Calorimetric analysis of AdcR and its interactions with zinc(II) and DNA

Zinc(II) ions play critical roles in all known life as structurally important stabilizing ions in proteins, catalytically active metals in enzymes, and signaling agents impacting physiological changes. To maintain homeostasis, the intracellular concentration of zinc(II) is strictly controlled by a family of metal-regulatory proteins in both prokaryotic and eukaryotic organisms. In S. pneumoniae, there are two proteins that share responsibility for Zn²⁺ homeostasis, one of them is the Adhesin Competence Repressor (AdcR) and it binds to a specific double-stranded DNA binding domain (dsDNA). AdcR has been structurally characterized containing two zinc(II) metal centers per monomeric unit. Here we report data collected from differential scanning calorimetry (DSC) experiments aimed to measure the structural stability of AdcR, the fully complimented Zn₂AdcR complex, and the protein/DNA complex Zn₂AdcR/dsDNA. Thermograms collected from DSC experiments yielded endothermic unfolding events for AdcR, Zn₂AdcR, and Zn₂AdcR/dsDNA complex at 55.6, 70.2, and 56.6 °C, respectively. A non-two state unfolding model best fits the data, giving ΔH terms associated with these thermal unfolding events of 5.1, 7.1, and 4.9 kcal/mol. These data allow for the development of a thermodynamic cycle connecting both zinc(II) and DNA binding to AdcR. Furthermore, pairing this newly reported data with known association constants for zinc(II) and DNA binding allowed for the generation of thermodynamic profiles for both zinc(II) binding to AdcR and Zn₂AdcR binding to DNA, which show both are decisively entropy-driven processes.