The Irving-Williams series and the 2-His-1-carboxylate facial triad: a thermodynamic study of Mn2+, Fe2+, and Co2+ binding to taurine/α-ketoglutarate dioxygenase (TauD)

Taurine/alpha-ketoglutarate (alpha KG) dioxygenase (TauD) is an E. coli nonheme Fe2+- and alpha KG-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 Fe2+ 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 Mn2+ and Co2+ binding to TauD, and these values are compared to the Fe2+ data reported earlier Henderson et al. (Inorg Chem 54: 2278-2283, 2015). The buffer-independent binding constants (K) were measured to be 1.6 x 10(6), 2.4 x 10(7), and 1.7 x 10(9), for Mn2+, Fe2+, and Co2+, respectively. The corresponding Delta GA degrees values were calculated to be - 8.4, - 10.1, and - 12.5 kcal/mol, respectively. The metal-binding enthalpy changes (Delta 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.