Investigating the influence of histidine residues on the metal ion binding ability of the wheat metallothionein γ-Ec-1 domain

While Zn(II) and Cd(II) have similar geochemical and environmental properties, their biological properties are distinctively different as Cd(II) ions have very limited metabolic significance and are mostly even toxic, while Zn(II) ions belong to the most essential micronutrients. One of the key proteins involved in intracellular Zn(II) and Cd(II) binding are metallothioneins (MTs), small cysteine-rich proteins ubiquitously found in many different organisms. In the past two decades, also MT sequences from diverse species that contain histidine residues have been found, and His-metal ion coordination has been shown. It is not clear, however, why in some MTs parts of the Cys residues are replaced by His, while most other MTs only contain Cys residues for metal ion binding. To address this question, we used the γ-domain of the early-cysteine labeled (Ec-1) metallothionein from common wheat as a model system because its enclosed M2Cys6 cluster represents the smallest metal-thiolate cluster possible with divalent metal ions. Based on the known three-dimensional structure of the γ-domain we set about to investigate the influence of a single Cys-to-His mutation on the structure and metal ion binding abilities of this domain. Combined data obtained by mass spectrometry, UV, as well as NMR spectroscopy suggest a preference for Zn(II) versus Cd(II) ions in the histidine containing binding site.