Biosorption of Cd2+ and Zn2+ by cell surface-engineered Saccharomyces cerevisiae

The carboxyl-terminal part of sexual adhesion glycoprotein α-agglutinin (AGα1Cp) was employed as an anchoring domain for surface display of short metal binding peptides of sequences N-Ser-(Gly-Cys-Gly-Cys-Pro-Cys-Gly-Cys)2-Gly-C (CP2 peptide) and N-Ser-(Gly-His-His-Pro-His)3-Gly-C (HP3 peptide) in the cell wall of potential biosorbent Saccharomyces cerevisiae. The constructed genetic fusions were efficiently expressed and found covalently attached to cell wall glucan and both O- and N-glycosylated. As a consequence of AGα1Cp overexpression the nitrogen content of cell wall increased by 30%, which was reflected by increased sorption capacity of both isolated cell walls and intact cells. Compared to parental cells, the surface display of mere anchoring domain doubled the amount of Cd2+ and Zn2+ adsorbed by modified biomass from solutions containing 10 to 200 μM metal ions. Displayed CP2 and HP3 sequences showed selectivity towards Cd2+ and Zn2+, respectively. The fusion of CP2 peptide to AGα1Cp sequence increased Cd2+ sorption by 30%, whilst accumulation of Zn2+ remained unaffected. The HP3 sequence, shown to form 3 high-affinity Zn2+ binding sites with apparent dissociation constant of 1.2×10−7, increased capability of engineered strain to adsorb Zn2+ by 20% but did not contribute to Cd2+ sorption. The Scatchard analysis of metal adsorption data suggest that the expression of AGα1Cp variants in S. cerevisiae increased the number of high-affinity covalent ligand–metal interactions, thereby enhancing metal recovery at low external concentrations. In addition, glycosylation of anchoring domain by polymannans, presumably phosphorylated, was likely to increase the ion-exchange capacity of modified biosorbent, which became effective at higher metal concentrations.