Zn(II) metabolism in prokaryotes

Thermodynamic studies of the mechanism of metal binding to the Escherichia coli zinc transporter YiiP

Sequence homology of the Escherichia coli YiiP places it within the family of cation diffusion facilitators, a family of membrane transporters that play a central role in regulating cellular zinc homeostasis. Here we describe the first thermodynamic and mechanistic studies of metal binding to a cation diffusion facilitator. Isothermal titration calorimetric analyses of the purified YiiP and binding competitions among Zn(2+), Cd(2+), and Hg(2+) revealed a mutually competitive binding site common to three metal ions and a set of noncompetitive binding sites, including one Cd(2+) site, one Hg(2+) site, and at least one Zn(2+) site, to which the binding of Zn(2+) exhibited partial inhibitions of both Cd(2+) and Hg(2+) bindings. Lowering the pH from 7.0 to 5.5 inhibited binding of Zn(2+) and Cd(2+) to the common site. Further, the enthalpy change of the Cd(2+) binding to the common site was found to be related linearly to the ionization enthalpy of the pH buffer with a slope corresponding to the release of 1.23 H(+) for each Cd(2+) binding. These H(+) effects are consistent with a coupled deprotonation process upon binding of Zn(2+) and Cd(2+). Modification of histidine residues by diethyl pyrocarbonate specifically inhibited Zn(2+) binding to the common binding site, indicating that the mechanism of binding-deprotonation coupling involves a histidine residue(s).

Zn, Cu and Co in cyanobacteria: selective control of metal availability

Homeostatic systems for essential and non-essential metals create the cellular environments in which the correct metals are acquired by metalloproteins while the incorrect ones are somehow avoided. Cyanobacteria have metal requirements often absent from other bacteria; copper in thylakoidal plastocyanin, zinc in carboxysomal carbonic anhydrase, cobalt in cobalamin but magnesium in chlorophyll, molybdenum in heterocystous nitrogenase, manganese in thylakoidal water-splitting oxygen-evolving complex. This article reviews: an intracellular trafficking pathway for inward copper supply, the sequestration of surplus zinc by metallothionein (also present in other bacteria) and the detection and export of excess cobalt. We consider the influence of homeostatic proteins on selective metal availability.

Bacterial silver resistance: molecular biology and uses and misuses of silver compounds

Resistance to silver compounds as determined by bacterial plasmids and genes has been defined by molecular genetics. Silver resistance conferred by the Salmonella plasmid pMGH100 involves nine genes in three transcription units. A sensor/responder (SilRS) two-component transcriptional regulatory system governs synthesis of a periplasmic Ag(I)-binding protein (SilE) and two efflux pumps (a P-type ATPase (SilP) plus a three-protein chemiosmotic RND Ag(I)/H+ exchange system (SilCBA)). The same genes were identified on five of 19 additional IncH incompatibility class plasmids but thus far not on other plasmids. Of 70 random enteric isolates from a local hospital, isolates from catheters and other Ag-exposed sites, and total genomes of enteric bacteria, 10 have recognizable sil genes. The centrally located six genes are found and functional in the chromosome of Escherichia coli K-12, and also occur on the genome of E. coli O157:H7. The use of molecular epidemiological tools will establish the range and diversity of such resistance systems in clinical and non-clinical sources. Silver compounds are used widely as effective antimicrobial agents to combat pathogens (bacteria, viruses and eukaryotic microorganisms) in the clinic and for public health hygiene. Silver cations (Ag+) are microcidal at low concentrations and used to treat burns, wounds and ulcers. Ag is used to coat catheters to retard microbial biofilm development. Ag is used in hygiene products including face creams, "alternative medicine" health supplements, supermarket products for washing vegetables, and water filtration cartridges. Ag is generally without adverse effects for humans, and argyria (irreversible discoloration of the skin resulting from subepithelial silver deposits) is rare and mostly of cosmetic concern.