Polypeptides for heavy-metal biosorption: capacity and specificity of two heterogeneous MerP proteins

MerP protein possesses a highly conserved domain with two cysteine residues for Hg2+ binding. The MerP protein originated from mer operon of Gram-positive bacterium Bacillus cereus RC607 possesses adjoining two cysteines while that of Gram-negative bacterium Pseudomonas sp. K-62 contains two cysteines separated by spacing two other residues. Both heterogeneous merP genes were cloned and over-expressed in Escherichia coli BL21 hosts. The resulting recombinant strains exhibited a six- to eightfold increase in Hg2+ resistance and an 10% increase in Hg2+ adsorption capacity. The merP over-expressed strain harboring Gram-positive merP had 142, 84 and 33% increase for Cu2+, Cd2+ and Pb2+ adsorption capability, respectively, over that of merP-free host cells. The strain carrying Gram-negative merP also increased 47, 55 and 12% for Cu2+, Cd2+ and Pb2+ adsorption, respectively. Multi-metal biosorption experiments showed that the affinity of metal biosorption by the recombinant strains decreased in the order of Cu>Pb>Cd. Peptides containing the amino acid sequences of metal-binding motif for both heterogeneous MerP proteins were chemically synthesized and covalently immobilized on Celite carriers to examine their metal-binding ability. For Bacillus MerP-originated peptides, the binding capacity was 0.72, 0.45 and 0.36 mol/mol peptide for Cu, Cd and Pb, respectively, while the capacity was 0.51 0.45 and 0.31 mol/mol peptide for Cu, Cd and Pb, respectively, for the peptide containing Pseudomonas MerP metal-binding motif. These results may show that MerP with adjoining cysteines seems to be more effective in binding metals than that with two separated cysteines.