The Irving–Williams series and the 2-His-1-carboxylate facial triad: a thermodynamic study of Mn 2+ , Fe 2+ , and Co 2+ binding to taurine/α-ketoglutarate dioxygenase (TauD)

Taurine/α-ketoglutarate (αKG) dioxygenase (TauD) is an E. coli nonheme Fe 2+ - and αKG-dependent metalloenzyme that catalyzes the hydroxylation of taurine, leading to the production of sulfite. The metal-dependent active site in TauD is formed by two histidine and one aspartate that coordinating to one face of an octahedral coordination geometry, known as the 2-His-1-carboxylate facial triad. This motif is found in many nonheme Fe 2+ proteins, but there is limited information on the thermodynamic parameters that govern metal-ion binding to this site. Here, we report data from calorimetry and related biophysical techniques 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 54: 2278–2283, 2015 ). The buffer-independent binding constants ( K ) were measured to be 1.6 × 10 6 , 2.4 × 10 7 , and 1.7 × 10 9 , for Mn 2+ , Fe 2+ , and Co 2+ , respectively. The corresponding Δ G ° values were calculated to be − 8.4, − 10.1, and − 12.5 kcal/mol, respectively. The metal-binding enthalpy changes (Δ H ) for these binding events are − 11.1 (± 0.1), − 12.2 (± 0.1), and − 16.0 (± 0.6) kcal/mol, respectively. These data are fully consistent with the Irving–Williams series, which show an increasing affinity for transition metal ions across the periodic table. It appears that the periodic increase in affinity, however, is a result of a complicated summation of enthalpy terms (including favorable metal-ion coordination processes and unfavorable ionization events) and related entropy terms.