Different Behavior of the Histidine Residue toward Cadmium and Zinc in a Cadmium-Specific Metallothionein from an Aquatic Fungus

Metallothioneins (MTs) are a large superfamily of ubiquitous cysteine-rich metalloproteins with main functions in metal ion homeostasis and detoxification. Neclu_MT1 is a metallothionein from the aquatic fungus Heliscus lugdunensis and so far the only known MT that is solely induced by CdII but not by ZnII or copper ions. In addition to eight cysteine residues, Neclu_MT1 also contains a less common single C-terminal histidine residue. To better understand the role of this histidine residue in metal ion binding, for the first time, potentiometric pH titrations are applied, revealing insights into the protonation and metal ion binding processes. Additional studies with absorption and NMR spectroscopy complement the finding that while the histidine residue is not crucial for the overall metal binding capacity, it does serve as a ligand in the ZnII but not in the CdII form of the protein.