How a microbe runs on metal.

What microbial metallomics is

Metallomics is the integrated study of the full complement of metal and metalloid species in a biological system — their identity, quantity, location, coordination chemistry, and function. The term was proposed by Hiroki Haraguchi around 2002–2004 as a companion "-omics" to genomics and proteomics. Its object of study is the metallome: the entirety of metal-containing molecules in a cell — not just free ions, but metalloproteins, metalloenzymes, metal–nucleic-acid complexes, and metal metabolites.

The microbial branch is distinct for three reasons. Microbes exploit a broader palette of metals — including nickel, cobalt, molybdenum and tungsten — in enzymes with no human counterpart. Free-living microbes face wildly fluctuating environmental metal supply and must buffer it internally. And in a host, the competition for metal becomes a decisive front in infection. The authoritative current synthesis of the bacterial side is Chen & Giedroc's Bacterial Metallostasis (Chemical Reviews, 2024).

The metalloproteome: why metals are non-negotiable

Metals are not an accessory to life's chemistry; they are its chemistry. Waldron, Rutherford, Ford & Robinson put it plainly in Nature (2009): nearly half of all enzymes must associate with a particular metal to function. Structural and bioinformatic surveys estimate that between a quarter and a half of all proteins bind a metal — commonly quoted as "about a third." Zinc alone is bound by roughly 5–6% of bacterial proteomes.

Those metals do the reactions carbon chemistry can't: iron carries electrons through respiration and builds the iron–sulfur clusters at the heart of central metabolism; zinc holds the shape of DNA-binding proteins and cleaves bonds in hydrolases; manganese and copper run antioxidant defenses; nickel, cobalt, molybdenum and tungsten power hydrogen, one-carbon, and nitrogen metabolism. Lose the metal, and the machine is just an inert chain of amino acids. Explore each one on The Metals.

Metallostasis: keeping the books balanced

Cells maintain the metalloproteome through metallostasis — the coordinated sensing, buffering, allocation, and trafficking of metals that keeps each metal's bioavailable concentration within tight limits. The counterintuitive fact at its core, established by Waldron & Robinson (Nature Reviews Microbiology, 2009), is that the concentration of free metal ions in the cytoplasm is extraordinarily low — for the tightest-binding metals, effectively less than one free ion per cell.

Metals are therefore never left to diffuse freely. They are handed off through buffered, ligand-mediated exchange — often by dedicated metallochaperones, soluble carriers that receive a specific metal and deliver it by direct protein-to-protein contact to a waiting enzyme or transporter. The Atx1/CopZ copper chaperones and the UreE and HypA/HypB nickel-maturation chaperones are canonical examples.

The Irving–Williams problem and mismetallation

Here is the central difficulty. The Irving–Williams series (1953) ranks the stability of divalent first-row transition-metal complexes, and for almost any ligand it runs:

Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺

Given equal access, a protein active site will grab the more competitive metals — especially copper and zinc — regardless of which metal it actually needs. A protein that requires the weakly competitive manganese or iron is at constant risk of mismetallation: capture by the wrong, more competitive metal. When that happens, the enzyme is poisoned in place.

Cells do not solve this by making every site intrinsically selective. They solve it by controlling relative metal availability, so the correct metal is the one present when the protein folds. Mismetallation occurs when that buffering capacity is overwhelmed — which is exactly why copper and zinc make such effective weapons in host defense (see below).

Metalloregulation: sensing to a single ion

To keep availability in range, bacteria read their internal metal levels with metalloregulatory proteins — allosteric switches whose DNA-binding activity is toggled by gaining or losing a specific metal. Giedroc and colleagues organize them into seven structural families: ArsR/SmtB, MerR, Fur, DtxR, NikR, CsoR/RcnR, and CopY. Functionally they follow two logics:

• Uptake repressors use the metal as a co-repressor: when the metal is plentiful, the metal-bound regulator switches off the import genes. Examples: Fur (iron), Zur (zinc), MntR (manganese), NikR (nickel), DtxR (iron, the diphtheria-toxin repressor).
• Efflux/detox activators do the opposite: under metal excess, the metal-bound regulator switches on export and sequestration genes. Examples: CueR and ZntR (copper and zinc efflux), CsoR and RcnR (copper; nickel/cobalt).

These sensors are astonishingly tuned — ZntR responds to free zinc in the femtomolar-to-nanomolar range. That is how a cell reads and defends its metal set-points. The iron regulator Fur, first named in E. coli by Hantke in 1981, alone governs more than ninety genes.

Metallophores: scavenging what's scarce

When a metal is scarce, microbes secrete metallophores — small, high-affinity chelators that grab the metal from the environment and deliver it back through dedicated importers. The archetype is the siderophore, an iron(III) chelator such as enterobactin, pyoverdine, or pyochelin, built by nonribosomal peptide synthetases. The concept now extends across the periodic table: chalkophores for copper (e.g., methanobactin), zincophores for zinc (e.g., staphylopine, pseudopaline), and reported nickel- and molybdenum-scavenging systems. Pseudomonas aeruginosa alone deploys three.

Transporters and metal sparing

Metals cross membranes through conserved families: ABC importers such as the high-affinity zinc uptake system ZnuABC; ZIP-family secondary importers; P-type (P1B-) ATPases such as CopA and ZntA that pump metals out; cation diffusion facilitators (CDF/ZnT); and RND efflux pumps such as CzcCBA that export zinc, cadmium and cobalt — often conferring antibiotic cross-resistance as a side effect.

When starvation bites, microbes practice metal sparing — remodeling the proteome to lower demand for the scarce metal by swapping in isozymes that use a different metal or none at all (for instance a zinc-independent ribosomal-protein paralog). Sparing, buffering, sensing and trafficking together are the four pillars of metallostasis.

The battle for metals

All of this becomes a weapon during infection. The host practices nutritional immunity: withholding iron, zinc and manganese with proteins like calprotectin, transferrin and lactoferrin — and, in the macrophage phagosome, doing the reverse by flooding the trapped microbe with toxic copper and zinc. Because copper and zinc sit at the competitive end of the Irving–Williams series, the surplus mismetalates the invader's enzymes and shreds its iron–sulfur clusters. Pathogens answer with the metallophores, importers and efflux systems above. We trace that arms race, and its clinical consequences, in Metals & the Microbiome and Applications.

Sources

• Waldron, Rutherford, Ford & Robinson. "Metalloproteins and metal sensing." Nature 460, 823–830 (2009). nature.com
• Waldron & Robinson. "How do bacterial cells ensure that metalloproteins get the correct metal?" Nat. Rev. Microbiol. 7, 25–35 (2009). nature.com
• Chen, Giedroc et al. "Bacterial Metallostasis." Chem. Rev. (2024). pubs.acs.org
• Giedroc. "Metal sensing in bacteria." J. Biol. Chem. (2019). jbc.org
• Haraguchi. "Metallomics as integrated biometal science." J. Anal. At. Spectrom. (2004). pubs.rsc.org