Analysis by isothermal titration calorimetry of Zn(II) binding to various apo-CzrAs and the apo-CzrA-DNA complex. (a) Representative titrations of Zn(II) binding to wild-type (WT), V66A, L68V and V66A/L68V CzrA dimers. (b) Thermodynamic parameters obtained for Zn(II) binding to CzrAs obtained from experiments like those shown in panel A. t, total, obtained by summing parameters obtained for the first (Zn1) and second (Zn2) Zn(II) binding steps. (c) Representative titrations of Zn(II) into CzrO complexes formed by wild-type (WT) and V66A/L68V CzrAs. These titrations correspond to the “top” and “bottom” of the heterotropic coupling equilibrium that defines this system. (d) Graphical illustration of the coupling energetics of wild-type vs. V66A/L68V CzrAs derived from the first (Zn1) and second (Zn2) binding steps. t, total. Conditions: 38 µM dimer, 3 mM NTA as a zinc competitor, or 38 µM dimer-DNA complex, with 1 mM NTA as competitor in 10 mM Hepes, 0.4 M NaCl, pH 7.0, 25.0 °C.