Mercuric ion binding abilities of MerP variants containing only one cysteine.

Using site-directed mutagenesis, Cys14 and Cys17 in MerP were replaced in turn by serine or alanine. All four variants were purified and partially characterized. The The mutant proteins all had one reactive thiol group left. In the absence of external thiols, the protein variants bound between two and four Hg2+, but unlike non-mutant MerP, none of the variants could bind Hg2+ when external thiol was added. This loss of the ability to specifically bind one Hg2+ per protein molecule shows that both cysteine residues 14 and 17 are necessary for binding of Hg2+ when there is competition from other thiol groups.