Structural and redox properties of the leaderless DsbE (CcmG) protein: both active-site cysteines of the reduced form are involved in its function in the Escherichia coli periplasm

The thiol/disulfide oxidoreductases play important roles in ensuring the correct formation of disulfide bonds, of which the DsbE protein, also called CcmG, is the one implicated in electron transfer for cytochrome c maturation in the periplasm of Escherichia coli. The soluble, N-terminally truncated DsbE was overexpressed and purified to homogeneity. Here we report the structural and redox properties of the leaderless form (DsbEL-). During the redox reaction, the protein undergoes a structural transformation resulting in a more stable reduced form, but this form shows very low reactivity in thiol/ disulfide exchange of cysteine residues and low activity in accelerating the reduction of insulin. The standard redox potential (E'0) for the active thiol/ disulfide was determined to be -0.186 V; only one of the two cysteines (Cys80) was suggested to be the active residue in the redox reaction. From the aspect of biochemical properties, DsbE can be regarded as a weak reductant in the Escherichia coli periplasm. This implies that the function of DsbE in cytochrome c maturation can be ascribed to its active-site cysteines and the structure of the reduced form.     

A Pro to His mutation in active site of thioredoxin increases its disulfide-isomerase activity 10-fold. New refolding systems for reduced or randomly oxidized ribonuclease.

Thioredoxin (Trx) from Escherichia coli was compared with bovine protein disulfide-isomerase (PDI) for its ability to catalyze native disulfide formation in either reduced or randomly oxidized (scrambled) ribonuclease A (RNase). On a molar basis, a 100-fold higher concentration of Trx than of PDI was required to give the same rate of native disulfide formation measured as recovery of RNase activity. A Pro-34 to His (P34H Trx) mutation in the active site of E. coli Trx (WCGPC), mimicking the two suggested active sites in PDI (WCGHC), increased the catalytic activity in disulfide formation about 10-fold. The mutant P34H Trx displayed a 35-mV higher redox potential (E'0) of the active site disulfide/dithiol relative to wild type Trx, making it more similar to the redox potential observed for PDI. This higher redox potential correlates well with the enhanced activity and suggests a role for the histidine side chain. Enzymatic isomerization of disulfides in scrambled, oxidized RNase requires the presence of a catalytic thiol such as GSH to initiate the thiol-disulfide interchange. Bovine thioredoxin reductase, together with NADPH, could replace GSH. For oxidative folding of reduced RNase in air with Trx, P34H Trx, or PDI, catalytic amounts of sodium selenite (1 microM) resulted in rapid disulfide formation and high yields of ribonuclease activity equivalent to previously known redox buffers of GSH and GSSG. These results demonstrate no obligatory role for glutathione in disulfide formation. A possible mechanism for the unknown thiol oxidative process accompanying folding and protein disulfide formation in vivo is discussed.     

Solution structure and backbone dynamics of the cysteine 103 to serine mutant of the N-terminal domain of DsbD from Neisseria meningitidis

The DsbD protein is essential for electron transfer from the cytoplasm to the periplasm of Gram-negative bacteria. Its N-terminal domain dispatches electrons coming from cytoplasmic thioredoxin (Trx), via its central transmembrane and C-terminal domains, to its periplasmic partners: DsbC, DsbE/CcmG, and DsbG. Previous structural studies described the latter proteins as Trx-like folds possessing a characteristic C-X-X-C motif able to generate a disulfide bond upon oxidation. The Escherichia coli nDsbD displays an immunoglobulin-like fold in which two cysteine residues (Cys103 and Cys109) allow a disulfide bond exchange with its biological partners.We have determined the structure in solution and the backbone dynamics of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitidis. Our results highlight significant structural changes concerning the beta-sheets and the local topology of the active site compared with the oxidized form of the E. coli nDsbD. The structure reveals a "cap loop" covering the active site, similar to the oxidized E. coli nDsbD X-ray structure. However, regions featuring enhanced mobility were observed both near to and distant from the active site, revealing a capacity of structural adjustments in the active site and in putative interaction areas with nDsbD biological partners. Results are discussed in terms of functional consequences.     

Regulation of the catalytic activity and structure of human thioredoxin 1 via oxidation and S-nitrosylation of cysteine residues

The mammalian cytosolic/nuclear thioredoxin system, comprising thioredoxin (Trx), selenoenzyme thioredoxin reductase (TrxR), and NADPH, is the major protein-disulfide reductase of the cell and has numerous functions. The active site of reduced Trx comprises Cys(32)-Gly-Pro-Cys(35) thiols that catalyze target disulfide reduction, generating a disulfide. Human Trx1 has also three structural Cys residues in positions 62, 69, and 73 that upon diamide oxidation induce a second Cys(62)-Cys(69) disulfide as well as dimers and multimers. We have discovered that after incubation with H(2)O(2) only monomeric two-disulfide molecules are generated, and they are inactive but able to regain full activity in an autocatalytic process in the presence of NADPH and TrxR. There are conflicting results regarding the effects of S-nitrosylation on Trx antioxidant functions and which residues are involved. We found that S-nitrosoglutathione-mediated S-nitrosylation at physiological pH is critically dependent on the redox state of Trx. Starting from fully reduced human Trx, both Cys(69) and Cys(73) were nitrosylated, and the active site formed a disulfide; the nitrosylated Trx was not a substrate for TrxR but regained activity after a lag phase consistent with autoactivation. Treatment of a two-disulfide form of Trx1 with S-nitrosoglutathione resulted in nitrosylation of Cys(73), which can act as a trans-nitrosylating agent as observed by others to control caspase 3 activity (Mitchell, D. A., and Marletta, M. A. (2005) Nat. Chem. Biol. 1, 154-158). The reversible inhibition of human Trx1 activity by H(2)O(2) and NO donors is suggested to act in cell signaling via temporal control of reduction for the transmission of oxidative and/or nitrosative signals in thiol redox control.     

NADPH依赖性硫氧还蛋白还原酶C在植物质体中的作用

硫氧还蛋白的氧化还原调节作用在生物界中普遍存在。它能够还原目标蛋白的二硫键,而自身的活性位点则被氧化。因此,对于新的催化循环,则需要由相应的还原酶将其再次还原成活性形式。硫氧还蛋白对维持高等植物的光合效率同样具有重要意义。叶绿体中的硫氧还蛋白分别由铁氧还蛋白依赖性硫氧还蛋白还原酶和NADPH依赖性硫氧还蛋白还原酶C(NTRC)两种酶还原。NTRC的本质是一种黄素蛋白,除了具有还原酶活性外,还整合了一个硫氧还蛋白结构域,在叶绿体和淀粉体的氧化还原调节中处于核心地位。这种特殊的双功能酶在卡尔文-本森循环、氧化戊糖磷酸途径、抗过氧化、四吡咯代谢、ATP和淀粉合成、生长素和光周期调控中扮演了多重角色。本综述总结了NTRC的生理功能,并讨论了该蛋白质对植物质体氧化还原稳态的调节机制。     

Thiol-disulfide exchange in an immunoglobulin-like fold: structure of the N-terminal domain of DsbD

Escherichia coli DsbD transports electrons across the plasma membrane, a pathway that leads to the reduction of protein disulfide bonds. Three secreted thioredoxin-like factors, DsbC, DsbE, and DsbG, reduce protein disulfide bonds whereby an active site C-X-X-C motif is oxidized to generate a disulfide bond. DsbD catalyzes the reduction of the disulfide of DsbC, DsbE, and DsbG but not of the thioredoxin-like oxidant DsbA. The reduction of DsbC, DsbE, and DsbG occurs by transport of electrons from cytoplasmic thioredoxin to the C-terminal thioredoxin-like domain of DsbD (DsbD(C)). The N-terminal domain of DsbD, DsbD(N), acts as a versatile adaptor in electron transport and is capable of forming disulfides with oxidized DsbC, DsbE, or DsbG as well as with reduced DsbD(C). Isolated DsbD(N) is functional in electron transport in vitro. Crystallized DsbD(N) assumes an immunoglobulin-like fold that encompasses two active site cysteines, C103 and C109, forming a disulfide bond between beta-strands. The disulfide of DsbD(N) is shielded from the environment and capped by a phenylalanine (F70). A model is discussed whereby the immunoglobulin fold of DsbD(N) may provide for the discriminating interaction with thioredoxin-like factors, thereby triggering movement of the phenylalanine cap followed by disulfide rearrangement.     

Selective redox regulation of cytokine receptor signaling by extracellular thioredoxin-1

The thiol-disulfide oxidoreductase thioredoxin-1 (Trx1) is known to be secreted by leukocytes and to exhibit cytokine-like properties. Extracellular effects of Trx1 require a functional active site, suggesting a redox-based mechanism of action. However, specific cell surface proteins and pathways coupling extracellular Trx1 redox activity to cellular responses have not been identified so far. Using a mechanism-based kinetic trapping technique to identify disulfide exchange interactions on the intact surface of living lymphocytes, we found that Trx1 catalytically interacts with a single principal target protein. This target protein was identified as the tumor necrosis factor receptor superfamily member 8 (TNFRSF8/CD30). We demonstrate that the redox interaction is highly specific for both Trx1 and CD30 and that the redox state of CD30 determines its ability to engage the cognate ligand and transduce signals. Furthermore, we confirm that Trx1 affects CD30-dependent changes in lymphocyte effector function. Thus, we conclude that receptor-ligand signaling interactions can be selectively regulated by an extracellular redox catalyst.     

Redox potential of human thioredoxin 1 and identification of a second dithiol/disulfide motif

Thioredoxin (Trx1) is a redox-active protein containing two active site cysteines (Cys-32 and Cys-35) that cycle between the dithiol and disulfide forms as Trx1 reduces target proteins. Examination of the redox characteristics of this active site dithiol/disulfide couple is complicated by the presence of three additional non-active site cysteines. Using the redox Western blot technique and matrix assisted laser desorption ionization time-of-flight mass spectrometry mass spectrometry, we determined the midpoint potential (E0) of the Trx1 active site (-230 mV) and identified a second redox-active dithiol/disulfide (Cys-62 and Cys-69) in an alpha helix proximal to the active site, which formed under oxidizing conditions. This non-active site disulfide was not a substrate for reduction by thioredoxin reductase and delayed the reduction of the active site disulfide by thioredoxin reductase. Within actively growing THP1 cells, most of the active site of Trx1 was in the dithiol form, whereas the non-active site was totally in the dithiol form. The addition of increasing concentrations of diamide to these cells resulted in oxidation of the active site at fairly low concentrations and oxidation of the non-active site at higher concentrations. Taken together these results suggest that the Cys-62-Cys-69 disulfide could provide a means to transiently inhibit Trx1 activity under conditions of redox signaling or oxidative stress, allowing more time for the sensing and transmission of oxidative signals.     

Identification and characterization of TRP14, a thioredoxin-related protein of 14 kDa. New insights into the specificity of thioredoxin function

We have identified and characterized a 14-kDa human thioredoxin (Trx)-related protein designated TRP14. This cytosolic protein was expressed in all tissues and cell types examined, generally in smaller amounts than Trx1. Although TRP14 contains five cysteines, only the two Cys residues in its WCPDC motif were exposed and redox sensitive. Unlike Trx1, which was an equally good substrate for both Trx reductase 1 (TrxR1) and TrxR2, oxidized TRP14 was reduced by TrxR1 but not by TrxR2. Biochemical characterization of TRP14 suggested that, like Trx1, TRP14 is a disulfide reductase; its active site cysteine is sufficiently nucleophilic with the pK(a) value of 6.1; and its redox potential (-257 mV) is similar to those of other cellular thiol reductants. However, although TRP14 reduced small disulfide-containing peptides, it did not reduce the disulfides of known Trx1 substrates, ribonucleotide reductase, peroxiredoxin, and methionine sulfoxide reductase. These results suggest that TRP14 and Trx1 might act on distinct substrate proteins.     

Thioredoxin 1 Is Inactivated Due to Oxidation Induced by Peroxiredoxin under Oxidative Stress and Reactivated by the Glutaredoxin System

The mammalian cytosolic thioredoxin system, comprising thioredoxin (Trx), Trx reductase, and NADPH, is the major protein-disulfide reductase of the cell and has numerous functions. Besides the active site thiols, human Trx1 contains three non-active site cysteine residues at positions 62, 69, and 73. A two-disulfide form of Trx1, containing an active site disulfide between Cys-32 and Cys-35 and a non-active site disulfide between Cys-62 and Cys-69, is inactive either as a disulfide reductase or as a substrate for Trx reductase. This could possibly provide a structural switch affecting Trx1 function during oxidative stress and redox signaling. We found that two-disulfide Trx1 was generated in A549 cells under oxidative stress. In vitro data showed that two-disulfide Trx1 was generated from oxidation of Trx1 catalyzed by peroxiredoxin 1 in the presence of H₂O₂. The redox Western blot data indicated that the glutaredoxin system protected Trx1 in HeLa cells from oxidation caused by ebselen, a superfast oxidant for Trx1. Our results also showed that physiological concentrations of glutathione, NADPH, and glutathione reductase reduced the non-active site disulfide in vitro. This reaction was stimulated by glutaredoxin 1 via the so-called monothiol mechanism. In conclusion, reversible oxidation of the non-active site disulfide of Trx1 is suggested to play an important role in redox regulation and cell signaling via temporal inhibition of its protein-disulfide reductase activity for the transmission of oxidative signals under oxidative stress.     

Activity assays of mammalian thioredoxin and thioredoxin reductase: Fluorescent disulfide substrates,mechanisms, and use with tissue samples

Thioredoxin (Trx) is a protein disulfide reductase that, together with nicotinamide adenine dinucleotide phosphate (NADPH) and thioredoxin reductase (TrxR), controls oxidative stress or redox signaling via thiol redox control. Human cytosolic Trx1 has Cys32 and Cys35 as the active site and three additional cysteine residues (Cys62, Cys69, and Cys73), which by oxidation generates inactive Cys62 to Cys69 two-disulfide Trx. This, combined with TrxR with a broad substrate specificity, complicates assays of mammalian Trx and TrxR. We sought to understand the autoregulation of Trx and TrxR and to generate new methods for quantification of Trx and TrxR. We optimized the synthesis of two fluorescent substrates, di-eosin–glutathione disulfide (Di-E–GSSG) and fluorescein isothiocyanate-labeled insulin (FiTC–insulin), which displayed higher fluorescence on disulfide reduction. Di-E–GSSG showed a very large increase in fluorescence quantum yield but had a relatively low affinity for Trx and was also a weak direct substrate for TrxR, in contrast to GSSG. FiTC–insulin was used to develop highly sensitive assays for TrxR and Trx. Reproducible conditions were developed for reactivation of modified Trx, commonly present in frozen or oxidized samples. Trx in cell extracts and tissue samples, including plasma and serum, were subsequently analyzed, showing highly reproducible results and allowing measurement of trace amounts of Trx.     

Gram-positive DsbE proteins function differently from Gram-negative DsbE homologs. A structure to function analysis of DsbE from Mycobacterium tuberculosis

Mycobacterium tuberculosis, a Gram-positive bacterium, encodes a secreted Dsb-like protein annotated as Mtb DsbE (Rv2878c, also known as MPT53). Because Dsb proteins in Escherichia coli and other bacteria seem to catalyze proper folding during protein secretion and because folding of secreted proteins is thought to be coupled to disulfide oxidoreduction, the function of Mtb DsbE may be to ensure that secreted proteins are in their correctly folded states. We have determined the crystal structure of Mtb DsbE to 1.1 A resolution, which reveals a thioredoxin-like domain with a typical CXXC active site. These cysteines are in their reduced state. Biochemical characterization of Mtb DsbE reveals that this disulfide oxidoreductase is an oxidant, unlike Gram-negative bacteria DsbE proteins, which have been shown to be weak reductants. In addition, the pK(a) value of the active site, solvent-exposed cysteine is approximately 2 pH units lower than that of Gram-negative DsbE homologs. Finally, the reduced form of Mtb DsbE is more stable than the oxidized form, and Mtb DsbE is able to oxidatively fold hirudin. Structural and biochemical analysis implies that Mtb DsbE functions differently from Gram-negative DsbE homologs, and we discuss its possible functional role in the bacterium.     

Structural and mechanistic insights into unusual thiol disulfide oxidoreductase

Cytoplasmic desulfothioredoxin (Dtrx) from the anaerobe Desulfovibrio vulgaris Hildenborough has been identified as a new member of the thiol disulfide oxidoreductase family. The active site of Dtrx contains a particular consensus sequence, CPHC, never seen in the cytoplasmic thioredoxins and generally found in periplasmic oxidases. Unlike canonical thioredoxins (Trx), Dtrx does not present any disulfide reductase activity, but it presents instead an unusual disulfide isomerase activity. We have used NMR spectroscopy to gain insights into the structure and the catalytic mechanism of this unusual Dtrx. The redox potential of Dtrx (-181 mV) is significantly less reducing than that of canonical Trx. A pH dependence study allowed the determination of the pK(a) of all protonable residues, including the cysteine and histidine residues. Thus, the pK(a) values for the thiol group of Cys(31) and Cys(34) are 4.8 and 11.3, respectively. The His(33) pK(a) value, experimentally determined for the first time, differs notably as a function of the redox states, 7.2 for the reduced state and 4.6 for the oxidized state. These data suggest an important role for His(33) in the molecular mechanism of Dtrx catalysis that is confirmed by the properties of mutant DtrxH33G protein. The NMR structure of Dtrx shows a different charge repartition compared with canonical Trx. The results presented are likely indicative of the involvement of this protein in the catalysis of substrates specific of the anaerobe cytoplasm of DvH. The study of Dtrx is an important step toward revealing the molecular details of the thiol-disulfide oxidoreductase catalytic mechanism.     

Measurement and meaning of cellular thiol:disufhide redox status

The functional group of cysteine is a thiol group (SH) that, due to its chemical reactivity, is able to undergo a wide array of modifications each with the potential to confer a different property or function to the molecule harboring this residue. Most of these modifications involve the reversible oxidation of the thiol to sulfenic acid (SOH), and disulfide, including intra- and intermolecular disulfides between polypeptides and glutathione (glutathionylation). The reversibility of these oxidations allows thiol groups to serve as versatile chemical and structural transducing elements in several low molecular mass metabolites and proteins. A plethora of cellular functions such as DNA and protein synthesis, protein secretion, cytoskeleton architecture, differentiation, apoptosis, and anti-oxidant defense, are recognized to be modulated, at certain stage, by thiol–disulfide exchange mechanisms of redox active thiol groups. All organisms are equipped with enzymatic systems composed by NADPH-dependent reductases, redoxins, and peroxidases that provide kinetic control of global thiol-redox homeostasis as well as target selectivity. These redox systems are distributed in different subcellular compartments and are not in equilibrium with each other. In consequence, measuring cellular thiol–disulfide status represents a challenge for studies aimed to obtain dynamic and spatio-temporal resolution. This review provides a summary of the methods and tools available to quantify the thiol redox status of cells.     

Study of the thiol/disulfide redox systems of the anaerobe Desulfovibrio vulgaris points out pyruvate:ferredoxin oxidoreductase as a new target for thioredoxin 1

Sulfate reducers have developed a multifaceted adaptative strategy to survive against oxidative stresses. Along with this oxidative stress response, we recently characterized an elegant reversible disulfide bond-dependent protective mechanism in the pyruvate:ferredoxin oxidoreductase (PFOR) of various Desulfovibrio species. Here, we searched for thiol redox systems involved in this mechanism. Using thiol fluorescent labeling, we show that glutathione is not the major thiol/disulfide balance-controlling compound in four different Desulfovibrio species and that no other plentiful low molecular weight thiol can be detected. Enzymatic analyses of two thioredoxins (Trxs) and three thioredoxin reductases allow us to propose the existence of two independent Trx systems in Desulfovibrio vulgaris Hildenborough (DvH). The TR1/Trx1 system corresponds to the typical bacterial Trx system. We measured a TR1 apparent K(m) value for Trx1 of 8.9 μM. Moreover, our results showed that activity of TR1 was NADPH-dependent. The second system named TR3/Trx3 corresponds to an unconventional Trx system as TR3 used preferentially NADH (K(m) for NADPH, 743 μM; K(m) for NADH, 5.6 μM), and Trx3 was unable to reduce insulin. The K(m) value of TR3 for Trx3 was 1.12 μM. In vitro experiments demonstrated that the TR1/Trx1 system was the only one able to reactivate the oxygen-protected form of Desulfovibrio africanus PFOR. Moreover, ex vivo pulldown assays using the mutant Trx1(C33S) as bait allowed us to capture PFOR from the DvH extract. Altogether, these data demonstrate that PFOR is a new target for Trx1, which is probably involved in the protective switch mechanism of the enzyme.     

Differential oxidation of thioredoxin-1, thioredoxin-2, and glutathione by metal ions

Metal toxicity often includes the generation of reactive oxygen species (ROS) and subsequent oxidative stress, but whether metals have different effects on the major thiol antioxidant systems is unknown. Here, we examine the effects of arsenic, cadmium, cesium, copper, iron, mercury, nickel, and zinc on glutathione (GSH), cytoplasmic thioredoxin-1 (Trx1), and mitochondrial thioredoxin-2 (Trx2) redox states. GSH/GSSG redox states were determined by HPLC, and Trx1 and Trx2 redox states were determined by Redox Western blot methods. Copper, iron, and nickel showed significant oxidation of GSH but relatively little oxidation of either Trx1 or Trx2. Arsenic, cadmium, and mercury showed little oxidation of GSH but significantly oxidized both Trx1 and Trx2. The magnitude of effects of arsenic, cadmium, and mercury was greater for the mitochondrial Trx2 (>60 mV) compared to the cytoplasmic Trx1 (20 to 40 mV). Apoptosis signal-regulating kinase 1 (ASK1) may be activated by two different pathways, one dependent upon GSH and glutaredoxin and the other independent of GSH and dependent upon thioredoxin. ASK1 activation and cell death were observed with metals that oxidized thioredoxins but not with metals that oxidized GSH. These findings show that metals have differential oxidative effects on the major thiol antioxidant systems and that activation of apoptosis may be associated with metal ions that oxidize thioredoxin and activate ASK1. The differential oxidation of the major thiol antioxidant systems by metal ions suggest that the distinct thiol/disulfide redox couples represented by GSH/GSSG and the thioredoxins may convey different levels of control in apoptotic and toxic signaling pathways.