Families of Soft-Metal-Ion-Transporting ATPases

Abstract

Life undoubtedly first arose in waters rich in metal ions.Since most metal ions are toxic in excess, it is reasonable to speculate that metal resistances probably evolved early, predating antibiotic resistances.This minireview focuses on transport ATPases that produce resistance to ions of the metals zinc, cadmium, lead, copper, silver, arsenic, and antimony in prokaryotes.Other aspects of transport and resistance to those metal ions have been reviewed recently (44,45,53,55,62) and will not be discussed here.Soft Lewis acids such as the transition metals Cu(I), Ag(I), Zn(II), and Cd(II) and the heavy metal Pb(II) have high polarizing power (a large ratio of ionic charge to the radius of the ion) and typically form strong bonds with soft Lewis bases (30).These include sulfur and nitrogen ligands such as cysteine and histidine residues in proteins.In contrast are the hard Lewis acids of groups 1a and 2a such as Na ϩ and Ca 2ϩ , which form ionic bonds with hard Lewis bases, for example the oxygen of protein carboxylates.The metalloids As(III) and Sb(III) of group 5a have properties similar to those of soft Lewis acids.For the purposes of this review, alkali and alkaline earth cations are termed hard metals, and transition metals, heavy metals, and metalloids are termed soft metals.Genes for resistance to inorganic salts of soft metals are found both on plasmids and in chromosomes (44,53,55).The physiological role of plasmid-encoded determinants is generally to confer resistance, while the role of chromosomally encoded systems may also include metal ion homeostasis.Copper and zinc ions are required for growth; there must be a fine balance between uptake and efflux to provide metal ion homeostasis.In the case of soft metals that are only toxic, specific uptake systems may not exist.Rather, those ions may be accumulated via other existing transport systems.