Calorimetric assessment of Fe(2+) binding to α-ketoglutarate/taurine dioxygenase: ironing out the energetics of metal coordination by the 2-His-1-carboxylate facial triad.

The thermodynamic properties of Fe(2+) binding to the 2-His-1-carboxylate facial triad in α-ketoglutarate/taurine dioxygenase (TauD) were explored using isothermal titration calorimetry. Direct titrations of Fe(2+) into TauD and chelation experiments involving the titration of ethylenediaminetetraacetic acid into Fe(2+)-TauD were performed under an anaerobic environment to yield a binding equilibrium of 2.4 (±0.1) × 10(7) (Kd = 43 nM) and a ΔG° value of -10.1 (±0.03) kcal/mol. Further analysis of the enthalpy/entropy contributions indicates a highly enthalpic binding event, where ΔH = -11.6 (±0.3) kcal/mol. Investigations into the unfavorable entropy term led to the observation of water molecules becoming organized within the Fe(2+)-TauD structure.