The Thermodynamics of Divalent Metals Binding to Taurine/α-Ketoglutarate-Dependent Oxygenase (TAUD)

Abstract

Taurine/α-ketoglutarate (αKG) dioxygenase (TauD) is a nonheme iron(II) and αKG dependent metalloenzyme, which catalyzes the hydroxylation of taurine leading to its decomposition into aminoacetaldehyde and sulfite, where sulfite is a key sulfur containing metabolite in E. coli. The nonheme iron(II) center in TauD is formed from two histidine side chain residues and a glutamic acid coordinating to one face of the octahedral coordination geometry. This common metal binding motif has been termed the 2-His-1-carboxylate facial triad and is found in a number of nonheme manganese, iron, and cobalt containing proteins. Here we have focused our efforts to measure the thermodynamic driving forces that lead to formation of these bioinorganic centers in biology, by studying divalent metal ion coordination to TauD using isothermal titration calorimetry. Titrations of metal complexes into the metal-free (apo) TauD and the corresponding chelation experiments were performed under anaerobic environment.The thermodynamic terms associated with cobalt(II), iron(II), and manganese(II) binding to apoTauD were deconvoluted from complex experiments, where the pH and buffer independent binding constant (K) were measured to be 2.9 × 109, 2.4 × 107, and 9.8 × 105, respectively. (The corresponding ΔG values were calculated to be −12.8 kcal/mol, −10.1 kcal/mol, and −8.2 kcal/mol, respectively.) Interestingly the measured enthalpy changes for these binding events (ΔH) are −17.8 kcal/mol, −12.8 kcal/mol, and −12.2 kcal/mol, respectively. These data are fully consistent with the Irving-Williams series, which suggest there is increasing affinity for transition metal ions from left to right across the periodic table. However, it seems this the increasing affinity is derived from increasing favorability of both the related ΔH and ΔS terms.