored αKG-Fe(II)TauD complex (λmax = 530 nm; ε530 = 140 M-1·cm-1) reacts with O2 in the absence of added taurine to generate a transient yellow species (λmax = 408 nm, minimum of 1600 M-1·cm-1), with apparent first-order rate constants for formation and decay of ∼0.25 s-1 and ∼0.5 min-1, that transforms to yield a greenish brown chromophore (λmax = 550 nm, 700 M-1·cm-1). The latter feature exhibits resonance Raman vibrations consistent with an Fe(III) catecholate species presumed to arise from enzymatic self-hydroxylation of a tyrosine residue. Significantly, 18O labeling studies reveal that the added oxygen atom derives from solvent rather than from O2. The transient yellow species, identified as a tyrosyl radical on the basis of EPR studies, is formed after αKG decomposition. Substitution of two active site tyrosine residues (Tyr73 and Tyr256) by site-directed mutagenesis identified Tyr73 as the likely site of formation of both the tyrosyl radical and the catechol-associated chromophore. The involvement of the tyrosyl radical in catalysis is excluded on the basis of the observed activity of the enzyme variants. We suggest that the Fe(IV) oxo species generally proposed (but not yet observed) as an intermediate for this family of enzymes reacts with Tyr73 when substrate is absent to generate Fe(III) hydroxide (capable of exchanging with solvent) and the tyrosyl radical, with the latter species participating in a multistep TauD self-hydroxylation reaction.