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Nickel and cobalt are essential micronutrients for many microorganisms and serve as enzyme cofactors that catalyze a diverse array of reactions. One complication is that high concentrations of these transition metal ions are toxic to cells, leading some prokaryotes to evolve sophisticated homeostatic mechanisms to regulate their transmembrane uptake or efflux. The biosynthesis of nickel and cobalt metalloenzymes requires the intracellular allocation of the metals to the appropriate apoproteins, often in an intricate process that involves the cooperative activity of accessory proteins. Here, we highlight the molecular physiology of nickel and cobalt cation metabolism in and summarize additional nickel- or cobalt-dependent processes and homeostatic mechanisms found in other microorganisms.

Metals such as zinc are required for life but can be toxic in excess. Other metals such as cadmium and lead have almost no known biological function, and can lead to cell damage and death even at low concentrations. Interestingly, all three metals are often recognized by the same gene regulators and membrane transporters. Therefore, an examination of the inherent chemical properties of these three metal ions is essential in understanding the basis of metal specificity displayed by target proteins responsible for metal homeostasis and resistance. The relationship between the chemical properties of these metals and similarities in structural responses they may elicit are discussed. The core elements regulating uptake, efflux, and sequestration of these metals are described and interpreted both biologically and chemically. Additional mechanisms aiding cell survival, such as precipitation of metal salts on the cell surface, are also mentioned.

One of the transition metals without a function in biological systems is silver. Silver interferes with the normal protein function of the organism and is extremely toxic because of its ability to bind to the metal binding sites in proteins. The use of silver as a biocide has long been known and today there is an increasing number of applications for silver, not only in hospitals but also in everyday life. In addition, silver resistant bacteria have been isolated from hospitals, silver mines, and silver-contaminated areas. There are different resistance mechanisms, as strains of appear to precipitate silver to remove it from the medium. In contrast, silver resistance in is plasmid-encoded and based on silver binding and export. Genome sequencing projects increasingly reveal the presence of this determinant, indicating a more widespread occurrence than previously expected.

Mercury has no beneficial biological role, and is highly toxic to all forms of life. Bacteria are involved in the global environmental cycling of mercury, both by reducing Hg to metallic Hg, which is less soluble in aqueous systems and therefore less bioavailable, and by oxidizing and methylating Hg species, and in the process making Hg more bioavailable and more highly toxic. The most thoroughly studied bacterial biotransformation of mercury is reduction by the widely distributed resistance operons found on plasmids and transposons in Gram-negative and -positive bacteria. The products of these resistance operons transport ionic Hg from outside the cell to the cellular cytoplasm, where mercuric reductase reduces divalent Hg to Hg, which is less toxic than Hg. Metallic mercury vapor, Hg, is volatile under aerobic conditions, leaves the cell by passive diffusion, and is volatilized from the growth environment. Sometimes, additional gene(s) determine organomercurial lyase, the enzyme that cleaves organomercurial compounds to inorganic Hg, which is then reduced to Hg. Two types of operons (“narrow spectrum” with inorganic Hg resistance and “broad spectrum” with both organomercurial and inorganic mercury resistances) confer high levels of resistance on host bacteria. The expression of resistance genes is primarily regulated by the MerR protein, which is the prototype of an increasing family of metal and other effector-responsive transcriptional activators. Methylation of inorganic Hg to CHHg is thought to occur nonenzymatically (perhaps even extracellularly) with microbially synthesized -adenosylmethionine as methyl donor.

This chapter will focus on recent progress on the mechanisms of metalloid uptake, metabolism, and detoxification in bacteria, archaea, and eukaryotic microbes. One of the initial challenges of the earliest cells would have been the ability to detoxify heavy metal ions, transition metal ions, and metalloids, including arsenic and antimony. The presence of arsenic resistance () genes in the genome of by far most living organisms sequenced to date illustrates firstly that genes must be ancient and secondly that arsenic is still ubiquitous in the environment, providing the selective pressure that maintains these genes in present-day organisms. Some early cells also probably could use arsenite as an electron acceptor, giving selective pressure for the evolution of respiratory arsenate reductase. As atmospheric O levels increased, arsenite was oxidized to arsenate abiotically. This provided an advantage for the evolution of arsenate reductases, some for arsenate respiration and energy production, and others for arsenate detoxification. Present-day selective pressure for metalloid resistance also comes from sources such as natural release of arsenic from volcanic activities, mining activities, the burning of coal, and other human activities. In addition is the use of arsenicals and antimonials as chemotherapeutic drugs for the treatment of parasitic diseases and cancer. Resistance to these drugs is becoming a major dilemma. Thus, an understanding of the molecular details of metalloid transport systems and detoxification enzymes is essential for the rational design of new drugs, and for treating drug-resistant microorganisms and tumor cells. Finally, this chapter will summarize recent identification of novel enzymes for arsenic reduction, oxidation, and methylation that expand the possibilities for metalloid metabolism and transformations.

The widespread industrial use of chromium has caused this heavy metal to be considered as a serious environmental pollutant. The most common forms of chromium in nature are the relatively innocuous trivalent form, Cr(III), and the more toxic hexavalent species, Cr(VI). Cr(VI) is usually present as the oxyanion chromate. Toxic effects of chromate for bacteria are associated with the inhibition of sulfate transport and with oxidative damage to biomolecules. The best studied bacterial mechanisms of resistance to chromate include reduction of Cr(VI) to the Cr(III) species and efflux of chromate from cell cytoplasm. Several chromate reductases have been identified in diverse bacterial species. Most characterized enzymes belong to the NAD(P)H-dependent flavoprotein family of reductases. Efflux of chromate by the ChrA membrane transporter, a plasmid-encoded protein, has been demonstrated in and species. Chromate efflux by ChrA consists of an energy-dependent process driven by the membrane potential. The CHR protein family, which includes putative ChrA homologs, currently contains about 135 sequences from all three domains of life. Other mechanisms of bacterial resistance to chromate involve the expression of components of the machinery for repair of DNA damage as well as free-radical scavenging enzymes.

Molybdenum (Mo) and tungsten (W) are trace elements that catalyze, upon binding to the appropriate cofactors, diverse and important redox reactions in the global carbon, nitrogen, and sulfur cycles. Mo is found in two forms of oxygen-labile metal cofactors, a pterin-based and a Fe-S-cluster-based scaffold, while W naturally only occurs in association with pterin cofactors and FeW-nitrogenases have been generated artificially. Both oxyanions enter the cell via an ABC-type high affinity uptake system and are subsequently processed by a multistep biosynthetic machinery forming either Mo- and W-pterin cofactors (Moco or Wco) in a large variety of Mo- and W-containing enzymes or the FeMo cofactor (FeMo-co) in nitrogenase-catalyzed nitrogen fixation. The functional diversity of pterin-based Mo and W cofactors is reflected by a large number of enzymes such as nitrate reductase, dimethyl sulfoxide reductase, formate dehydrogenase, aldehyde oxidoreductase and CO dehydrogenase. In these enzymes Mo and W are bound via thiolates to one or two unique tricyclic pterin moieties, commonly referred to as molybdopterin but the term “metal binding pterin” (MPT) is more appropriate due to its association with both, Mo and W. It is commonly believed, but still not demonstrated, that Moco and Wco are synthesized by a similar and highly conserved pathway. Synthesis of the Moco can be divided into four major steps, according to the biosynthetic intermediates cyclic pyranopterin monophosphate, MPT, and adenylated MPT. Differences in the final metal insertion step(s) between Moco and Wco synthesis will be discussed. In contrast, FeMo-co biosynthesis is less understood in terms of reaction intermediate and mechanisms of different reactions catalyzed by the involved proteins. It starts with the formation of Fe-S cluster core structures that are assembled and arranged to a topology similar to mature FeMo-co. In the next steps, Mo and homocitrate are transferred before the mature cofactor is inserted into nitrogenase. Finally, a brief overview about Mo- and W-pterin enzymes as well as FeMo- and FeW-nitrogenases is given.