Localization effect on the metal biosorption capability of recombinant mammalian and fish metallothioneins in Escherichia coli

In this study, we examined the expression of mammalian and fish metallothioneins (MTs) in Escherichia coli as a strategy to enhance metal biosorption efficiency of bacterial biosorbents for lead (Pb), copper (Cu), cadmium (Cd), and zinc (Zn). In addition, MT proteins were expressed in either the cytoplasmic or periplasmic compartment of host cells to explore the localization effect on metal biosorption. The results showed that MT expression led to a significant increase (5-210%) in overall biosorption efficiency (η_ads), especially for biosorption of Cd. The MT-driven improvement in metal biosorption relied more on the increase in the biosorption rates (r2, a kinetic property) than on the equilibrium biosorption capacities (q_max, a thermodynamic property), despite a 10-45% and 30-80% increase in q_max of Cd and Zn, respectively. Periplasmic expression of MTs appeared to be more effective in facilitating the metal-binding ability than the cytoplasmlic MT expression. Notably, disparity of the impacts on biosorption ability was observed for the origin of MT proteins, as human MT (MT1A) was the most effective biosorption stimulator compared to MTs originating from mouse (MT1) and fish (OmMT). Moreover, the overall biosorption efficiency (η_ads) of the MT-expressing recombinant biosorbents was found to be adsorbate-dependent: the η_ads values decreased in the order of Cd > Cu > Zn > Pb.