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. (C) 2003 Elsevier Inc. All rights reserved.