ABSTRACT
In this chapter the mechanisms of resistance to ions of the heavy metals zinc, cadmium, lead, copper, arsenic, and antimony in bacteria will be described. In addition, the pathways of arsenical resistance in the prokaryote Saccharomyces cerevisiae will be discussed. Although often grouped as heavy metals, the ions of these metals are better characterized as soft metal ions, which are those with high polarizing power (a large ratio of ionic charge to the radius of the ion), in contrast to the hard metal ions of Groups I and II such as Na+ and Ca2+. This distinction between hard and soft metals is important biologically. When considering how these metals interact with biological molecules such as proteins, hard metals most frequently bind to proteins weakly through ionic interactions with functional groups such as carboxylates of glutamate or aspartate residues. In contrast, soft metals form much stronger, nearly covalent, bonds with functional groups such as the thiolates of cysteine residues and the imidazolium nitrogens of histidine residues. These strong interactions with proteins account for much of the biological toxicity of soft metals.
