A fusion protein was engineered from maltose binding protein (pmal) and human metallothionein (MT). The recombinant protein (pmal-MT) expressed in E. coli was purified, and immobilized on Chitopearl™ resin. As expected from a tertiary structure of metallothionein, the pmal-MT ligand adsorbed 12.1 cadmium molecules per one molecule of the ligand at pH 5.2. We have found that the pmal-MT ligand also bound 26.6 gallium molecules per one molecule of the ligand at pH 6.5. Adsorption isotherms for the both ions were correlated by Langmuir-type equation. Two types of binding sites have been elucidated based on HSAB (hard and soft acid and base) theory: gallium ion specifically binds to amino acid residues containing oxygen and nitrogen atoms, while cadmium ion binds to specific binding sites formed by multiple cysteine residues. The pmal-MT protein bound these metals in the concentration range of 0.2 - 1.0 mM, and the bound metal ions could be eluted under relatively mild condition (pH 2.0). The pmal-MT Chitopearl™ resin was stable and could be used repeatedly without loss of binding activity. Thus, this new protein-based ligand would be useful for recovery of toxic heavy metals and/or valuable metal ions from various aqueous solutions.
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