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Taurine/α-ketoglutarate (αKG) dioxygenase (TauD) is a nonheme Fe(2+) and αKG dependent metalloenzyme, which catalyzes the hydroxylation of taurine eventually leading to sulfite production in Escherichia coli. The metal-dependent active site in TauD is formed by two histidine and an aspartate side-chains coordinating to one face of the octahedral coordination geometry, known as the 2-His-1-carboxylate facial triad. This motif is found in many nonheme Fe(2+) proteins, yet limited information is known regarding the thermodynamic parameters that govern transition metal ions binding to this site. Here we report data associated with three cytosolic metal ions competing for this site. Isothermal titration calorimetry and related biophysical techniques were used to generate complete thermodynamic profiles of Mn(2+) and Co(2+) binding to TauD, and these values are compared to the Fe(2+) data reported earlier [Henderson et al., Inorg. Chem. 2015 54, 2278–2283]. The pH and buffer independent binding constant (K) were measured to be 1.1 × 10(6), 2.4 × 10(7), and 2.9 × 10(9), for Mn(2+), Fe(2+), and Co(2+), respectively. The corresponding ΔG values were calculated to be −8.1, −10.1, and −12.9 kcal/mol, respectively. The measured enthalpy changes for these binding events (ΔH) are −12.2 (± 0.1), −12.8 (± 0.1), and −17.8 (± 0.6) kcal/mol, respectively. These data are fully consistent with the Irving-Williams series, which shows an increasing affinity for transition metal ions from left to right across the periodic table. It appears that the periodic increase in affinity, however, is a result of increasing enthalpic favorability (ΔH) and variation in the penalty associated with entropy (ΔS) associated with these processes. |
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