Thermodynamics of zinc binding to rhodanese

The binding of zinc ion (Zn 2+) to rhodanese at two pH values was studied by microcalorimetry and the free energy, enthalpy, and entropy changes determined. Binding exhibited rather large endothermic enthalpy changes quite similar to those observed for zinc-model compound interactions. The large positive entropy changes which accompany binding appear to be a feature common to Zn 2+-apocarbonic anhydrase systems as well. The correlations between Zn 2+ interaction with model compounds resembling protein side chains and the thermodynamic values obtained for Zn 2+-protein interactions suggest that endothermic enthalpies of binding should commonly be observed under slightly acidic to basic conditions. It is found that commercial rhodanese binds Zn 2+ with moderate to weak affinity by a process that is entropy driven much like that of other Zn 2+-protein interactions.