The thermodynamic properties of Fe2+ binding to the 2-His-1-carboxylate facial triad in a-ketoglutarate/taurine dioxygenase (TauD) were explored using isothermal titration calorimetry. Direct titrations of Fe2+ into TauD and chelation experiments involving the titration of ethylenediaminetetraacetic acid into Fe2+-TauD were performed under an anaerobic environment to yield a binding equilibrium of 2.4 (+/- 0.1) x 10 (7) (K-d = 43 nM) and a Delta G degrees value of -10.1 (+/- 0.03) kcal/mol. Further analysis of the enthalpy/entropy contributions indicates a highly enthalpic binding event, where Delta H = -11.6 (+/- 0.3) kcal/mol. Investigations into the unfavorable entropy term led to the observation of water molecules becoming organized within the Fe2+-TauD structure.