Glutathione and Transition-Metal Homeostasis inEscherichia coli

Abstract

Glutathione (GSH) and its derivative phytochelatin are important binding factors in transition-metal homeostasis in many eukaryotes. Here, we demonstrate that GSH is also involved in chromate, Zn(II), Cd(II), and Cu(II) homeostasis and resistance in Escherichia coli. While the loss of the ability to synthesize GSH influenced metal tolerance in wild-type cells only slightly, GSH was important for residual metal resistance in cells without metal efflux systems. In mutant cells without the P-type ATPase ZntA, the additional deletion of the GSH biosynthesis system led to a strong decrease in resistance to Cd(II) and Zn(II). Likewise, in mutant cells without the P-type ATPase CopA, the removal of GSH led to a strong decrease of Cu(II) resistance. The precursor of GSH, ␥-glutamylcysteine (␥EC), was not able to compensate for a lack of GSH. On the contrary, ␥EC-containing cells were less copper and cadmium tolerant than cells that contained neither ␥EC nor GSH. Thus, GSH may play an important role in trace-element metabolism not only in higher organisms but also in bacteria.Under aerobic growth conditions, either glutathione (GSH; L-␥-glutamyl-L-cysteine-glycine) or the small 12-kDa protein thioredoxin (TrxB) is essential to maintain a reduced environment in the cytosol of Escherichia coli cells (5,27,60,65). Since E. coli thioredoxin reductase can transfer electrons from NADH to glutaredoxin 4 (Grx4, GrxD, or YdhD) and Grx4 can reduce Grx1 (GrxA) and Grx3 (GrxC) (14), E. coli is able to catalyze the reduction of disulfides without GSH. Thus, GSH by itself is not essential for the survival of this bacterium (17).The cellular GSH is kept almost completely reduced (2, 30): the reduced GSH-oxidized GSH (GSSG) couple has a standard redox potential at pH 7.0 of Ϫ240 mV (66). Using a potential of about Ϫ260 mV in vivo (29) and the Nernst equation results in the calculation of a GSH concentration of about 5 mM and a GSSG concentration of about 5 M. Therefore, any change in the GSH concentration is likely to influence the cellular metabolism by changing the redox potential of the cytoplasm and maybe also that of the periplasm (59). A decrease of the GSH concentration by half would increase the cytoplasmic redox potential by 18 mV.GSH is also involved in the osmoadaptation of E. coli (39). As a response to highly osmotic conditions, a mutant strain unable to synthesize trehalose as an osmoprotectant accumulates GSH to a concentration about 10-fold that under normal conditions. The first and quickest response of E. coli to changing osmotic conditions is to change the cytoplasmic potassium concentration (39), and indeed, GSH is needed for the regulation of this pool (13), probably through interaction with the GSH-gated potassium efflux system KefCYabF (43). Moreover, GSH is involved in the detoxification of methylglyoxal (13), although there are GSH-independent pathways of methylglyoxal degradation (44), and resistance to chlorine compounds (7, 67). In bacteria other than E. coli, GSH is essential for thiamine synthesis (19) and ...

Bacterial copper storage proteins

“…A role for these proteins in Cu(I) storage is currently the most logical suggestion for their function, but in many cases what they are storing Cu for remains unknown. The presence of bacterial Cu storage proteins seems consistent with a number of other observations: (1) that bacterial Cu-import systems exist (6,7,17,21,52,56,77,95), including into the cytosol; (2) that endogenous pools of the metal are available in bacteria (11,15,16,18,96); and (3) that E. coli grown in both LB and minimal medium accumulates Cu (97). It also highlights that there are alternative mechanisms to using different cellular compartments to prevent mis-metallation of proteins by Cu (37).…”

“…Copper availability appears to be largely constrained by the use of high affinity sites in proteins (12)(13)(14), although 'pools' of Cu bound by other molecules are important (4,5,11,(15)(16)(17)(18).…”

Functional Partnership of the Copper Export Machinery and Glutathione Balance in Human Cells

“…Cell proliferation in low glutathione requires both Atox1 and ATP7B, indicating that copper trafficking through the secretion pathway plays a crucial role in cell proliferation under these conditions. An analogous link between copper and glutathione was also observed in bacteria (40), suggesting an early emergence of this molecular symbiosis in evolution. Why do the Atox1 Ϫ/Ϫ cells lose viability in low glutathione?…”

Probing the coordination properties of glutathione with transition metal ions (Cr2+,Mn2+,Fe2+,Co2+, Ni2+,Cu2+,Zn2+,Cd2+,Hg2+) by density functional theory

“…Moreover, GSH, possessing all kinds of biological donor atoms (two carboxyls, one thiol, one amino and two pairs of carbonyls and amide donors within two peptide bonds) is a versatile ligand, forming stable complexes with both hard and soft metal cations [22,23]. Thus, the coordination chemistry of glutathione is also important as it serves as a model system for binding metal cations of larger peptide and protein molecules [24,25].…”