Utilizing Isothermal Titration Calorimetry to Quantify the Thermodynamics of Cadmium Mimicry: A Comparison to Calcium

Abstract

Isothermal titration calorimetry (ITC) is a powerful calorimetric method which can determine all thermodynamic parameters (K, [Delta]H, [Delta]G, and [Delta]S) for a binding event in a single experiment. More recently, ITC has become popular for investigations of metal-ligand and metal-biomolecule interactions. However, these studies require carefully chosen experimental conditions and proper post-hoc analysis for valid conclusions to be made. Of particular importance is accounting for metal-buffer equilibria, however, these thermodynamic data are not always available in the literature. By utilizing well-defined ligands, ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), and accounting for the equilibria involved in metal ion interactions, ITC has been utilized to extract the thermodynamic parameters of previously undefined calcium and cadmium buffer interactions (Tris, Bis-Tris, MES, and MOPS). Utilizing these data, buffer and pH independent values were extracted from the ITC investigations of both calcium and cadmium binding to human cardiac troponin C, the regulatory protein that controls cardiac contraction. Data presented may have important implications for cadmium toxicity. The methodologies used for data extraction are meant to serve as a model for those who wish to utilize ITC for future metal binding investigations.