Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Abstract

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb²⁺ and Zn²⁺ binding to metallothionein-3 (MT-3). Pb²⁺ binds to zinc-replete Zn₇MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn₇MT-3 and Pb₇MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb²⁺ binding to both apo MT-3 and Zn₇MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.