Silver(I), Mercury(II), Cadmium(II), and Zinc(II) Target Exposed Enzymic Iron-Sulfur Clusters when They Toxify Escherichia coli Original paper

What was studied?

This study aimed to explore how certain soft metals, specifically silver (Ag), mercury (Hg), cadmium (Cd), and zinc (Zn), target and damage [4Fe-4S] clusters, particularly in dehydratase enzymes, within Escherichia coli (E. coli). The researchers investigated the inactivation of fumarase A, a member of the dehydratase enzyme family, and other iron-sulfur dehydratases upon exposure to these metals. The study examined the inactivation of these enzymes both in vitro and in vivo, testing the hypothesis that soft metals exert their toxicity by directly damaging the iron-sulfur clusters in the enzymes, thus disrupting bacterial growth and metabolism.

Who was studied?

The study primarily focused on Escherichia coli (E. coli), a widely studied model organism, particularly the K-12 strain of E. coli. The researchers used several derivatives of this strain, including mutants with specific genetic modifications, to investigate how soft metals impact microbial cells. These strains included those lacking certain efflux systems, such as the ZntA-deficient mutant, to assess their sensitivity to metals like zinc. The experiments were performed under both aerobic and anaerobic conditions to determine the full scope of metal toxicity. Additionally, purified fumarase A was employed to study the direct effects of the metals on enzymatic activity and [4Fe-4S] cluster integrity.

Most important findings

The study revealed that Ag(I), Hg(II), Cd(II), and Zn(II) significantly inactivated fumarase A, a key enzyme in the dehydratase family, by targeting its [4Fe-4S] clusters. These metal ions destroyed the clusters by displacing the iron atoms, thereby impairing enzyme activity. The degree of damage varied across metals: Ag(I) and Hg(II) were the most potent, inactivating the enzyme at low micromolar concentrations, while Cd(II) and Zn(II) required higher doses to induce similar effects. The study also found that metals such as Mn(II), Co(II), Ni(II), and Pb(II) did not damage the [4Fe-4S] clusters even at millimolar concentrations. Electron paramagnetic resonance (EPR) analysis confirmed that the metals disrupted the iron-sulfur clusters by releasing iron, with Ag(I) causing the most extensive damage, leading to a complete loss of the [4Fe-4S] clusters.

Moreover, when E. coli cells were exposed to these metals in vivo, similar results were observed. Ag(I), Hg(II), Cd(II), and Zn(II) caused the inactivation of dehydratases, which could be reversed by rebuilding the iron-sulfur clusters with ferrous iron and dithiothreitol (DTT), supporting the hypothesis that the damage was specifically to the [4Fe-4S] clusters. The study also highlighted that the metal-induced damage was selective; other enzymes, such as malate dehydrogenase and NADH dehydrogenase, which do not rely on iron-sulfur clusters, were not affected.

Key implications

This study provides critical insights into the molecular mechanisms behind the toxicity of soft metals like silver, mercury, cadmium, and zinc, particularly in their impact on bacterial dehydratase enzymes. By targeting the [4Fe-4S] clusters in these enzymes, the metals disrupt key metabolic processes in bacteria, leading to growth inhibition. This mechanism suggests that the ability of these metals to damage microbial cells can be linked to their affinity for sulfur and their ability to target enzymes with exposed iron-sulfur clusters. The findings are significant for understanding metal toxicity in both environmental and clinical contexts, as soft metals are commonly used as antimicrobial agents. Furthermore, the study underscores the potential for metal-induced oxidative stress due to the disruption of iron homeostasis in cells.