Transition Metal Homeostasis

Abstract

This chapter focuses on transition metals. All transition metal cations are toxic-those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers. Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity (flux control), with control of gene expression playing only a minor role. If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.g., ABC transport systems or P-type ATPases. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled. The toxicity threshold of an ion is defined by its ability to produce radicals (copper, iron, chromate), to bind to sulfide and thiol groups (copper, zinc, all cations of the second and third transition period), or to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe(III) compounds, combined with the ability to cause dangerous Fenton reactions. This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm. Toxic metals like Cd2+ bind to thiols and sulfide, preventing assembly of iron complexes and releasing the metal from iron-sulfur clusters. In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. E. coli solves this problem probably by preventing copper uptake, combined with rapid efflux if the metal happens to enter the cytoplasm.

The Components of the Unique Zur Regulon of Cupriavidus metallidurans Mediate Cytoplasmic Zinc Handling

“…The constitutively synthesized cation diffusion facilitator (CDF) (TC2.A.4) family member ZitB (8,9) counteracts uptake by the export of surplus zinc. It is likely that a flux-controlled kinetic flow equilibrium is at the core of zinc homeostasis in this enterobacterium (10,11). Low zinc conditions result in the synthesis of the primary ABC transporter (TC3.A.1) ZnuABC, which is controlled by the Zur regulator of the Fur protein family (12)(13)(14)(15).…”

Synthesis and Antibacterial Activity of Metal(loid) Nanostructures by Environmental Multi-Metal(loid) Resistant Bacteria and Metal(loid)-Reducing Flavoproteins

“…In turn, silver is mainly found in the 1+ oxidation state [Ag (I)], and less commonly as [Ag (II)], [Ag (III)], or metallic silver (Ag 0 ) (Burriel et al, 2006 ). Finally, gold is found as metallic gold (Au 0 ) as well as in six other oxidation states, 1+ [Au (I)] and 3+ [Au (III)] being the most commonly found in nature (Nies and Grass, 2009 ).…”

Direct and Indirect Inhibition of Salmonella Peptide Deformylase by Nitric Oxide

“…divalent metal ions follow the Irving-Williams series (12). Transition metal toxicity is prevented by regulated mechanisms of metal uptake, sequestration, and efflux (8,13,14). To prevent metals with a high binding affinity such as zinc from inappropriately binding to sites intended for more weakly binding metals, specific metallochaperones may be employed, and metal availability may be regulated both spatially and temporally (14,15).…”