Glutaredoxin

The yeast Saccharomyces cerevisiae contains two glutaredoxin genes that are required for protection against reactive oxygen species

Glutaredoxins are small heat-stable proteins that act as glutathione-dependent disulfide oxidoreductases. Two genes, designated GRX1 and GRX2, which share 40-52% identity and 61-76% similarity with glutaredoxins from bacterial and mammalian species, were identified in the yeast Saccharomyces cerevisiae. Strains deleted for both GRX1 and GRX2 were viable but lacked heat-stable oxidoreductase activity using beta-hydroxyethylene disulfide as a substrate. Surprisingly, despite the high degree of homology between Grx1 and Grx2 (64% identity), the grx1 mutant was unaffected in oxidoreductase activity, whereas the grx2 mutant displayed only 20% of the wild-type activity, indicating that Grx2 accounted for the majority of this activity in vivo. Expression analysis indicated that this difference in activity did not arise as a result of differential expression of GRX1 and GRX2. In addition, a grx1 mutant was sensitive to oxidative stress induced by the superoxide anion, whereas a strain that lacked GRX2 was sensitive to hydrogen peroxide. Sensitivity to oxidative stress was not attributable to altered glutathione metabolism or cellular redox state, which did not vary between these strains. The expression of both genes was similarly elevated under various stress conditions, including oxidative, osmotic, heat, and stationary phase growth. Thus, Grx1 and Grx2 function differently in the cell, and we suggest that glutaredoxins may act as one of the primary defenses against mixed disulfides formed following oxidative damage to proteins.

Reduction of the periplasmic disulfide bond isomerase, DsbC, occurs by passage of electrons from cytoplasmic thioredoxin

The Escherichia coli periplasmic protein DsbC is active both in vivo and in vitro as a protein disulfide isomerase. For DsbC to attack incorrectly formed disulfide bonds in substrate proteins, its two active-site cysteines should be in the reduced form. Here we present evidence that, in wild-type cells, these two cysteines are reduced. Further, we show that a pathway involving the cytoplasmic proteins thioredoxin reductase and thioredoxin and the cytoplasmic membrane protein DsbD is responsible for the reduction of these cysteines. Thus, reducing potential is passed from cytoplasmic electron donors through the cytoplasmic membrane to DsbC. This pathway does not appear to utilize the cytoplasmic glutathione-glutaredoxin pathway. The redox state of the active-site cysteines of DsbC correlates quite closely with its ability to assist in the folding of proteins with multiple disulfide bonds. Analysis of the activity of mutant forms of DsbC in which either or both of these cysteines have been altered further supports the role of DsbC as a disulfide bond isomerase.

Cyanate causes depletion of ascorbate in organisms

Ascorbate-dehydroascorbate redox cycle plays a key role in protecting organisms from an excess of oxidants. Recently, we found a novel reaction of dehydroascorbate with cyanate under the conditions of neutral pH and ordinary temperature. In this report, we demonstrated that through this irreversible reaction, cyanate causes the depletion of ascorbate in the matrix, where the ascorbate-dehydroascorbate redox cycle revolves. When the leaves of weed (Erigeron canadensis) were soaked in sodium cyanate solution generally used as a herbicide, the depletion of ascorbate as well as dehydroascorbate in them was observed, followed by the change in color from green to brown. These results suggest that a possible way of cyanate toxicity is to inflict oxidative stress on organisms.

Isolation, expression, and regulation of the pgr1(+) gene encoding glutathione reductase absolutely required for the growth of Schizosaccharomyces pombe

The pgr1(+) gene encoding glutathione reductase (GR, EC 1.6.4.2) was isolated from Schizosaccharomyces pombe using a polymerase chain reaction fragment as a probe. The gene consists of two exons and an intron of 55 nucleotides, encoding a polypeptide of 465 amino acids (50,238 Da) with conserved residues characteristic of GR. The transcriptional start site was localized at 239 nucleotides upstream from the ATG initiation codon. The level of transcript as well as the GR enzyme activity increased more than 11-fold when the cloned pgr1(+) gene was expressed on a multicopy plasmid. This overexpression conferred on S. pombe cells more resistance against menadione, a redox cycling agent, but not against H2O2. The level of pgr1(+) transcripts increased by treatment with oxidants such as menadione, cumene hydroperoxide, and diamide. It also increased by treatment with high osmolarity, heat shock, or at the stationary growth phase. The deletion of the pap1(+) gene encoding an AP-1 homolog in S. pombe caused reduction in the pgr1(+) gene expression. Furthermore, Deltapap1 cells lost the inducibility of pgr1(+) gene expression by the above stresses, implying that Pap1 is involved in general stress-inducible gene expression. When the pgr1(+) gene was disrupted, the haploid spores were not viable. Repression of nmt1 promoter-driven pgr1(+) expression by thiamine caused cessation of growth, which was rescued by the episomal pgr1(+) gene. These results indicate that GR activity, which efficiently reduces GSSG, is essentially required for the growth of S. pombe, unlike in Saccharomyces cerevisiae or Escherichia coli.

Characterization of Escherichia coli NrdH. A glutaredoxin-like protein with a thioredoxin-like activity profile

Ribonucleotides are converted to deoxyribonucleotides by ribonucleotide reductases. Either thioredoxin or glutaredoxin is a required electron donor for class I and II enzymes. Glutaredoxins are reduced by glutathione, thioredoxins by thioredoxin reductase. Recently, a glutaredoxin-like protein, NrdH, was isolated as the functional electron donor for a NrdEF ribonucleotide reductase, a class Ib enzyme, from Lactococcus lactis. The absence of glutathione in this bacterium raised the question of the identity of the intracellular reductant for NrdH. Homologues of NrdH are present in the genomes of Escherichia coli and Salmonella typhimurium, upstream of the genes for the poorly transcribed nrdEF, separated from it by an open reading frame (nrdI) coding for a protein of unknown function. Overexpression of E. coli NrdH protein shows that it is a functional hydrogen donor with higher specificity for the class Ib (NrdEF) than for the class Ia (NrdAB) ribonucleotide reductase. Furthermore, this glutaredoxin-like enzyme is reduced by thioredoxin reductase and not by glutathione. We suggest that several uncharacterized glutaredoxin-like proteins present in the genomes of organisms lacking GSH, including archae, will also react with thioredoxin reductase and be related to the ancestors from which the GSH-dependent glutaredoxins have evolved by the acquisition of a GSH-binding site. We also show that NrdI, encoded by all nrdEF operons, has a stimulatory effect on ribonucleotide reduction.

The role of the thioredoxin and glutaredoxin pathways in reducing protein disulfide bonds in the Escherichia coli cytoplasm

In Escherichia coli, two pathways use NADPH to reduce disulfide bonds that form in some cytoplasmic enzymes during catalysis: the thioredoxin system, which consists of thioredoxin reductase and thioredoxin, and the glutaredoxin system, composed of glutathione reductase, glutathione, and three glutaredoxins. These systems may also reduce disulfide bonds which form spontaneously in cytoplasmic proteins when E. coli is grown aerobically. We have investigated the role of both systems in determining the thiol-disulfide balance in the cytoplasm by determining the ability of protein disulfide bonds to form in mutants missing components of these systems. We find that both the thioredoxin and glutaredoxin systems contribute to reducing disulfide bonds in cytoplasmic proteins. In addition, these systems can partially substitute for each other in vivo since double mutants missing parts of both systems generally allow substantially more disulfide bond formation than mutants missing components of just one system. Some of these double mutants were found to require the addition of a disulfide reductant to the medium to grow well aerobically. Thus, E. coli requires either a functional thioredoxin or glutaredoxin system to reduce disulfide bonds which appear after each catalytic cycle in the essential enzyme ribonucleotide reductase and perhaps to reduce non-native disulfide bonds in cytoplasmic proteins. Our results suggest the existence of a novel thioredoxin in E. coli.

Cloning, overexpression, and characterization of glutaredoxin 2, an atypical glutaredoxin from Escherichia coli

Glutaredoxin 2 (Grx2) from Escherichia coli catalyzes GSH-disulfide oxidoreductions via two redox-active cysteine residues, but in contrast to glutaredoxin 1 (Grx1) and glutaredoxin 3 (Grx3), is not a hydrogen donor for ribonucleotide reductase. To characterize Grx2, a chromosomal fragment containing the E. coli Grx2 gene (grxB) was cloned and sequenced. grxB (645 base pairs) is located between the rimJ and pyrC genes while an open reading frame immediately upstream grxB encodes a novel transmembrane protein of 402 amino acids potentially belonging to class II of substrate export transporters. The deduced amino acid sequence for Grx2 comprises 215 residues with a molecular mass of 24.3 kDa. There is almost no similarity between the amino acid sequence of Grx2 and Grx1 or Grx3 (both 9-kDa proteins) with the exception of the active site which is identical in all three glutaredoxins (C9PYC12 for Grx2). Only limited similarities were noted to glutathione S-transferases (Grx2 amino acids 16-72), and protein disulfide isomerases from different organisms (Grx2 amino acids 70-180). Grx2 was overexpressed and purified to homogeneity and its activity was compared with those of Grx1 and Grx3 using GSH, NADPH, and glutathione reductase in the reduction of 0.7 mM beta-hydroxyethyl disulfide. The three glutaredoxins had similar apparent Km values for GSH (2-3 mM) but Grx2 had the highest apparent kcat (554 s-1). Expression of two truncated forms of Grx2 (1-114 and 1-133) which have predicted secondary structures similar to Grx1 (betaalphabetaalphabetabetaalpha) gave rise to inclusion bodies. The mutant proteins were resolubilized and purified but lacked GSH-disulfide oxidoreductase activity. The latter should therefore require the participation of amino acid residues from the COOH-terminal half of the molecule and is probably not confined to a Grx1-like NH2-terminal subdomain. Grx2 being radically different from the presently known glutaredoxins in terms of molecular weight, amino acid sequence, catalytic activity, and lack of a consensus GSH-binding site is the first member of a novel class of glutaredoxins.

Modification of liver cytosol enzyme activities promoted in vitro by reduced sulfur species generated from cystine with gamma-cystathionase

Liver cytosolic gamma-cystathionase catalyzes the generation of reduced sulfur species, referred to as "bound sulfur,' in the presence of cystine. Incubating a rat liver cytosol fraction in the presence of cystine or oxidized glutathione inactivated certain cytosolic enzyme activities. The activities of cytosolic phosphofructokinase (PFK) and pyruvate kinase rapidly decreased at pH 7.4 during incubation with a lower concentration of cystine than during incubation with oxidized glutathione. Hexokinase and 11 other enzymes in the system were affected minimally or not at all. Adding dithiothreitol to the system reactivated the modified enzymes. Inactivated PFK activity could also be recovered when reduced glutathione or NADPH was added to the cytosol fraction. In these reconstitution systems, purified rat liver PFK was directly inactivated with cystine trisulfide (one of the low molecular types of bound sulfur), but not by cystine (below 0.1 mM). Purified PFK was also inactivated by incubation with cystine plus gamma-cystathionase freshly prepared from cytosol. This was not observed, however, when gamma-cystathionase was pretreated with a specific inhibitor, D,L-propargylglycine. The cystine-dependent inactivation of PFK observed in liver cytosol is shown to be caused mainly by the reaction between bound sulfur and the enzyme, but not by the direct thiol/disulfide exchange. Thus, in vitro modification of the cytosolic enzymes by bound sulfur generated from cystine with gamma-cystathionase has high potency and relatively specific.

Nitric oxide modification of rat brain neurogranin. Identification of the cysteine residues involved in intramolecular disulfide bridge formation using site-directed mutagenesis

Neurogranin (Ng) is a neuron-specific protein kinase C-selective substrate, which binds calmodulin (CaM) in the dephosphorylated form at low levels of Ca2+. This protein contains redox active Cys residues that are readily oxidized by several nitric oxide (NO) donors and other oxidants to form intramolecular disulfide. Identification of the Cys residues of rat brain Ng, Cys3, Cys4, Cys9, and Cys51, involved in NO-mediated intramolecular disulfide bridge formation was examined by site-directed mutagenesis. Mutation of all four Cys residues or single mutation of Cys51 blocked the oxidant-mediated intramolecular disulfide formation as monitored by the downward mobility shift under nonreducing SDS-polyacrylamide gel electrophoresis. Single mutation of Cys3, Cys4, or Cys9 or double mutation of any pair of these three Cys residues did not block such intramolecular disulfide formation, although the rates of oxidation of these mutant proteins were different. Thus, Cys51 is an essential pairing partner in NO-mediated intramolecular disulfide formation in Ng. Cys3, Cys4, and Cys9 individually could pair with Cys51, and the order of reactivity was Cys9 > Cys4 > Cys3, suggesting that Cys9 and Cys51 form the preferential disulfide bridge. In all cases tested, the intramolecularly disulfide bridged Ng proteins displayed dramatically attenuated CaM-binding affinity and approximately 2-3-fold weaker protein kinase C substrate phosphorylation activity. The data indicate that the N-terminal Cys3, Cys4, and Cys9 are in close proximity to the C-terminal Cys51 in solution. The disulfide bridge between the N- and C-terminal domains of Ng renders the central CaM-binding and phosphorylation site domain in a fixed conformation unfavorable for binding to CaM and as a substrate of protein kinase C.

Sulfate reduction in higher plants: molecular evidence for a novel 5'-adenylylsulfate reductase

Sulfate-assimilating organisms reduce inorganic sulfate for Cys biosynthesis. There are two leading hypotheses for the mechanism of sulfate reduction in higher plants. In one, adenosine 5'-phosphosulfate (APS) (5'-adenylysulfate) sulfotransferase carries out reductive transfer of sulfate from APS to reduced glutathione. Alternatively, the mechanism may be similar to that in bacteria in which the enzyme, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase, catalyzes thioredoxin (Trx)-dependent reduction of PAPS. Three classes of cDNA were cloned from Arabidopsis thaliana termed APR1, -2, and -3, that functionally complement a cysH, PAPS reductase mutant strain of Escherichia coli. The coding sequence of the APR clones is homologous with PAPS reductases from microorganisms. In addition, a carboxyl-terminal domain is homologous with members of the Trx superfamily. Further genetic analysis showed that the APR clones can functionally complement a mutant strain of E. coli lacking Trx, and an APS kinase, cysC. mutant. These results suggest that the APR enzyme may be a Trx-independent APS reductase. Cell extracts of E. coli expressing APR showed Trx-independent sulfonucleotide reductase activity with a preference for APS over PAPS as a substrate. APR-mediated APS reduction is dependent on dithiothreitol, has a pH optimum of 8.5, is stimulated by high ionic strength, and is sensitive to inactivation by 5'-adenosinemonophosphate (5'-AMP). 2'-AMP, or 3'-phosphoadenosine-5'-phosphate (PAP), a competitive inhibitor of PAPS reductase, do not affect activity. The APR enzymes may be localized in different cellular compartments as evidenced by the presence of an amino-terminal transit peptide for plastid localization in APR1 and APR3 but not APR2. Southern blot analysis confirmed that the APR clones are members of a small gene family, possibly consisting of three members.