Acute Toxicity of Divalent Mercury to Bacteria Explained by the Formation of Dicysteinate and Tetracysteinate Complexes Bound to Proteins in Escherichia coli and Bacillus subtilis

Abstract

Bacteria are the most abundant organisms on Earth and also the major life form affected by mercury (Hg) poisoning in aquatic and terrestrial food webs. For this reason, identifying the intracellular molecular forms and trafficking pathways of Hg in bacteria at environmentally relevant concentrations is critical to controlling Hg toxicity. We applied high energy-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy to bacteria with intracellular concentrations of Hg as low as 0.7 ng/mg (ppm). Gram-positive Bacillus subtilis and gram-negative Escherichia coli were exposed to three Hg species: HgCl2, Hg-dicysteinate (Hg(Cys)2), and Hg-dithioglycolate (Hg(TGA)2). In all cases, Hg was transformed into new two-and four-coordinate cysteinate complexes, interpreted to be bound, respectively, to the consensus metal binding CXXC motif and zinc finger domains of proteins, with glutathione acting as a transfer ligand. Replacement of zinc cofactors essential to gene regulatory proteins with Hg would inhibit vital functions such as DNA transcription and repair and is suggested to be a main cause of Hg genotoxicity.

Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury

“…Figure illustrates Hg Lα1 HERFD–XAS, using α-HgS, showing the nonresonant X-ray emission Hg Lα1 spectrum (Figure A) and comparing standard Hg L III XAS with the Hg Lα1 HERFD–XAS (Figure B). The dramatic increase in spectroscopic resolution obtained with HERFD−XAS, relative to conventional XAS, and apparent in the figure is in agreement with earlier results from other groups. − , Below, we compare Hg Lα1 HERFD–XAS for a wide range of coordination environments and discuss the advantages and limitations of the method as a spectroscopic probe for mercury speciation in complex samples, with a particular focus on environmental and toxicological applications.…”

“…The method can eliminate both core-hole and exchange broadening, affording substantial enhancement in spectroscopic resolution. Several studies using Hg Lα1 HERFD–XAS have been previously reported by others. − Indeed, this measurement is arguably the most experimentally convenient HERFD–XAS, with an incident energy that is readily accessible on most hard X-ray beamlines and employing the fifth harmonic of Si(111) (the simplest cut) that gives an inherently high reflectivity and narrow Darwin width (∼2.5″) . Moreover, as discussed below, resolution gains with Hg Lα1 HERFD–XAS over conventional Hg L III XAS can be substantial, and for a combination of these reasons, Hg Lα1 HERFD–XAS is often chosen as a first experiment when commissioning HERFD–XAS equipment .…”

Chemical Forms of Mercury in Blue Marlin Billfish: Implications for Human Exposure

“…Mercury exerts its toxicity via complexation to cysteinyl (Cys) and selenocysteinyl (Sec) metal binding sites in proteins. , The binding of Hg to Sec residues reduces the Se supply for the biosynthesis of vital selenoproteins. It is usually considered that Hg is detoxified as HgSe and therefore is without toxicological consequences when the molar difference [Se] – [Hg] > 0, or the molar ratio [Hg] mol /[Se] mol < 1, leaving sufficient bioavailable Se for seleoprotein synthesis and activity. , The [Hg] mol /[Se] mol ratio represents the fraction of Se to total Se bound to Hg.…”

Chemical Forms of Mercury in Pilot Whales Determined from Species-Averaged Mercury Isotope Signatures

“…Recently, advancement in the application of high energy-resolution X-ray absorption (HR-XANES) spectroscopy in wildlife identified that MeHg is detoxified to nontoxic mercury selenide (HgSe) through an intermediary Hg-tetraselenolate (Hg­(Sec) 4 ) species. − The stepwise transformation of MeHg observed in bird and fish tissues (MeHg → Hg­(Sec) 4 → HgSe), likely initiated by selenoprotein P (SelP), provided the first in vivo mechanistic information for the observation of HgSe in the liver and extrahepatic tissues of marine mammals − and seabirds using standard resolution X-ray absorption spectroscopy and electron microscopy. Although these two techniques provide critical structural information on mercury, − they provide limited insight into biochemical processes essential for understanding the toxicokinetics of toxic MeHg in organisms (i.e., fate, tissue-specific exchange).…”