Lead(II) Binding in Natural and Artificial Proteins Original paper

What was reviewed?

This chapter synthesizes structural, spectroscopic, and functional evidence for lead binding in proteins, spanning natural targets and de novo–designed scaffolds to define how Pb(II) engages biological ligands and perturbs macromolecular function. The review emphasizes lead’s thiophilicity, preference for Cys-rich coordination, and frequent adoption of hemidirected trigonal-pyramidal PbS₃ sites whose stereochemistry reflects the 6s² lone pair. Using δ-aminolevulinic acid dehydratase (ALAD), calmodulin, zinc-finger domains, and bacterial metalloregulatory networks (pbr, znt, cad) as exemplars, it links metal-site substitution or opportunistic binding to heme biosynthesis failure, signaling distortion, transcriptional misregulation, and selective pressure for microbial efflux and sequestration. It further exploits three-stranded coiled coils and related peptide bundles to isolate first-sphere rules for Pb(II) geometry and affinity, integrating UV/Vis, EXAFS/XAS, and ^207Pb NMR to benchmark PbS₃ fingerprints and quantify competition with Zn(II) and Ca(II).

Who was reviewed?

The evidence base includes human and mammalian protein systems (ALAD in erythrocytes; Ca(II)-sensor calmodulin; diverse zinc-finger transcription factors), Gram-negative and Gram-positive bacterial metal-resistance operons from Cupriavidus metallidurans, Escherichia coli, and Staphylococcus aureus (PbrR/PbrA/PbrT, ZntR/ZntA, CadC/CadA), and minimal peptide scaffolds engineered to present Cys₃ pockets within three-stranded coiled-coils. Clinical and environmental contexts appear through observations such as the predominance of ALAD-bound Pb in blood and the extraordinarily tight Pb(II) inhibition of ALAD activity, while microbial contexts appear via operon-encoded uptake, efflux, and periplasmic precipitation that together remodel local metal speciation.

Most important findings

Across systems, Pb(II) outcompetes native metals at Cys-rich loci and locks into hemidirected PbS₃ coordination; in ALAD, Pb(II) substitutes for Zn(II) at a Cys₃ site with femtomolar Ki, blocks substrate activation, and drives both heme failure and ALA accumulation, with a large fraction of whole-blood lead bound to ALAD in exposed workers. Calmodulin’s EF-hands can be filled by Pb(II) with higher affinity than Ca(II), provoking aberrant activation at very low concentrations and, under crystallization conditions, additional surface Pb(II) contacts that highlight the ion’s opportunism. Canonical and noncanonical zinc-finger motifs show that Pb(II) binding favors Cys-rich sites, misfolds domains, and diminishes DNA binding for factors such as GATA, TFIIIA, and Sp1.

In bacteria, lead-sensing transcription factors and P-type ATPases orchestrate resistance: PbrR selectively detects Pb(II) and induces an operon that couples uptake, cytosolic chaperoning, efflux, and periplasmic phosphate precipitation; ZntA and CadA export Pb(II) alongside Zn/Cd, while ZntR and CadC transduce Pb(II) occupancy into promoter remodeling. Designed Cys₃ coiled-coils reproduce natural PbS₃ spectra and reveal site-specific preferences (e.g., modest preference for “d” over “a” heptad layers), pKa coupling of thiolate formation, and extremely high apparent affinities, thereby decoupling second-sphere sterics from first-sphere chemistry. Collectively, these findings explain how Pb(II) displaces essential metals to alter enzymology and gene control and how microbial resistance circuits and sulfur-rich ligands sequester Pb(II), processes that, at community scale, are expected to shift competitive fitness under lead exposure through altered availability, export, and precipitation.

Key implications

For clinicians integrating microbiome-aware toxicology, the reviewed mechanisms indicate that host targets (ALAD, Ca(II) sensors, zinc-fingers) explain systemic morbidity, while bacterial Pb(II) sensing and efflux define how lead exposure selects for metal-resistant taxa and changes the metal landscape around biofilms and mucosa. Because Pb(II) binding is both avid and structurally specific, even low-level exposure can disrupt heme synthesis and signaling, and community composition may track with operon-encoded export and periplasmic precipitation capacity. These protein-level rules also rationalize biomonitoring (ALAD activity, blood Pb partitions) and suggest that therapeutic strategies must consider not only chelation but also the microbial metal economy that shapes colonization and inflammation at barrier sites.