Generating disulfides with the Quiescin-sulfhydryl oxidases

Redox regulation of UPR signalling and mitochondrial ER contact sites

Mitochondria and the endoplasmic reticulum (ER) have a synergistic relationship and are key regulatory hubs in maintaining cell homeostasis. Communication between these organelles is mediated by mitochondria ER contact sites (MERCS), allowing the exchange of material and information, modulating calcium homeostasis, redox signalling, lipid transfer and the regulation of mitochondrial dynamics. MERCS are dynamic structures that allow cells to respond to changes in the intracellular environment under normal homeostatic conditions, while their assembly/disassembly are affected by pathophysiological conditions such as ageing and disease. Disruption of protein folding in the ER lumen can activate the Unfolded Protein Response (UPR), promoting the remodelling of ER membranes and MERCS formation. The UPR stress receptor kinases PERK and IRE1, are located at or close to MERCS. UPR signalling can be adaptive or maladaptive, depending on whether the disruption in protein folding or ER stress is transient or sustained. Adaptive UPR signalling via MERCS can increase mitochondrial calcium import, metabolism and dynamics, while maladaptive UPR signalling can result in excessive calcium import and activation of apoptotic pathways. Targeting UPR signalling and the assembly of MERCS is an attractive therapeutic approach for a range of age-related conditions such as neurodegeneration and sarcopenia. This review highlights the emerging evidence related to the role of redox mediated UPR activation in orchestrating inter-organelle communication between the ER and mitochondria, and ultimately the determination of cell function and fate.

Escherichia coli Cytoplasmic Expression of Disulfide-Bonded Proteins: Side-by-Side Comparison between Two Competing Strategies

The production of disulfide bond-containing recombinant proteins in Escherichia coli has traditionally been done by either refolding from inclusion bodies or by targeting the protein to the periplasm. However, both approaches have limitations. Two broad strategies were developed to allow the production of proteins with disulfide bonds in the cytoplasm of E. coli: i) engineered strains with deletions in the disulfide reduction pathways, e.g. SHuffle, and ii) the co-expression of oxidative folding catalysts, e.g. CyDisCo. However, to our knowledge, the relative effectiveness of these strategies has not been properly evaluated. Here, we systematically compare the purified yields of 14 different proteins of interest (POI) that contain disulfide bonds in their native state when expressed in both systems. We also compared the effects of different background strains, commonly used promoters, and two media types: defined and rich autoinduction. In rich autoinduction media, POI which can be produced in a soluble (non-native) state without a system for disulfide bond formation were produced in higher purified yields from SHuffle, whereas all other proteins were produced in higher purified yields using CyDisCo. In chemically defined media, purified yields were at least 10x higher in all cases using CyDisCo. In addition, the quality of the three POI tested was superior when produced using CyDisCo.

Effects of redox modulation on quiescin/sulfhydryl oxidase activity of melanoma cells

Secreted quiescin/sulfhydryl oxidase (QSOX) is overexpressed in many tumor cell lines, including melanoma, and is usually associated with a pro-invasive phenotype. Our previous work described that B16-F10 cells enter in a quiescent state as a protective mechanism against damage generated by reactive oxygen species (ROS) during melanogenesis stimulation. Our present results show that QSOX activity was two-fold higher in cells with stimulated melanogenesis when compared to control cells. Considering that glutathione (GSH) is one of the main factor responsible for controlling redox homeostasis in cells, this work also aimed to investigate the relationship between QSOX activity, GSH levels and melanogenesis stimulation in B16-F10 murine melanoma cell line. The redox homeostasis was impaired by treating cells with GSH in excess or depleting its intracellular levels through BSO treatment. Interestingly, GSH-depleted cells without stimulation of melanogenesis kept high levels of viability, suggesting a possible adaptive mechanism of survival even under low GSH levels. They also showed lower extracellular activity of QSOX, and higher QSOX intracellular immunostaining, suggesting that this enzyme was less excreted from cells and corroborating with a diminished extracellular QSOX activity. On the other hand, cells under melanogenesis stimulation showed a lower GSH/GSSG ratio (8:1) in comparison with control (non-stimulated) cells (20:1), indicating a pro-oxidative state after stimulation. This was accompanied by decreased cell viability after GSH-depletion, no alterations in QSOX extracellular activity, but higher QSOX nucleic immunostaining. We suggest that melanogenesis stimulation and redox impairment caused by GSH-depletion enhanced the oxidative stress in these cells, contributing to additional alterations of its metabolic adaptive response.

Caveolae-mediated endocytosis of extracellular QSOX1b modulates the migration of fibroblasts

Quiescin/sulfhydryl oxidase (QSOX1) is a secreted flavoprotein that modulates cellular proliferation, migration and adhesion, roles attributed to its ability to organize the extracellular matrix. We previously showed that exogenously added QSOX1b induces smooth muscle cells migration in a process that depends on its enzymatic activity and that is mediated by hydrogen peroxide derived from Nox1, a catalytic subunit of NAD(P)H oxidases. Here, we report that exogenous QSOX1b also stimulates the migration of L929 fibroblasts and that this effect is regulated by its endocytosis. The use of endocytosis inhibitors and caveolin 1-knockdown demonstrated that this endocytic pathway is caveola-mediated. QSOX1b colocalized with Nox1 in intracellular vesicles, as detected by confocal fluorescence, suggesting that extracellular QSOX1b is endocytosed with the transmembrane Nox1. These results reveal that endosomal QSOX1b is a novel intracellular redox regulator of cell migration.

Identification of three new microsatellites and their effects on body measurement traits in pigs using time of flight-mass spectrometry (TOF-MS)

The body status of livestock affects their physiological function and productive performances. Microsatellites, one of the most used DNA markers, have been found to be associated with pig productive traits. However, their identifications and effects on body measurement traits of the Chinese Qinghai Bamei pig still uncovered. According to our previous sequencing data, in this study, three novel microsatellites were found in this breed. Using time of flight-mass spectrometry (TOF-MS) method, these microsatellites were further identified in a large Bamei pig population. TOF-MS spectra showed that there are three microsatellites loci, named P1, P2 and P3. These microsatellites were linkage equilibrium based on the values of D' and r² tests. Association results demonstrated that P1 locus was associated with the body length, body height and chest width and the beneficial genotype was 150-/150-bp (p < 0.05); and P2 locus was associated with the body height (p < 0.05), and the 145-/145-bp, 145-/147-bp and 145-/149-bp were claimed as favorable genotypes and 145-bp allele was considered as the favorable allele. These findings suggested that P1 and P2 microsatellites might be considered as the candidate genetic markers to select pigs with superior body sizes, especially in local breed.

Characterization of a Sulfhydryl Oxidase From Plasmodium berghei as a Target for Blocking Parasite Transmission

Quiescin sulfhydryl oxidase (QSOX), present in a wide variety of eukaryotic species, catalyzes the insertion of disulfide bonds into unfolded, reduced proteins. Here we characterized the QSOX protein from the rodent malaria parasite Plasmodium berghei (PbQSOX), which is conserved in all sequenced malaria parasite species. The PbQSOX protein was not expressed in asexual erythrocytic stages, but was most abundantly expressed in ookinetes. Indirect immunofluorescence assays revealed PbQSOX was not only localized in cytoplasm of gametocytes, gametes and ookinetes, but also expressed on the surface of gametes and ookinetes. Western blot identified extracellular presence of PbQSOX in the culture medium of ookinetes suggestive of secretion. Pbqsox deletion (Δpbqsox) did not affect asexual intraerythrocytic development, but reduced exflagellation of male gametocytes as well as formation and maturation of ookinetes. Pbqsox deletion also led to a significant increase in the reduced thiol groups of ookinete surface proteins, suggesting that it may play a role in maintaining the integrity of disulfide bonds of surface proteins, which might be needed for ookinete development. Mosquitoes that fed on Δpbqsox-infected mice showed a significant reduction in ookinete and oocyst numbers compared to those fed on wild-type parasite-infected mice. Further, both polyclonal mouse antisera and a monoclonal antibody against the recombinant PbQSOX exhibited substantial transmission-blocking activities in in vitro and mosquito feeding assays, suggesting QSOX is a potential target for blocking parasite transmission.

Aberrant Upregulation of Compensatory Redox Molecular Machines May Contribute to Sperm Dysfunction in Infertile Men with Unilateral Varicocele: A Proteomic Insight

Aims: To understand the molecular pathways involved in oxidative stress (OS)-mediated sperm dysfunction against a hypoxic and hyperthermic microenvironment backdrop of varicocele through a proteomic approach. Results: Protein selection (261) based on their role in redox homeostasis and/or oxidative/hyperthermic/hypoxic stress response from the sperm proteome data set of unilateral varicocele (UV) in comparison with fertile control displayed 85 to be differentially expressed. Upregulation of cellular oxidant detoxification and glutathione and reduced nicotinamide adenine dinucleotide (NADH) metabolism accompanied with downregulation of protein folding, energy metabolism, and heat stress responses were observed in the UV group. Ingenuity pathway analysis (IPA) predicted suppression of oxidative phosphorylation (OXPHOS) (validated by Western blotting [WB]) along with augmentation in OS and mitochondrial dysfunction in UV. The top affected networks indicated by IPA involved heat shock proteins (HSPs: HSPA2 and HSP90B1). Their expression profile was corroborated by immunocytochemistry and WB. Hypoxia-inducible factor 1A as an upstream regulator of HSPs was predicted by MetaCore. Occurrence of reductive stress in UV spermatozoa was corroborated by thiol redox status. Innovation: This is the first evidence of a novel pathway showing aberrant redox homeostasis against chronic hypoxic insult in varicocele leading to sperm dysfunction. Conclusions: Upregulation of antioxidant system and dysfunctional OXPHOS would have shifted the redox balance of biological redox couples (GSH/GSSG, NAD⁺/NADH, and NADP⁺/NADPH) to a more reducing state leading to reductive stress. Chronic reductive stress-induced OS may be involved in sperm dysfunction in infertile men with UV, where the role of HSPs cannot be ignored. Intervention with antioxidant therapy warrants proper prior investigation.

Golgi pH, Ion and Redox Homeostasis: How Much Do They Really Matter?

Exocytic and endocytic compartments each have their own unique luminal ion and pH environment that is important for their normal functioning. A failure to maintain this environment - the loss of homeostasis - is not uncommon. In the worst case, all the main Golgi functions, including glycosylation, membrane trafficking and protein sorting, can be perturbed. Several factors contribute to Golgi homeostasis. These include not only ions such as H⁺, Ca²⁺, Mg²⁺, Mn²⁺, but also Golgi redox state and nitric oxide (NO) levels, both of which are dependent on the oxygen levels in the cells. Changes to any one of these factors have consequences on Golgi functions, the nature of which can be dissimilar or similar depending upon the defects themselves. For example, altered Golgi pH homeostasis gives rise to Cutis laxa disease, in which glycosylation and membrane trafficking are both affected, while altered Ca²⁺ homeostasis due to the mutated SCPA1 gene in Hailey-Hailey disease, perturbs various protein sorting, proteolytic cleavage and membrane trafficking events in the Golgi. This review gives an overview of the molecular machineries involved in the maintenance of Golgi ion, pH and redox homeostasis, followed by a discussion of the organelle dysfunction and disease that frequently result from their breakdown. Congenital disorders of glycosylation (CDGs) are discussed only when they contribute directly to Golgi pH, ion or redox homeostasis. Current evidence emphasizes that, rather than being mere supporting factors, Golgi pH, ion and redox homeostasis are in fact key players that orchestrate and maintain all Golgi functions.

Quiescin Sulfhydryl Oxidase 1 (QSOX1) Secreted by Lung Cancer Cells Promotes Cancer Metastasis

As lung cancer shows the highest mortality in cancer-related death, serum biomarkers are demanded for lung cancer diagnosis and its treatment. To discover lung cancer protein biomarkers, secreted proteins from primary cultured lung cancer and adjacent normal tissues from patients were subjected to LC/MS⁻MS proteomic analysis. Quiescin sulfhydryl oxidase (QSOX1) was selected as a biomarker candidate from the enriched proteins in the secretion of lung cancer cells. QSOX1 levels were higher in 82% (51 of 62 tissues) of lung cancer tissues compared to adjacent normal tissues. Importantly, QSOX1 serum levels were significantly higher in cancer patients (p < 0.05, Area Under curve (AUC) = 0.89) when measured by multiple reaction monitoring (MRM). Higher levels of QSOX1 were also uniquely detected in lung cancer tissues, among several other solid cancers, by immunohistochemistry. QSOX1-knock-downed Lewis lung cancer (LLC) cells were less viable from oxidative stress and reduced migration and invasion. In addition, LLC mouse models with QSOX1 knock-down also proved that QSOX1 functions in promoting cancer metastasis. In conclusion, QSOX1 might be a lung cancer tissue-derived biomarker and be involved in the promotion of lung cancers, and thus can be a therapeutic target for lung cancers.

Gaussia princeps luciferase: a bioluminescent substrate for oxidative protein folding

Gaussia princeps luciferase (GLuc) generates an intense burst of blue light when exposed to coelenterazine in the absence of ATP. Here we show that this 5-disulfide containing enzyme can be used as a facile and convenient substrate for studies of oxidative protein folding. Reduced GLuc (rGLuc), with 10 free cysteine residues, is completely inactive as a luciferase but >60% bioluminescence activity, compared to controls, can be recovered using a range of oxidizing regimens in the absence of the exogenous shuffling activity of protein disulfide isomerase (PDI). The sulfhydryl oxidase QSOX1 can be assayed using rGLuc in a simple bioluminescence plate reader format. Similarly, low concentrations of rGLuc can be oxidized by millimolar levels of dehydroascorbate, hydrogen peroxide or much lower concentrations of sodium tetrathionate. The oxidative refolding of rGLuc in the presence of a range of glutathione redox buffers is only marginally accelerated by micromolar levels of PDI. This modest rate enhancement probably results from a relatively simple disulfide connectivity in native GLuc; reflecting two homologous domains each carrying two disulfide bonds with a single interdomain disulfide. When GLuc is reoxidized under denaturing conditions the resulting scrambled protein (sGLuc) can be used in a sensitive bioluminescence assay for reduced PDI in the absence of added exogenous thiols. Finally, the general facility by which rGLuc can recover bioluminescent activity in vitro provides a sensitive method for the assessment of inhibitors of oxidative protein folding.

More than a syllable in fib-ROS-is: The role of ROS on the fibrotic extracellular matrix and on cellular contacts

Fibrosis is characterized by excess deposition of extracellular matrix (ECM). However, the ECM changes during fibrosis not only quantitatively but also qualitatively. Thus, the composition is altered as the expression of various ECM proteins changes. Moreover, also posttranslational modifications, secretion, deposition and crosslinkage as well as the proteolytic degradation of ECM components run differently during fibrosis. As several of these processes involve redox reactions and some of them are even redox-regulated, reactive oxygen species (ROS) influence fibrotic diseases. Redox regulation of the ECM has not been studied intensively, although evidences exist that the alteration of the ECM, including the redox-relevant processes of its formation and degradation, may be of key importance not only as a cause but also as a consequence of fibrotic diseases. Myofibroblasts, which have differentiated from fibroblasts during fibrosis, produce most of the ECM components and in return obtain important environmental cues of the ECM, including their redox-dependent fibrotic alterations. Thus, myofibroblast differentiation and fibrotic changes of the ECM are interdependent processes and linked with each other via cell-matrix contacts, which are mediated by integrins and other cell adhesion molecules. These cell-matrix contacts are also regulated by redox processes and by ROS. However, most of the redox-catalyzing enzymes are localized within cells. Little is known about redox-regulating enzymes, especially the ones that control the formation and cleavage of redox-sensitive disulfide bridges within the extracellular space. They are also important players in the redox-regulative crosstalk between ECM and cells during fibrosis.

Chemistry and Enzymology of Disulfide Cross-Linking in Proteins

Cysteine thiols are among the most reactive functional groups in proteins, and their pairing in disulfide linkages is a common post-translational modification in proteins entering the secretory pathway. This modest amino acid alteration, the mere removal of a pair of hydrogen atoms from juxtaposed cysteine residues, contrasts with the substantial changes that characterize most other post-translational reactions. However, the wide variety of proteins that contain disulfides, the profound impact of cross-linking on the behavior of the protein polymer, the numerous and diverse players in intracellular pathways for disulfide formation, and the distinct biological settings in which disulfide bond formation can take place belie the simplicity of the process. Here we lay the groundwork for appreciating the mechanisms and consequences of disulfide bond formation in vivo by reviewing chemical principles underlying cysteine pairing and oxidation. We then show how enzymes tune redox-active cofactors and recruit oxidants to improve the specificity and efficiency of disulfide formation. Finally, we discuss disulfide bond formation in a cellular context and identify important principles that contribute to productive thiol oxidation in complex, crowded, dynamic environments.

Maternal nicotine exposure leads to decreased cardiac protein disulfide isomerase and impaired mitochondrial function in male rat offspring

Smoking throughout pregnancy can lead to complications during gestation, parturition and neonatal development. Thus, nicotine replacement therapies are a popular alternative thought to be safer than cigarettes. However, recent studies in rodents suggest that fetal and neonatal nicotine exposure alone results in cardiac dysfunction and high blood pressure. While it is well known that perinatal nicotine exposure causes increased congenital abnormalities, the mechanisms underlying longer-term deficits in cardiac function are not completely understood. Recently, our laboratory demonstrated that nicotine impairs placental protein disulfide isomerase (PDI) triggering an increase in endoplasmic reticulum stress, leading us to hypothesize that this may also occur in the heart. At 3 months of age, nicotine-exposed offspring had 45% decreased PDI levels in the absence of endoplasmic reticulum stress. Given the association of PDI and superoxide dismutase enzymes, we further observed that antioxidant superoxide dismutase-2 levels were reduced by 32% in these offspring concomitant with a 26-49% decrease in mitochondrial complex proteins (I, II, IV and V) and tissue inhibitor of metalloproteinase-4, a critical matrix metalloprotease for cardiac contractility and health. Collectively, this study suggests that perinatal nicotine exposure decreases PDI, which can promote oxidative damage and mitochondrial damage, associated with a premature decline in cardiac function.

Identification, characterization and purification of porcine Quiescin Q6-Sulfydryl Oxidase 2 protein

Background

Post-spermiogenesis membrane surface modifications rely on molecules present in the reproductive tracts. Two isoforms (isoform 1 and 2) from Quiescin Q6-Sulfydryl Oxidase protein family have been identified in the male reproductive tract of rodent species. However, unlike isoform 1, scarce information is available for isoform 2, likely due to its lower expression level and lack of proper purification methods to obtain sufficient protein quantity for further assays.

Results

This study demonstrated the presence of short and long forms of Quiescin Q6-Sulfydryl Oxidase 2 in boar, likely representing the secretory (short form) and transmembrane (long form) forms of Quiescin Q6-Sulfydryl Oxidase 2. Immunohistochemistry studies revealed the presence of Quiescin Q6-Sulfydryl Oxidase 2 in a broad range of porcine tissues; the pronounced vesicle-contained Quiescin Q6-Sulfydryl Oxidase 2 at the apical region of epididymis and seminal vesicles epithelium suggested its involvement in sperm physiology and its participation in semen formation. The majority of porcine Quiescin Q6-Sulfydryl Oxidase 2 could be purified via either antibody affinity column or be salted out using 10%–40% ammonium sulfate. Higher amount of low molecular weight Quiescin Q6-Sulfydryl Oxidase 2 observed in the seminal vesicle likely represents the secretory form of Quiescin Q6-Sulfydryl Oxidase 2 and reflects an exuberant secretory activity in this organ.

Conclusions

We demonstrated for the first time, the presence of Quiescin Q6-Sulfydryl Oxidase 2 in porcine species; moreover, two forms of Quiescin Q6-Sulfydryl Oxidase 2 were identified and exhibited distinct molecular weights and properties during protein purification processes. This study also provided feasible Quiescin Q6-Sulfydryl Oxidase 2 purification methods from slaughterhouse materials that could potentially allow obtaining sufficient amount of Quiescin Q6-Sulfydryl Oxidase 2 for future functional investigations.

Endoplasmic reticulum chaperones tweak the mitochondrial calcium rheostat to control metabolism and cell death

The folding of secretory proteins is a well-understood mechanism, based on decades of research on endoplasmic reticulum (ER) chaperones. These chaperones interact with newly imported polypeptides close to the ER translocon. Classic examples for these proteins include the immunoglobulin binding protein (BiP/GRP78), and the lectins calnexin and calreticulin. Although not considered chaperones per se, the ER oxidoreductases of the protein disulfide isomerase (PDI) family complete the folding job by catalyzing the formation of disulfide bonds through cysteine oxidation. Research from the past decade has demonstrated that ER chaperones are multifunctional proteins. The regulation of ER-mitochondria Ca²⁺ crosstalk is one of their additional functions, as shown for calnexin, BiP/GRP78 or the oxidoreductases Ero1α and TMX1. This function depends on interactions of this group of proteins with the ER Ca²⁺ handling machinery. This novel function makes perfect sense for two reasons: i. It allows ER chaperones to control mitochondrial apoptosis instantly without a lengthy bypass involving the upregulation of pro-apoptotic transcription factors via the unfolded protein response (UPR); and ii. It allows the ER protein folding machinery to fine-tune ATP import via controlling the speed of mitochondrial oxidative phosphorylation. Therefore, the role of ER chaperones in regulating ER-mitochondria Ca²⁺ flux identifies the progression of secretory protein folding as a central regulator of cell survival and death, at least in cell types that secrete large amount of proteins. In other cell types, ER protein folding might serve as a sentinel mechanism that monitors cellular well-being to control cell metabolism and apoptosis. The selenoprotein SEPN1 is a classic example for such a role. Through the control of ER-mitochondria Ca²⁺-flux, ER chaperones and folding assistants guide cellular apoptosis and mitochondrial metabolism.

Tumor-initiating cells of breast and prostate origin show alterations in the expression of genes related to iron metabolism

The importance of iron in the growth and progression of tumors has been widely documented. In this report, we show that tumor-initiating cells (TICs), represented by spheres derived from the MCF7 cell line, exhibit higher intracellular labile iron pool, mitochondrial iron accumulation and are more susceptible to iron chelation. TICs also show activation of the IRP/IRE system, leading to higher iron uptake and decrease in iron storage, suggesting that level of properly assembled cytosolic iron-sulfur clusters (FeS) is reduced. This finding is confirmed by lower enzymatic activity of aconitase and FeS cluster biogenesis enzymes, as well as lower levels of reduced glutathione, implying reduced FeS clusters synthesis/utilization in TICs. Importantly, we have identified specific gene signature related to iron metabolism consisting of genes regulating iron uptake, mitochondrial FeS cluster biogenesis and hypoxic response (ABCB10, ACO1, CYBRD1, EPAS1, GLRX5, HEPH, HFE, IREB2, QSOX1 and TFRC). Principal component analysis based on this signature is able to distinguish TICs from cancer cells in vitro and also Leukemia-initiating cells (LICs) from non-LICs in the mouse model of acute promyelocytic leukemia (APL). Majority of the described changes were also recapitulated in an alternative model represented by MCF7 cells resistant to tamoxifen (TAMR) that exhibit features of TICs. Our findings point to the critical importance of redox balance and iron metabolism-related genes and proteins in the context of cancer and TICs that could be potentially used for cancer diagnostics or therapy.

Ebselen inhibits QSOX1 enzymatic activity and suppresses invasion of pancreatic and renal cancer cell lines

Quiescin sulfhydryl oxidase 1 (QSOX1) is a highly conserved disulfide bond-generating enzyme that is overexpressed in diverse tumor types. Its enzymatic activity promotes the growth and invasion of tumor cells and alters extracellular matrix composition. In a nude mouse-human tumor xenograft model, tumors containing shRNA for QSOX1 grew significantly more slowly than controls, suggesting that QSOX1 supports a proliferative phenotype in vivo. High throughput screening experiments identified ebselen as an in vitro inhibitor of QSOX1 enzymatic activity. Ebselen treatment of pancreatic and renal cancer cell lines stalled tumor growth and inhibited invasion through Matrigel in vitro. Daily oral treatment with ebselen resulted in a 58% reduction in tumor growth in mice bearing human pancreatic tumor xenografts compared to controls. Mass spectrometric analysis of ebselen-treated QSOX1 mechanistically revealed that C165 and C237 of QSOX1 covalently bound to ebselen. This report details the anti-neoplastic properties of ebselen in pancreatic and renal cancer cell lines. The results here offer a “proof-of-principle” that enzymatic inhibition of QSOX1 may have clinical relevancy.

Mia40 is a facile oxidant of unfolded reduced proteins but shows minimal isomerase activity

Mia40 participates in oxidative protein folding within the mitochondrial intermembrane space (IMS) by mediating the transfer of reducing equivalents from client proteins to FAD-linked oxidoreductases of the Erv1 family (lfALR in mammals). Here we investigate the specificity of the human Mia40/lfALR system towards non-cognate unfolded protein substrates to assess whether the efficient introduction of disulfides requires a particular amino acid sequence context or the presence of an IMS targeting signal. Reduced pancreatic ribonuclease A (rRNase), avian lysozyme, and riboflavin binding protein are all competent substrates of the Mia40/lfALR system, although they lack those sequence features previously thought to direct disulfide bond formation in cognate IMS substrates. The oxidation of rRNase by Mia40 does not limit overall turnover of unfolded substrate by the Mia40/lfALR system. Mia40 is an ineffective protein disulfide isomerase when its ability to restore enzymatic activity from scrambled RNase is compared to that of protein disulfide isomerase. Mia40's ability to bind amphipathic peptides is evident by avid binding to the isolated B-chain during the insulin reductase assay. In aggregate these data suggest that the Mia40/lfALR system has a broad sequence specificity and that potential substrates may be protected from adventitious oxidation by kinetic sequestration within the mitochondrial IMS.

Structures and functions of protein disulfide isomerase family members involved in proteostasis in the endoplasmic reticulum

The endoplasmic reticulum (ER) is an essential cellular compartment in which an enormous number of secretory and cell surface membrane proteins are synthesized and subjected to cotranslational or posttranslational modifications, such as glycosylation and disulfide bond formation. Proper maintenance of ER protein homeostasis (sometimes termed proteostasis) is essential to avoid cellular stresses and diseases caused by abnormal proteins. Accumulating knowledge of cysteine-based redox reactions catalyzed by members of the protein disulfide isomerase (PDI) family has revealed that these enzymes play pivotal roles in productive protein folding accompanied by disulfide formation, as well as efficient ER-associated degradation accompanied by disulfide reduction. Each of PDI family members forms a protein-protein interaction with a preferential partner to fulfill a distinct function. Multiple redox pathways that utilize PDIs appear to function synergistically to attain the highest quality and productivity of the ER, even under various stress conditions. This review describes the structures, physiological functions, and cooperative actions of several essential PDIs, and provides important insights into the elaborate proteostatic mechanisms that have evolved in the extremely active and stress-sensitive ER.

Oxidative protein folding: from thiol-disulfide exchange reactions to the redox poise of the endoplasmic reticulum

This review examines oxidative protein folding within the mammalian endoplasmic reticulum (ER) from an enzymological perspective. In protein disulfide isomerase-first (PDI-first) pathways of oxidative protein folding, PDI is the immediate oxidant of reduced client proteins and then addresses disulfide mispairings in a second isomerization phase. In PDI-second pathways the initial oxidation is PDI-independent. Evidence for the rapid reduction of PDI by reduced glutathione is presented in the context of PDI-first pathways. Strategies and challenges are discussed for determination of the concentrations of reduced and oxidized glutathione and of the ratios of PDI(red):PDI(ox). The preponderance of evidence suggests that the mammalian ER is more reducing than first envisaged. The average redox state of major PDI-family members is largely to almost totally reduced. These observations are consistent with model studies showing that oxidative protein folding proceeds most efficiently at a reducing redox poise consistent with a stoichiometric insertion of disulfides into client proteins. After a discussion of the use of natively encoded fluorescent probes to report the glutathione redox poise of the ER, this review concludes with an elaboration of a complementary strategy to discontinuously survey the redox state of as many redox-active disulfides as can be identified by ratiometric LC-MS-MS methods. Consortia of oxidoreductases that are in redox equilibrium can then be identified and compared to the glutathione redox poise of the ER to gain a more detailed understanding of the factors that influence oxidative protein folding within the secretory compartment.

Cellular disulfide bond formation in bioactive peptides and proteins

Bioactive peptides play important roles in metabolic regulation and modulation and many are used as therapeutics. These peptides often possess disulfide bonds, which are important for their structure, function and stability. A systematic network of enzymes--a disulfide bond generating enzyme, a disulfide bond donor enzyme and a redox cofactor--that function inside the cell dictates the formation and maintenance of disulfide bonds. The main pathways that catalyze disulfide bond formation in peptides and proteins in prokaryotes and eukaryotes are remarkably similar and share several mechanistic features. This review summarizes the formation of disulfide bonds in peptides and proteins by cellular and recombinant machinery.

Disulfide bond generation in mammalian blood serum: detection and purification of quiescin-sulfhydryl oxidase

A sensitive new plate-reader assay has been developed showing that adult mammalian blood serum contains circulating soluble sulfhydryl oxidase activity that can introduce disulfide bonds into reduced proteins with the reduction of oxygen to hydrogen peroxide. The activity was purified 5000-fold to >90% homogeneity from bovine serum and found by mass spectrometry to be consistent with the short isoform of quiescin-sulfhydryl oxidase 1 (QSOX1). This FAD-dependent enzyme is present at comparable activity levels in fetal and adult commercial bovine sera. Thus cell culture media that are routinely supplemented with either fetal or adult bovine sera will contain this facile catalyst of protein thiol oxidation. QSOX1 is present at approximately 25 nM in pooled normal adult human serum. Examination of the unusual kinetics of QSOX1 toward cysteine and glutathione at low micromolar concentrations suggests that circulating QSOX1 is unlikely to significantly contribute to the oxidation of these monothiols in plasma. However, the ability of QSOX1 to rapidly oxidize conformationally mobile protein thiols suggests a possible contribution to the redox status of exofacial and soluble proteins in blood plasma. Recent proteomic studies showing that plasma QSOX1 can be utilized in the diagnosis of pancreatic cancer and acute decompensated heart failure, together with the overexpression of this secreted enzyme in a number of solid tumors, suggest that the robust QSOX assay developed here may be useful in the quantitation of enzyme levels in a wide range of biological fluids.

Expression level of quiescin sulfhydryl oxidase 1 (QSOX1) in neuroblastomas

Neuroblastoma is the most common extracranial solid malignant tumor observed during childhood. Although these tumors can sometimes regress spontaneously or respond well to treatment in infants, genetic alterations that influence apoptosis can, in some cases, confer resistance to chemotherapy or result in relapses and adversely affect prognosis for these patients. The aim of this study was to correlate immunohistochemical expression of the protein quiescin sulfhydryl oxidase 1 (QSOX1) in samples obtained from untreated neuroblastomas with the patients’ clinical and pathological prognostic factors and clinical course. Neuroblastoma samples (n=23) obtained from histology blocks were arrayed into tissue microarrays and analysed by immunohistochemistry. The cases were classified according to the following clinical and pathological prognostic factors: age at diagnosis greater or less than/equal to 18 months; location of the lesion at diagnosis (abdominal or extra-abdominal); presence or absence of bone-marrow infiltration; tumor differentiation (well or poorly differentiated); Shimada histopathologic classification (favourable or unfavourable); state of the tumor extracellular matrix (Schwannian-stroma rich or poor); amplification of the MYCN oncogene; and clinical course (dead or alive with or without relapses/residual lesions). Twelve of the cases were female, 9 children were over 18 months old, 9 cases presented with extra-abdominal tumors and 9 cases exhibited tumors with unfavourable histologies. Fifteen patients underwent bone-marrow biopsy, and 4 of these were positive for metastasis. Nine patients died. The higher immunohistochemical expression of QSOX1 was more common in well-differentiated samples (P=0.029), in stroma-rich samples (P=0.029) and in samples from patients with a high prevalence of relapses/residual disease. The functions of QSOX1 include extracellular matrix maturation and the induction of apoptosis. Therefore, QSOX1 may be involved in neuroblastoma differentiation and regression and may thus function as a biomarker for identifying risk groups for this neoplasm.

QSOX1 inhibits autophagic flux in breast cancer cells

The QSOX1 protein (Quiescin Sulfhydryl oxidase 1) catalyzes the formation of disulfide bonds and is involved in the folding and stability of proteins. More recently, QSOX1 has been associated with tumorigenesis and protection against cellular stress. It has been demonstrated in our laboratory that QSOX1 reduces proliferation, migration and invasion of breast cancer cells in vitro and reduces tumor growth in vivo. In addition, QSOX1 expression has been shown to be induced by oxidative or ER stress and to prevent cell death linked to these stressors. Given the function of QSOX1 in these two processes, which have been previously linked to autophagy, we wondered whether QSOX1 might be regulated by autophagy inducers and play a role in this catabolic process. To answer this question, we used in vitro models of breast cancer cells in which QSOX1 was overexpressed (MCF-7) or extinguished (MDA-MB-231). We first showed that QSOX1 expression is induced following amino acid starvation and maintains cellular homeostasis. Our results also indicated that QSOX1 inhibits autophagy through the inhibition of autophagosome/lysosome fusion. Moreover, we demonstrated that inhibitors of autophagy mimic the effect of QSOX1 on cell invasion, suggesting that its role in this process is linked to the autophagy pathway. Previously published data demonstrated that extinction of QSOX1 promotes tumor growth in NOG mice. In this study, we further demonstrated that QSOX1 null tumors present lower levels of the p62 protein. Altogether, our results demonstrate for the first time a role of QSOX1 in autophagy in breast cancer cells and tumors.

Going through the barrier: coupled disulfide exchange reactions promote efficient catalysis in quiescin sulfhydryl oxidase

The quiescin sulfhydryl oxidase (QSOX) family of enzymes generates disulfide bonds in peptides and proteins with the reduction of oxygen to hydrogen peroxide. Determination of the potentials of the redox centers in Trypanosoma brucei QSOX provides a context for understanding catalysis by this facile oxidant of protein thiols. The CXXC motif of the thioredoxin domain is comparatively oxidizing (E'0 of -144 mV), consistent with an ability to transfer disulfide bonds to a broad range of thiol substrates. In contrast, the proximal CXXC disulfide in the ERV (essential for respiration and vegetative growth) domain of TbQSOX is strongly reducing (E'0 of -273 mV), representing a major apparent thermodynamic barrier to overall catalysis. Reduction of the oxidizing FAD cofactor (E'0 of -153 mV) is followed by the strongly favorable reduction of molecular oxygen. The role of a mixed disulfide intermediate between thioredoxin and ERV domains was highlighted by rapid reaction studies in which the wild-type CGAC motif in the thioredoxin domain of TbQSOX was replaced by the more oxidizing CPHC or more reducing CGPC sequence. Mixed disulfide bond formation is accompanied by the generation of a charge transfer complex with the flavin cofactor. This provides thermodynamic coupling among the three redox centers of QSOX and avoids the strongly uphill mismatch between the formal potentials of the thioredoxin and ERV disulfides. This work identifies intriguing mechanistic parallels between the eukaryotic QSOX enzymes and the DsbA/B system catalyzing disulfide bond generation in the bacterial periplasm and suggests that the strategy of linked disulfide exchanges may be exploited in other catalysts of oxidative protein folding.

Human augmenter of liver regeneration: probing the catalytic mechanism of a flavin-dependent sulfhydryl oxidase

Augmenter of liver regeneration is a member of the ERV family of small flavin-dependent sulfhydryl oxidases that contain a redox-active CxxC disulfide bond in redox communication with the isoalloxazine ring of bound FAD. These enzymes catalyze the oxidation of thiol substrates with the reduction of molecular oxygen to hydrogen peroxide. This work studies the catalytic mechanism of the short, cytokine form of augmenter of liver regeneration (sfALR) using model thiol substrates of the enzyme. The redox potential of the proximal disulfide in sfALR was found to be approximately 57 mV more reducing than the flavin chromophore, in agreement with titration experiments. Rapid reaction studies show that dithiothreitol (DTT) generates a transient mixed disulfide intermediate with sfALR signaled by a weak charge-transfer interaction between the thiolate of C145 and the oxidized flavin. The subsequent transfer of reducing equivalents to the flavin ring is relatively slow, with a limiting apparent rate constant of 12.4 s(-1). However, reoxidation of the reduced flavin by molecular oxygen is even slower (2.3 s(-1) at air saturation) and thus largely limits turnover at 5 mM DTT. The nature of the charge-transfer complexes observed with DTT was explored using a range of simple monothiols to mimic the initial nucleophilic attack on the proximal disulfide. While β-mercaptoethanol is a very poor substrate of sfALR (∼0.3 min(-1) at 100 mM thiol), it rapidly generates a mixed disulfide intermediate allowing the thiolate of C145 to form a strong charge-transfer complex with the flavin. Unlike the other monothiols tested, glutathione is unable to form charge-transfer complexes and is an undetectable substrate of the oxidase. These data are rationalized on the basis of the stringent steric requirements for thiol-disulfide exchange reactions. The inability of the relatively bulky glutathione to attain the in-line geometry required for efficient disulfide exchange in sfALR may be physiologically important in preventing the oxidase from catalyzing the potentially harmful oxidation of intracellular glutathione.

Solubilization and identification of hen eggshell membrane proteins during different times of chicken embryo development using the proteomic approach

A fertilized chicken egg is a unit of life. During hatching, transport of nutrients, including calcium, have been reported from the egg components to the developing embryo. Calcium is mobilized from the eggshell with the involvement of Ca(2+)-binding proteins. In addition, other unknown proteins may also play some important roles during embryo developing process. Therefore identification and prediction of biological functions of eggshell membrane (ESM) proteins during chick embryo development was conducted by proteome analysis. Comparison of different lysis solutions indicated that the highest ability to extract ESM proteins could be obtained with 1 % sodium dodecyl sulfate in 5 mM Tris-HCl buffer pH 8.8 containing 0.1 % 2-mercaptoethanol. In this study fertilized Cornish chicken eggs were incubated at 37 °C in humidified incubators for up to 21 days. At selected times (days 1, 9, 15 and 21), samples were taken and the ESMs were carefully separated by hand, washed with distilled water, and air-dried at room temperature. The ESM proteins were then solubilized and analyzed by proteome analysis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis combined with high performance liquid chromatography and mass spectrometry revealed 62 proteins in the ESM; only keratin is known ESM protein, 8 of which are egg white proteins and related while 53 others have not previously been reported. Some differences in the types of proteins and their molecular functions were noted in ESM at different incubation times. One protein which was present only at days 15 and 21 of egg incubation was identified as a calcium binding protein i.e. EGF like repeats and discoidin I like domain 3 (EDIL3 homologous protein).

Biosynthetic pathway for the epipolythiodioxopiperazine acetylaranotin in Aspergillus terreus revealed by genome-based deletion analysis

Epipolythiodioxopiperazines (ETPs) are a class of fungal secondary metabolites derived from diketopiperazines. Acetylaranotin belongs to one structural subgroup of ETPs characterized by the presence of a seven-membered 4,5-dihydrooxepine ring. Defining the genes involved in acetylaranotin biosynthesis should provide a means to increase the production of these compounds and facilitate the engineering of second-generation molecules. The filamentous fungus Aspergillus terreus produces acetylaranotin and related natural products. Using targeted gene deletions, we have identified a cluster of nine genes (including one nonribosomal peptide synthetase gene, ataP) that is required for acetylaranotin biosynthesis. Chemical analysis of the wild-type and mutant strains enabled us to isolate 17 natural products from the acetylaranotin biosynthesis pathway. Nine of the compounds identified in this study are natural products that have not been reported previously. Our data have allowed us to propose a biosynthetic pathway for acetylaranotin and related natural products.

Diversification of quiescin sulfhydryl oxidase in a preserved framework for redox relay

BACKGROUND

The enzyme family Quiescin Sulfhydryl Oxidase (QSOX) is defined by the presence of an amino-terminal thioredoxin-fold (Trx) domain and a carboxy-terminal Erv family sulfhydryl oxidase domain. QSOX enzymes, which generate disulfide bonds and transfer them to substrate proteins, are present in a wide variety of eukaryotic species including metazoans and plants, but are absent from fungi. Plant and animal QSOXs differ in their active-site amino acid sequences and content of non-catalytic domains. The question arises, therefore, whether the Trx-Erv fusion has the same mechanistic significance in all QSOX enzymes, and whether shared features distinguish the functional domains of QSOX from other instances in which these domains occur independently. Through a study of QSOX phylogeny and an analysis of QSOX sequence diversity in light of recently determined three-dimensional structures, we sought insight into the origin and evolution of this multi-domain redox alliance.

RESULTS

An updated collection of QSOX enzymes was used to confirm and refine the differences in domain composition and active-site sequence motif patterns of QSOXs belonging to various eukaryotic phyla. Beyond the expected phylogenetic distinction of animal and plant QSOX enzymes, trees based on individual redox-active QSOX domains show a particular distinction of the Trx domain early in plant evolution. A comparison of QSOX domains with Trx and Erv domains from outside the QSOX family revealed several sequence and structural features that clearly differentiate QSOXs from other enzymes containing either of these domains. Notably, these features, present in QSOXs of various phyla, localize to the interface between the Trx and Erv domains observed in structures of QSOX that model interdomain redox communication.

CONCLUSIONS

The infrastructure for interdomain electron relay, previously identified for animal and parasite QSOXs, is found broadly across the QSOX family, including the plant enzymes. We conclude that the conserved three-dimensional framework of the QSOX catalytic domains accommodates lineage-specific differences and paralog diversification in the amino acid residues surrounding the redox-active cysteines. Our findings indicate that QSOX enzymes are characterized not just by the presence of the two defining domain folds but also by features that promote coordinated activity.

Elevated transcription of the gene QSOX1 encoding quiescin Q6 sulfhydryl oxidase 1 in breast cancer

The q arm of chromosome 1 is frequently amplified at the gene level in breast cancer. Since the significance of this is unclear we investigated whether 1q genes are overexpressed in this disease. The cDNA levels of 1q-located genes were analysed in a search for overexpressed genes. 26 genes mapping to the 1q arm show highly significant (P≤0.01) overexpression of transcripts in breast cancer compared to normal breast tissue. Amongst those showing the highest levels of overexpression in both expressed sequence tag (EST) and serial analysis of gene expression (SAGE) databases was enzyme quiescin Q6 sulfhydryl oxidase 1 (QSOX1). We investigated QSOX1 cDNA derived from T47D breast carcinoma cells by RT-PCR and 3′-RACE PCR and identified a novel extended form of QSOX1 transcript, containing a long 3′UTR, nearly double the size of the previously reported QSOX1 cDNA, and confirmed its 3′ end nucleotide sequence using RACE-PCR. We also used quantitative real-time PCR to analyse a panel of cDNAs derived from 50 clinically-graded normal and malignant breast tissue samples for the expression of QSOX1 mRNAs. QSOX1 transcription was elevated in an increasing proportion in the grade 2 and grade 3 tumours (graded according to the Nottingham prognostic index), with 10 of the 15 grade 3 tumours (67%) examined exceeding the normal range. There was a significant correlation between relative transcript level and clinical grade (P≤0.01) for all qPCR primer sets tested. QSOX1 mRNA levels, based on SAGE expression data, did not correlate with either Estrogen Receptor (ER) or Epidermal Growth Factor Receptor 2 (ErbB-2 or HER2/neu) expression. Our data indicate that QSOX1 is a potential new prognostic marker which may prove of use in the staging of breast tumours and the stratification of breast cancer patients.

Phylogenetics and enzymology of plant quiescin sulfhydryl oxidase

Quiescin Sulfhydryl Oxidase (QSOX), a catalyst of disulfide bond formation, is found in both plants and animals. Mammalian, avian, and trypanosomal QSOX enzymes have been studied in detail, but plant QSOX has yet to be characterized. Differences between plant and animal QSOXs in domain composition and active-site sequences raise the question of whether these QSOXs function by the same mechanism. We demonstrate that Arabidopsis thaliana QSOX produced in bacteria is folded and functional as a sulfhydryl oxidase but does not exhibit the interdomain electron transfer observed for its animal counterpart. Based on this finding, further exploration into the respective roles of the redox-active sites in plant QSOX and the reason for their concatenation is warranted.

Redox-assisted protein folding systems in eukaryotic parasites

SIGNIFICANCE

The cysteine (Cys) residues of proteins play two fundamentally important roles. They serve as sites of post-translational redox modifications as well as influence the conformation of the protein through the formation of disulfide bonds.

RECENT ADVANCES

Redox-related and redox-associated protein folding in protozoan parasites has been found to be a major mode of regulation, affecting myriad aspects of the parasitic life cycle, host-parasite interactions, and the disease pathology. Available genome sequences of various parasites have begun to complement the classical biochemical and enzymological studies of these processes. In this article, we summarize the reversible Cys disulfide (S-S) bond formation in various classes of strategically important parasitic proteins, and its structural consequence and functional relevance.

CRITICAL ISSUES

Molecular mechanisms of folding remain under-studied and often disconnected from functional relevance.

FUTURE DIRECTIONS

The clinical benefit of redox research will require a comprehensive characterization of the various isoforms and paralogs of the redox enzymes and their concerted effect on the structure and function of the specific parasitic client proteins.

Exploring the smallest active fragment of HsQSOX1b and finding a highly efficient oxidative engine

Human quiescin-sulfhydryl oxidase 1 isoform b (HsQSOX1b) is a highly efficient, multiple-domain enzyme that directly inserts disulfide bonds into client protein. However, previous studies have focused mainly on the catalytic activity of the whole protein rather than its domain structure. In this research, we dissected the structure and function of HsQSOX1b and explored its mechanism as a highly efficient sulfhydryl oxidase by analyzing the truncated variants. The results showed that the first HsQSOX1b thioredoxin domain was essential for thiol oxidase activity. The smallest active fragment (SAQ) was identified to consist of a helix-rich region (HRR) and an essential for respiration and viability/augmenter of liver regeneration (ERV/ALR) domain, which remained highly active to oxidize an artificial non-thiol substrate but not small molecular and protein thiols. Our study clearly demonstrated that SAQ is a highly efficient oxidative engine, which shows high efficiency in the de novo disulfide formation and oxygen reduction and that this more efficient oxidative engine is necessary for the highly efficient catalysis of QSOXs compared to Erv1 and Erv2. This study will help address the roles of different HsQSOX1b domains in de novo disulfide formation and encourage the engineering of more efficient QSOX variants for the in vitro folding of disulfide-containing proteins.

Protein substrate discrimination in the quiescin sulfhydryl oxidase (QSOX) family

This work explores the substrate specificity of the quiescin sulfhydryl oxidase (QSOX) family of disulfide-generating flavoenzymes to provide enzymological context for investigation of the physiological roles of these facile catalysts of oxidative protein folding. QSOX enzymes are generally unable to form disulfide bonds within well-structured proteins. Use of a temperature-sensitive mutant of ubiquitin-conjugating enzyme 4 (Ubc4') as a model substrate shows that QSOX activity correlates with the unfolding of Ubc4' monitored by circular dichroism. Fusion of Ubc4' with the more stable glutathione-S-transferase domain demonstrates that QSOX can selectively introduce disulfides into the less stable domain of the fusion protein. In terms of intermolecular disulfide bond generation, QSOX is unable to cross-link well-folded globular proteins via their surface thiols. However, the construction of a septuple mutant of RNase A, retaining a single cysteine residue, demonstrates that flexible protein monomers can be directly coupled by the oxidase. Steady- and pre-steady-state kinetic experiments, combined with static fluorescence approaches, indicate that while QSOX is an efficient catalyst for disulfide bond formation between mobile elements of structure, it does not appear to have a significant binding site for unfolded proteins. These aspects of protein substrate discrimination by QSOX family members are rationalized in terms of the stringent steric requirements for disulfide exchange reactions.

Flavin-linked Erv-family sulfhydryl oxidases release superoxide anion during catalytic turnover

Typically, simple flavoprotein oxidases couple the oxidation of their substrates with the formation of hydrogen peroxide without release of significant levels of the superoxide ion. However, two evolutionarily related single-domain sulfhydryl oxidases (Erv2p; a yeast endoplasmic reticulum resident protein and augmenter of liver regeneration, ALR, an enzyme predominantly found in the mitochondrial intermembrane) release up to ~30% of the oxygen they reduce as the superoxide ion. Both enzymes oxidize dithiol substrates via a redox-active disulfide adjacent to the flavin cofactor within the helix-rich Erv domain. Subsequent reduction of the flavin is followed by transfer of reducing equivalents to molecular oxygen. Superoxide release was initially detected using tris(3-hydroxypropyl)phosphine (THP) as an alternative reducing substrate to dithiothreitol (DTT). THP, and other phosphines, showed anomalously high turnover numbers with Erv2p and ALR in the oxygen electrode, but oxygen consumption was drastically suppressed upon the addition of superoxide dismutase. The superoxide ion initiates a radical chain reaction promoting the aerobic oxidation of phosphines with the formation of hydrogen peroxide. Use of a known flux of superoxide generated by the xanthine/xanthine oxidase system showed that one superoxide ion stimulates the reduction of 27 and 4.5 molecules of oxygen using THP and tris(2-carboxyethyl)phosphine (TCEP), respectively. This superoxide-dependent amplification of oxygen consumption by phosphines provides a new kinetic method for the detection of superoxide. Superoxide release was also observed by a standard chemiluminescence method using a luciferin analogue (MCLA) when 2 mM DTT was employed as a substrate of Erv2p and ALR. The percentage of superoxide released from Erv2p increased to ~65% when monomeric mutants of the normally homodimeric enzyme were used. In contrast, monomeric multidomain quiescin sulfhydryl oxidase enzymes that also contain an Erv FAD-binding fold release only 1-5% of their total reduced oxygen species as the superoxide ion. Aspects of the mechanism and possible physiological significance of superoxide release from these Erv-domain flavoproteins are discussed.

Quiescin sulfhydryl oxidase (QSOX) is expressed in the human atheroma core: possible role in apoptosis

Quiescin sulfhydryl oxidases (QSOXs) catalyze the formation of disulfide bonds in peptides and proteins, and in vertebrates comprise two proteins: QSOX1 and QSOX2. QSOX1, the most extensively studied type, has been implicated in protein folding, production of extracellular matrix, redox regulation, protection from apoptosis, angiogenesis, and cell differentiation. Atherosclerosis is an immunopathological condition in which redox processes, apoptosis, cell differentiation, and matrix secretion/maturation have critical roles. Considering these data, we hypothesized that QSOX1 could be involved in this disease, possibly reducing apoptosis and angiogenesis inside the plaque. QSOX1 labeling in normal human carotid vessels showed predominant expression by endothelium, subendothelium, and adventitia. In atherosclerotic plaques, however, QSOX1 was highly expressed in macrophages at the lipid core. QSOX1 expression was also studied in terms of mRNA and protein in cell types present in plaques under apoptotic or activating stimuli, emulating conditions found in the atherosclerotic process. QSOX1 mRNA increased in endothelial cells and macrophages after the induction of apoptosis. At the protein level, the correlation between apoptosis and QSOX1 expression was not evident in all cell types, possibly because of a rapid secretion of QSOX1. Our results propose for the first time possible roles for QSOX1 in atherosclerosis, being upregulated in endothelial cells and macrophages by apoptosis and cell activation, and possibly controlling these processes, as well as angiogenesis. The quantitative differences in QSOX1 induction may depend on the cell type and also on local factors.

Transannular disulfide formation in gliotoxin biosynthesis and its role in self-resistance of the human pathogen Aspergillus fumigatus

Gliotoxin (1), the infamous representative of the group of epipolythiodioxopiperazines (ETPs), is a virulence factor of the human pathogenic fungus Aspergillus fumigatus. The unique redox-sensitive transannular disulfide bridge is critical for deleterious effects caused by redox cycling and protein conjugation in the host. Through a combination of genetic, biochemical, and chemical analyses, we found that 1 results from GliT-mediated oxidation of the corresponding dithiol. In vitro studies using purified GliT demonstrate that the FAD-dependent, homodimeric enzyme utilizes molecular oxygen as terminal electron acceptor with concomitant formation of H(2)O(2). In analogy to the thiol-disulfide oxidoreductase superfamily, a model for dithiol-disulfide exchange involving the conserved CxxC motif is proposed. Notably, while all studied disulfide oxidases invariably form intra- or interchenar disulfide bonds in peptides, GliT is the first studied enzyme producing an epidithio bond. Furthermore, through sensitivity assays using wild type, Delta gliT mutant, and complemented strain, we found that GliT confers resistance to the producing organism. A phylogenetic study revealed that GliT falls into a clade of yet fully uncharacterized fungal gene products deduced from putative ETP biosynthesis gene loci. GliT thus not only represents the prototype of ETP-forming enzymes in eukaryotes but also delineates a novel mechanism for self-resistance.

Multiple catalytically active thioredoxin folds: a winning strategy for many functions

The Thioredoxin (Trx) fold is a versatile protein scaffold consisting of a four-stranded β-sheet surrounded by three α-helices. Various insertions are possible on this structural theme originating different proteins, which show a variety of functions and specificities. During evolution, the assembly of different Trx fold domains has been used many times to build new multi-domain proteins able to perform a large number of catalytic functions. To clarify the interaction mode of the different Trx domains within a multi-domain structure and how their combination can affect catalytic performances, in this review, we report on a structural and functional analysis of the most representative proteins containing more than one catalytically active Trx domain: the eukaryotic protein disulfide isomerases (PDIs), the thermophilic protein disulfide oxidoreductases (PDOs) and the hybrid peroxiredoxins (Prxs).

Oxidative protein folding and the Quiescin-sulfhydryl oxidase family of flavoproteins

Flavin-linked sulfhydryl oxidases participate in the net generation of disulfide bonds during oxidative protein folding in the endoplasmic reticulum. Members of the Quiescin-sulfhydryl oxidase (QSOX) family catalyze the facile direct introduction of disulfide bonds into unfolded reduced proteins with the reduction of molecular oxygen to generate hydrogen peroxide. Current progress in dissecting the mechanism of QSOX enzymes is reviewed, with emphasis on the CxxC motifs in the thioredoxin and Erv/ALR domains and the involvement of the flavin prosthetic group. The tissue distribution and intra- and extracellular location of QSOX enzymes are discussed, and suggestions for the physiological role of these enzymes are presented. The review compares the substrate specificity and catalytic efficiency of the QSOX enzymes with members of the Ero1 family of flavin-dependent sulfhydryl oxidases: enzymes believed to play key roles in disulfide generation in yeast and higher eukaryotes. Finally, limitations of our current understanding of disulfide generation in metazoans are identified and questions posed for the future.

Crystal structures of human Ero1α reveal the mechanisms of regulated and targeted oxidation of PDI

In the endoplasmic reticulum (ER) of eukaryotic cells, Ero1 flavoenzymes promote oxidative protein folding through protein disulphide isomerase (PDI), generating reactive oxygen species (hydrogen peroxide) as byproducts. Therefore, Ero1 activity must be strictly regulated to avoid futile oxidation cycles in the ER. Although regulatory mechanisms restraining Ero1α activity ensure that not all PDIs are oxidized, its specificity towards PDI could allow other resident oxidoreductases to remain reduced and competent to carry out isomerization and reduction of protein substrates. In this study, crystal structures of human Ero1α were solved in its hyperactive and inactive forms. Our findings reveal that human Ero1α modulates its oxidative activity by properly positioning regulatory cysteines within an intrinsically flexible loop, and by fine-tuning the electron shuttle ability of the loop through disulphide rearrangements. Specific PDI targeting is guaranteed by electrostatic and hydrophobic interactions of Ero1α with the PDI b'-domain through its substrate-binding pocket. These results reveal the molecular basis of the regulation and specificity of protein disulphide formation in human cells.

Structure of the human sulfhydryl oxidase augmenter of liver regeneration and characterization of a human mutation causing an autosomal recessive myopathy

The sulfhydryl oxidase augmenter of liver regeneration (ALR) binds FAD in a helix-rich domain that presents a CxxC disulfide proximal to the isoalloxazine ring of the flavin. Head-to-tail interchain disulfide bonds link subunits within the homodimer of both the short, cytokine-like, form of ALR (sfALR), and a longer form (lfALR) which resides in the mitochondrial intermembrane space (IMS). lfALR has an 80-residue N-terminal extension with an additional CxxC motif required for the reoxidation of reduced Mia40 during oxidative protein folding within the IMS. Recently, Di Fonzo et al. [Di Fonzo, A., Ronchi, D., Lodi, T., Fassone, E., Tigano, M., Lamperti, C., Corti, S., Bordoni, A., Fortunato, F., Nizzardo, M., Napoli, L., Donadoni, C., Salani, S., Saladino, F., Moggio, M., Bresolin, N., Ferrero, I., and Comi, G. P. (2009) Am. J. Hum. Genet. 84, 594-604] described an R194H mutation of human ALR that led to cataract, progressive muscle hypotonia, and hearing loss in three children. The current work presents a structural and enzymological characterization of the human R194H mutant in lf- and sfALR. A crystal structure of human sfALR was determined by molecular replacement using the rat sfALR structure. R194 is located at the subunit interface of sfALR, close to the intersubunit disulfide bridges. The R194 guanidino moiety participates in three H-bonds: two main-chain carbonyl oxygen atoms (from R194 itself and from C95 of the intersubunit disulfide of the other protomer) and with the 2'-OH of the FAD ribose. The R194H mutation has minimal effect on the enzyme activity using model and physiological substrates of short and long ALR forms. However, the mutation adversely affects the stability of both ALR forms: e.g., by decreasing the melting temperature by about 10 degrees C, by increasing the rate of dissociation of FAD from the holoenzyme by about 45-fold, and by strongly enhancing the susceptibility of sfALR to partial proteolysis and to reduction of its intersubunit disulfide bridges by glutathione. Finally, a comparison of the TROSY-HSQC 2D NMR spectra of wild-type sfALR and its R194H mutant reveals a significant increase in conformational flexibility in the mutant protein. In sum, these in vitro data document the major impact of the seemingly conservative R194H mutation on the stability of dimeric ALR and complement the in vivo observations of Di Fonzo et al.

Fluorescent protein-based redox probes

Redox biochemistry is increasingly recognized as an integral component of cellular signal processing and cell fate decision making. Unfortunately, our capabilities to observe and measure clearly defined redox processes in the natural context of living cells, tissues, or organisms are woefully limited. The most advanced and promising tools for specific, quantitative, dynamic and compartment-specific observations are genetically encoded redox probes derived from green fluorescent protein (GFP). Within only few years from their initial introduction, redox-sensitive yellow FP (rxYFP), redox-sensitive GFPs (roGFPs), and HyPer have generated enormous interest in applying these novel tools to monitor dynamic redox changes in vivo. As genetically encoded probes, these biosensors can be specifically targeted to different subcellular locations. A critical advantage of roGFPs and HyPer is their ratiometric fluorogenic behavior. Moreover, the probe scaffold of redox-sensitive fluorescent proteins (rxYFP and roGFPs) is amenable to molecular engineering, offering fascinating prospects for further developments. In particular, the engineering of redox relays between roGFPs and redox enzymes allows control of probe specificity and enhancement of sensitivity. Genetically encoded redox probes enable the functional analysis of individual proteins in cellular redox homeostasis. In addition, redox biosensor transgenic model organisms offer extended opportunities for dynamic in vivo imaging of redox processes.