Post-translational modification of Cu/Zn superoxide dismutase under anaerobic conditions

MSD2, an apoplastic Mn-SOD, contributes to root skotomorphogenic growth by modulating ROS distribution in Arabidopsis

Reactive oxygen species (ROS) play essential roles as a second messenger in various physiological processes in plants. Due to their oxidative nature, ROS can also be harmful. Thus, the generation and homeostasis of ROS are tightly controlled by multiple enzymes. Membrane-localized NADPH oxidases are well known to generate ROS during developmental and stress responses, but the metabolic pathways of the superoxide (O2^-) generated by them in the apoplast are poorly understood, and the identity of the apoplastic superoxide dismutase (SOD) is unknown in Arabidopsis. Here, we show that a putative manganese SOD, MSD2 is secreted and possesses a SOD activity that can be inhibited by nitration at tyrosine 68. The expression of MSD2 in roots is light condition-dependent, suggesting that MSD2 may act on ROS metabolism in roots during the light-to-dark transition. Root architecture is governed by ROS distribution that exhibits opposite gradient of H₂O₂ and O2^-, which is indeed altered in etiolated msd2 mutants and accompanied by changes in the onset of differentiation. These results provide a missing link in our understanding of ROS metabolism and suggest that MSD2 plays a role in root skotomorphogenesis by regulating ROS distribution, thereby playing a pivotal role in plant growth and development.

Sod1 integrates oxygen availability to redox regulate NADPH production and the thiol redoxome

Cu/Zn superoxide dismutase (Sod1) is a key antioxidant enzyme, and its importance is underscored by the fact that its ablation in cell and animal models results in oxidative stress; metabolic defects; and reductions in cell proliferation, viability, and lifespan. Curiously, Sod1 detoxifies superoxide radicals (O₂^•−) in a manner that produces an oxidant as byproduct, hydrogen peroxide (H₂O₂). While much is known about the necessity of scavenging O₂^•−, it is less clear what the physiological roles of Sod1-derived H₂O₂ are. We discovered that Sod1-derived H₂O₂ plays an important role in antioxidant defense by stimulating the production of NADPH, a vital cellular reductant required for reactive oxygen species scavenging enzymes, as well as redox regulating a large network of enzymes.

Cu/Zn superoxide dismutase (Sod1) is a highly conserved and abundant antioxidant enzyme that detoxifies superoxide (O₂^•−) by catalyzing its conversion to dioxygen (O₂) and hydrogen peroxide (H₂O₂). Using Saccharomyces cerevisiae and mammalian cells, we discovered that a major aspect of the antioxidant function of Sod1 is to integrate O₂ availability to promote NADPH production. The mechanism involves Sod1-derived H₂O₂ oxidatively inactivating the glycolytic enzyme, GAPDH, which in turn reroutes carbohydrate flux to the oxidative phase of the pentose phosphate pathway (oxPPP) to generate NADPH. The aerobic oxidation of GAPDH is dependent on and rate-limited by Sod1. Thus, Sod1 senses O₂ via O₂^•− to balance glycolytic and oxPPP flux, through control of GAPDH activity, for adaptation to life in air. Importantly, this mechanism for Sod1 antioxidant activity requires the bulk of cellular Sod1, unlike for its role in protection against O₂^•− toxicity, which only requires <1% of total Sod1. Using mass spectrometry, we identified proteome-wide targets of Sod1-dependent redox signaling, including numerous metabolic enzymes. Altogether, Sod1-derived H₂O₂ is important for antioxidant defense and a master regulator of metabolism and the thiol redoxome.

Superoxide Dismutase 1 in Health and Disease: How a Frontline Antioxidant Becomes Neurotoxic

Cu/Zn superoxide dismutase (SOD1) is a frontline antioxidant enzyme catalysing superoxide breakdown and is important for most forms of eukaryotic life. The evolution of aerobic respiration by mitochondria increased cellular production of superoxide, resulting in an increased reliance upon SOD1. Consistent with the importance of SOD1 for cellular health, many human diseases of the central nervous system involve perturbations in SOD1 biology. But far from providing a simple demonstration of how disease arises from SOD1 loss-of-function, attempts to elucidate pathways by which atypical SOD1 biology leads to neurodegeneration have revealed unexpectedly complex molecular characteristics delineating healthy, functional SOD1 protein from that which likely contributes to central nervous system disease. This review summarises current understanding of SOD1 biology from SOD1 genetics through to protein function and stability.

Recent Developments in Enzymatic Antioxidant Defence Mechanism in Plants with Special Reference to Abiotic Stress

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.

Signaling Toward Reactive Oxygen Species-Scavenging Enzymes in Plants

Reactive oxygen species (ROS) are signaling molecules essential for plant responses to abiotic and biotic stimuli as well as for multiple developmental processes. They are produced as byproducts of aerobic metabolism and are affected by adverse environmental conditions. The ROS content is controlled on the side of their production but also by scavenging machinery. Antioxidant enzymes represent a major ROS-scavenging force and are crucial for stress tolerance in plants. Enzymatic antioxidant defense occurs as a series of redox reactions for ROS elimination. Therefore, the deregulation of the antioxidant machinery may lead to the overaccumulation of ROS in plants, with negative consequences both in terms of plant development and resistance to environmental challenges. The transcriptional activation of antioxidant enzymes accompanies the long-term exposure of plants to unfavorable environmental conditions. Fast ROS production requires the immediate mobilization of the antioxidant defense system, which may occur via retrograde signaling, redox-based modifications, and the phosphorylation of ROS detoxifying enzymes. This review aimed to summarize the current knowledge on signaling processes regulating the enzymatic antioxidant capacity of plants.

SOD1, more than just an antioxidant

During cellular respiration, radicals, such as superoxide, are produced, and in a large concentration, they may cause cell damage. To combat this threat, the cell employs the enzyme Cu/Zn Superoxide Dismutase (SOD1), which converts the radical superoxide into molecular oxygen and hydrogen peroxide, through redox reactions. Although this is its main function, recent studies have shown that the SOD1 has other functions that deviates from its original one including activation of nuclear gene transcription or as an RNA binding protein. This comprehensive review looks at the most important aspects of human SOD1 (hSOD1), including the structure, properties, and characteristics as well as transcriptional and post-translational modifications (PTM) that the enzyme can receive and their effects, and its many functions. We also discuss the strategies currently used to analyze it to better understand its participation in diseases linked to hSOD1 including Amyotrophic Lateral Sclerosis (ALS), cancer, and Parkinson.

Mitochondrial manganese superoxide dismutase knock-down increases oxidative stress and caspase-3 activity in the white shrimp Litopenaeus vannamei exposed to high temperature, hypoxia, and reoxygenation

Shrimp are increasingly exposed to warmer temperatures and lower oxygen concentrations in their habitat due to climate change. These conditions may lead to oxidative stress and apoptosis. We studied the effects of high temperature, hypoxia, reoxygenation, and the combination of these factors on lipid peroxidation, protein carbonylation, and caspase-3 activity in gills of white shrimp Litopenaeus vannamei. Silencing of mitochondrial manganese superoxide dismutase (mMnSOD) was used to determine the role of this enzyme in response to the abiotic stressors described above, to avoid oxidative damage and apoptosis. In addition, mMnSOD gene expression and mitochondrial SOD activity were evaluated to determine the efficiency of silencing this enzyme. The results showed that there was no effect of the abiotic stress conditions on the thiobarbituric acid reactive substances (TBARS), but protein carbonylation increased in all the oxidative stress treatments and caspase-3 activity decreased in hypoxia at 28 °C. On the other hand, mMnSOD-silenced shrimp experienced higher oxidative stress, since TBARS, carbonylated proteins and caspase-3 activity increased in some silenced treatments. Unexpectedly, mitochondrial SOD activity increased in some of the silenced treatments as well. Altogether, these results suggest that mMnSOD has a key role in shrimp for the prevention of oxidative damage development and induction of apoptosis in response to hypoxia, reoxygenation, high temperature, and their interactions, as conditions derived from climate change.

Mechanisms of SOD1 regulation by post-translational modifications

SOD1 is commonly known for its ROS scavenging activity, but recent work has uncovered additional roles in modulating metabolism, maintaining redox balance, and regulating transcription. This new paradigm of expanded SOD1 function raises questions regarding the regulation of SOD1 and the cellular partitioning of its biological roles. Despite decades of research on SOD1, much of which focuses on its pathogenic role in amyotrophic lateral sclerosis, relatively little is known about its regulation by post-translational modifications (PTMs). However, over the last decade, advancements in mass spectrometry have led to a boom in PTM discovery across the proteome, which has also revealed new mechanisms of SOD1 regulation by PTMs and an array of SOD1 PTMs with high likelihood of biological function. In this review, we address emerging mechanisms of SOD1 regulation by post-translational modifications, many of which begin to shed light on how the various functions of SOD1 are regulated within the cell.

Extra-mitochondrial Cu/Zn superoxide dismutase (Sod1) is dispensable for protection against oxidative stress but mediates peroxide signaling in Saccharomyces cerevisiae

Cu/Zn Superoxide Dismutase (Sod1) is a highly conserved and abundant metalloenzyme that catalyzes the disproportionation of superoxide radicals into hydrogen peroxide and molecular oxygen. As a consequence, Sod1 serves dual roles in oxidative stress protection and redox signaling by both scavenging cytotoxic superoxide radicals and producing hydrogen peroxide that can be used to oxidize and regulate the activity of downstream targets. However, the relative contributions of Sod1 to protection against oxidative stress and redox signaling are poorly understood. Using the model unicellular eukaryote, Baker's yeast, we found that only a small fraction of the total Sod1 pool is required for protection against superoxide toxicity and that this pool is localized to the mitochondrial intermembrane space. On the contrary, we find that much larger amounts of extra-mitochondrial Sod1 are critical for peroxide-mediated redox signaling. Altogether, our results force the re-evaluation of the physiological role of bulk Sod1 in redox biology; namely, we propose that the vast majority of Sod1 in yeast is utilized for peroxide-mediated signaling rather than superoxide scavenging.

Nitration of MnSOD in the Carotid Body and Adrenal Gland Induced by Chronic Intermittent Hypoxia

Chronic intermittent hypoxia (CIH), main feature of obstructive sleep apnea, produces nitro-oxidative stress, which contributes to potentiate carotid body (CB) chemosensory discharges and sympathetic-adrenal-axis activity, leading to hypertension. The MnSOD enzymatic activity, a key enzyme on oxidative stress control, is reduced by superoxide-induced nitration. However, the effects of CIH-induced nitration on MnSOD enzymatic activity in the CB and adrenal gland are not known. We studied the effects of CIH on MnSOD protein and immunoreactive (MnSOD-ir) levels in the CB, adrenal gland and superior cervical ganglion (SCG), and on 3-nitrotyrosine (3-NT-ir), CuZnSOD (CuZnSOD-ir), MnSOD nitration, and its enzymatic activity in the CB and adrenal gland from male Sprague-Dawley rats exposed to CIH for 7 days. CIH increased 3-NT-ir in CB and adrenal gland, whereas MnSOD-ir increased in the CB and in adrenal cortex, but not in the whole adrenal medulla or SCG. CIH nitrated MnSOD in the CB and adrenal medulla, but its activity decreased in the adrenal gland. CuZnSOD-ir remained unchanged in both tissues. All changes observed were prevented by ascorbic acid treatment. Present results show that CIH for 7 days produced MnSOD nitration, but failed to reduce its activity in the CB, because of the increased protein level.

A role for Candida albicans superoxide dismutase enzymes in glucose signaling

The Saccharomyces cerevisiae and Candida albicans yeasts have evolved to differentially use glucose for fermentation versus respiration. S. cerevisiae is Crabtree positive, where glucose represses respiration and promotes fermentation, while the opportunistic fungal pathogen C. albicans is Crabtree negative and does not repress respiration with glucose. We have previously shown that glucose control in S. cerevisiae involves the antioxidant enzyme Cu/Zn superoxide dismutase (SOD1), where H₂O₂ generated by SOD1 stabilizes the casein kinase YCK1 for glucose sensing. We now demonstrate that C. albicans SODs also participate in glucose regulation. C. albicans expresses two cytosolic SODs, Cu/Zn SOD1 and Mn containing SOD3, and both complemented a S. cerevisiae sod1Δ mutant in stabilizing YCK1. Moreover, in C. albicans cells, both SODs functioned to repress glucose transporter genes in response to glucose. However, the action of SODs in glucose control has diverged in the two yeasts. In S. cerevisiae, SOD1 specifically functions in the glucose sensing pathway involving YCK1 and the RGT1 repressor, but the analogous YCK/RGT1 pathway in C. albicans shows no control by SOD enzymes. Instead C. albicans SODs work in the glucose repression pathway involving the MIG1 transcriptional repressor. In C. albicans, the SODs repress glucose uptake, while in S. cerevisiae, SOD1 activates glucose uptake, in accordance with the divergent modes for glucose utilization in these two distantly related yeasts.

Oxygen-dependent activation of Cu,Zn-superoxide dismutase-1

Copper zinc superoxide dismutase (Sod1) is a critical enzyme in limiting reactive oxygen species in both the cytosol and the mitochondrial intermembrane space.

Riboflavin transport and metabolism in humans

Recent studies elucidated how riboflavin transporters and FAD forming enzymes work in humans and create a coordinated flavin network ensuring the maintenance of cellular flavoproteome. Alteration of this network may be causative of severe metabolic disorders such as multiple acyl-CoA dehydrogenase deficiency (MADD) or Brown-Vialetto-van Laere syndrome. A crucial step in the maintenance of FAD homeostasis is riboflavin uptake by plasma and mitochondrial membranes. Therefore, studies on recently identified human plasma membrane riboflavin transporters are presented, together with those in which still unidentified mitochondrial riboflavin transporter(s) have been described. A main goal of future research is to fill the gaps still existing as for some transcriptional, functional and structural details of human FAD synthases (FADS) encoded by FLAD1 gene, a novel "redox sensing" enzyme. In the frame of the hypothesis that FADS, acting as a "FAD chaperone", could play a crucial role in the biogenesis of mitochondrial flavo-proteome, several basic functional aspects of flavin cofactor delivery to cognate apo-flavoenzyme are also briefly dealt with. The establishment of model organisms performing altered FAD homeostasis will improve the molecular description of human pathologies. The molecular and functional studies of transporters and enzymes herereported, provide guidelines for improving therapies which may have beneficial effects on the altered metabolism.

Cu/Zn superoxide dismutase and the proton ATPase Pma1p of Saccharomyces cerevisiae

In eukaryotes, the Cu/Zn containing superoxide dismutase (SOD1) plays a critical role in oxidative stress protection as well as in signaling. We recently demonstrated a function for Saccharomyces cerevisiae Sod1p in signaling through CK1γ casein kinases and identified the essential proton ATPase Pma1p as one likely target. The connection between Sod1p and Pma1p was explored further by testing the impact of sod1Δ mutations on cells expressing mutant alleles of Pma1p that alter activity and/or post-translational regulation of this ATPase. We report here that sod1Δ mutations are lethal when combined with the T912D allele of Pma1p in the C-terminal regulatory domain. This "synthetic lethality" was reversed by intragenic suppressor mutations in Pma1p, including an A906G substitution that lies within the C-terminal regulatory domain and hyper-activates Pma1p. Surprisingly the effect of sod1Δ mutations on Pma1-T912D is not mediated through the Sod1p signaling pathway involving the CK1γ casein kinases. Rather, Sod1p sustains life of cells expressing Pma1-T912D through oxidative stress protection. The synthetic lethality of sod1Δ Pma1-T912D cells is suppressed by growing cells under low oxygen conditions or by treatments with manganese-based antioxidants. We now propose a model in which Sod1p maximizes Pma1p activity in two ways: one involving signaling through CK1γ casein kinases and an independent role for Sod1p in oxidative stress protection.

The effects of dietary restriction on oxidative stress in rodents

Oxidative stress is observed during aging and in numerous age-related diseases. Dietary restriction (DR) is a regimen that protects against disease and extends life span in multiple species. However, it is unknown how DR mediates its protective effects. One prominent and consistent effect of DR in a number of systems is the ability to reduce oxidative stress and damage. The purpose of this review is to comprehensively examine the hypothesis that dietary restriction reduces oxidative stress in rodents by decreasing reactive oxygen species (ROS) production and increasing antioxidant enzyme activity, leading to an overall reduction of oxidative damage to macromolecules. The literature reveals that the effects of DR on oxidative stress are complex and likely influenced by a variety of factors, including sex, species, tissue examined, types of ROS and antioxidant enzymes examined, and duration of DR. Here we present a comprehensive review of the existing literature on the effect of DR on mitochondrial ROS generation, antioxidant enzymes, and oxidative damage. In a majority of studies, dietary restriction had little effect on mitochondrial ROS production or antioxidant activity. On the other hand, DR decreased oxidative damage in the majority of cases. Although the effects of DR on endogenous antioxidants are mixed, we find that glutathione levels are the most likely antioxidant to be increased by dietary restriction, which supports the emerging redox-stress hypothesis of aging.

Yeast copper-zinc superoxide dismutase can be activated in the absence of its copper chaperone

Copper-zinc superoxide dismutase (Sod1) is an abundant intracellular enzyme that catalyzes the disproportionation of superoxide to give hydrogen peroxide and dioxygen. In most organisms, Sod1 acquires copper by a combination of two pathways, one dependent on the copper chaperone for Sod1 (CCS), and the other independent of CCS. Examples have been reported of two exceptions: Saccharomyces cerevisiae, in which Sod1 appeared to be fully dependent on CCS, and Caenorhabditis elegans, in which Sod1 was completely independent of CCS. Here, however, using overexpressed Sod1, we show there is also a significant amount of CCS-independent activation of S. cerevisiae Sod1, even in low-copper medium. In addition, we show CCS-independent oxidation of the disulfide bond in S. cerevisiae Sod1. There appears to be a continuum between CCS-dependent and CCS-independent activation of Sod1, with yeast falling near but not at the CCS-dependent end.

Species-specific activation of Cu/Zn SOD by its CCS copper chaperone in the pathogenic yeast Candida albicans

Candida albicans is a pathogenic yeast of important public health relevance. Virulence of C. albicans requires a copper and zinc containing superoxide dismutase (SOD1), but the biology of C. albicans SOD1 is poorly understood. To this end, C. albicans SOD1 activation was examined in baker's yeast (Saccharomyces cerevisiae), a eukaryotic expression system that has proven fruitful for the study of SOD1 enzymes from invertebrates, plants, and mammals. In spite of the 80% similarity between S. cerevisiae and C. albicans SOD1 molecules, C. albicans SOD1 is not active in S. cerevisiae. The SOD1 appears incapable of productive interactions with the copper chaperone for SOD1 (CCS1) of S. cerevisiae. C. albicans SOD1 contains a proline at position 144 predicted to dictate dependence on CCS1. By mutation of this proline, C. albicans SOD1 gained activity in S. cerevisiae, and this activity was independent of CCS1. We identified a putative CCS1 gene in C. albicans and created heterozygous and homozygous gene deletions at this locus. Loss of CCS1 resulted in loss of SOD1 activity, consistent with its role as a copper chaperone. C. albicans CCS1 also restored activity to C. albicans SOD1 expressed in S. cerevisiae. C. albicans CCS1 is well adapted for activating its partner SOD1 from C. albicans, but not SOD1 from S. cerevisiae. In spite of the high degree of homology between the SOD1 and CCS1 molecules in these two fungal species, there exists a species-specific barrier in CCS-SOD interactions which may reflect the vastly different lifestyles of the pathogenic versus the noninfectious yeast.

Mechanistic aspects of hSOD1 maturation from the solution structure of Cu(I) -loaded hCCS domain 1 and analysis of disulfide-free hSOD1 mutants

Superoxide dismutase 1 (SOD1) maturation within the cell is mainly accomplished with the SOD1-specific chaperone, CCS, a dimeric protein with three distinct domains in each monomer. We recently showed that the first domain of human CCS (hCCSD1) is responsible for copper transfer to its protein partner, human SOD1 (hSOD1). The NMR solution structure of the copper(I)-loaded form of hCCSD1 reported here contributes further to characterization of the copper-transfer mechanism to hSOD1. NMR spectroscopy was also used to examine the hSOD1 mutants C57A, C146A, and C57A/C146A, which are unable to form the structurally conserved disulfide bond in SOD1, in order to investigate the role of these cysteines during hSOD1 copper acquisition. Together, the information on both hCCS and hSOD1, along with a sequence analysis of eukaryotic CCSD1, allows us to propose important mechanistic aspects regarding the copper-transfer process from hCCS to hSOD1.

SOD1 integrates signals from oxygen and glucose to repress respiration

Cu/Zn superoxide dismutase (SOD1) is an abundant enzyme that has been best studied as a regulator of antioxidant defense. Using the yeast Saccharomyces cerevisiae, we report that SOD1 transmits signals from oxygen and glucose to repress respiration. The mechanism involves SOD1-mediated stabilization of two casein kinase 1-gamma (CK1γ) homologs, Yck1p and Yck2p, required for respiratory repression. SOD1 binds a C-terminal degron we identified in Yck1p/Yck2p and promotes kinase stability by catalyzing superoxide conversion to peroxide. The effects of SOD1 on CK1γ stability are also observed with mammalian SOD1 and CK1γ and in a human cell line. Therefore, in a single circuit, oxygen, glucose, and reactive oxygen can repress respiration through SOD1/CK1γ signaling. Our data therefore may provide mechanistic insight into how rapidly proliferating cells and many cancers accomplish glucose-mediated repression of respiration in favor of aerobic glycolysis.