Cadmium Toxicity in Glutathione Mutants ofEscherichia coli

Abstract

The higher affinity of Cd 2؉ for sulfur compounds than for nitrogen and oxygen led to the theoretical consideration that cadmium toxicity should result mainly from the binding of Cd 2؉ to sulfide, thiol groups, and sulfur-rich complex compounds rather than from Cd 2؉ replacement of transition-metal cations from nitrogenor oxygen-rich biological compounds. This hypothesis was tested by using Escherichia coli for a global transcriptome analysis of cells synthesizing glutathione (GSH; wild type), ␥-glutamylcysteine (⌬gshB mutant), or neither of the two cellular thiols (⌬gshA mutant). The resulting data, some of which were validated by quantitative reverse transcription-PCR, were sorted using the KEGG (Kyoto Encyclopedia of Genes and Genomes) orthology system, which groups genes hierarchically with respect to the cellular functions of their respective products. The main difference among the three strains concerned tryptophan biosynthesis, which was up-regulated in wild-type cells upon cadmium shock and strongly up-regulated in ⌬gshA cells but repressed in ⌬gshB cells containing ␥-glutamylcysteine instead of GSH. Overall, however, all three E. coli strains responded to cadmium shock similarly, with the up-regulation of genes involved in protein, disulfide bond, and oxidative damage repair; cysteine and iron-sulfur cluster biosynthesis; the production of proteins containing sensitive iron-sulfur clusters; the storage of iron; and the detoxification of Cd 2؉ by efflux. General energy conservation pathways and iron uptake were down-regulated. These findings indicated that the toxic action of Cd 2؉ indeed results from the binding of the metal cation to sulfur, lending support to the hypothesis tested.With the exception of that of copper, the affinities of the borderline metals of the first transition group for the ligands oxygen and sulfur increase in the same manner as those of the metals of the second group, in order from Mn 2ϩ to Zn 2ϩ . In contrast, Cd 2ϩ (and Pb 2ϩ ) has a much higher affinity for sulfur than for oxygen (40). Thus, Cd 2ϩ toxicity should be the result mainly of the affinity of Cd 2ϩ for sulfur. If in a physiological complex, the first-shell ligands around a transition-metal cation are mainly nitrogen or oxygen, Cd 2ϩ should not be able to replace that particular cation. Cadmium should replace the cations, however, if the first shell is composed of sulfur atoms predominantly. Theoretically, cadmium toxicity should be the result of the binding of Cd 2ϩ to sulfide, generated during the biosynthesis of cysteine and of iron-sulfur centers (FeS centers); binding to thiol groups, e.g., of proteins; and the replacement of other transition-metal cations from such sulfur-rich complex compounds.Cellular thiols may interfere with this mode of action of cadmium toxicity. The main cellular thiol in cyanobacteria and proteobacteria is glutathione (GSH), which is absent in many other prokaryotes that contain other thiol compounds like mycothiol and ergothioneine (14, 39). GSH (L-␥-glutamyl-Lcysteine-glycine) is esse...

YqhD: a broad-substrate range aldehyde reductase with various applications in production of biorenewable fuels and chemicals

“…Cd 2+ has a high affinity for sulfide, resulting in limitation of cysteine biosynthesis (Helbig et al 2008), similar to the effect seen following furfural challenge (Miller et al 2009a). …”

“…yqhD was activated 4-fold following Cd 2+ treatment of E. coli strain W3110 (Helbig et al 2008). Cd 2+ has a high affinity for sulfide, resulting in limitation of cysteine biosynthesis (Helbig et al 2008), similar to the effect seen following furfural challenge (Miller et al 2009a).…”

“…This Cd 2+ -mediated activation of yqhD was still observed in the absence of gshA or gshB, critical components of the biosynthesis pathway for E. coli's main thiol, glutathione (Helbig et al 2008). …”

“…In addition to its thiol group, glutathione, like isatin, contains two ketone groups. While the conservation of Cd 2+ -mediated activation in the absence of gshA or gshB suggests that yqhD regulation is not related to these two enzymes, deletion of gshA or gshB resulted in increased abundance of yqhD relative to the wild-type, both for untreated samples and for those treated with Cd 2+ , suggesting that there is a link between glutathione production and expression of yqhD (Helbig et al 2008) Note that other studies found that neither increased expression nor deletion of yqhD altered Cd 2+ sensitivity in E. coli BW25113, indicating that activation of yqhD does not significantly contribute to Cd 2+ resistance (Perez et al 2008). Finally, treatment with 1 mM S-nitrosoglutathione (GSNO) increased yqhD transcript abundance by 2-fold (Mukhopadhyay et al 2004).…”

A P-type ATPase importer that discriminates between essential and toxic transition metals

“…Transition metals such as zinc, copper, and iron are essential to many physiological processes but are also toxic at elevated concentrations. Other metals, such as cadmium, silver, and mercury, exhibit acute toxicity by binding to macromolecules and perturbing their physiological interactions (1,2). A diversity of membrane-embedded transporters translocate metals across the cell membrane, underlining the importance and the delicacy of this process.…”