The bicarbonate/carbon dioxide pair increases hydrogen peroxide-mediated hyperoxidation of human peroxiredoxin 1

The multifaceted nature of peroxiredoxins in chemical biology

Peroxiredoxins (Prx), thiol-dependent peroxidases, were first identified as H2O2 detoxifiers, and more recently as H2O2 sensors, intermediates in redox-signaling pathways, metabolism modulators, and chaperones. The multifaceted nature of Prx is not only dependent on their peroxidase activity but also strongly associated with specific protein-protein interactions that are being identified, and where the Prx oligomerization dynamics plays a role. Their oxidation by a peroxide substrate forms a sulfenic acid that opens a route to channel the redox signal to diverse protein targets. Recent research underscores the importance of different Prx isoforms in the cellular processes behind disease development with potential therapeutic applications.

Carbon dioxide/bicarbonate is required for sensitive inactivation of mammalian glyceraldehyde-3-phosphate dehydrogenase by hydrogen peroxide

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) contains an active site Cys and is one of the most sensitive cellular enzymes to oxidative inactivation and redox regulation. Here, we show that inactivation by hydrogen peroxide is strongly enhanced in the presence of carbon dioxide/bicarbonate. Inactivation of isolated mammalian GAPDH by H2O2 increased with increasing bicarbonate concentration and was sevenfold faster in 25 mM (physiological) bicarbonate compared with bicarbonate-free buffer of the same pH. H2O2 reacts reversibly with CO2 to form a more reactive oxidant, peroxymonocarbonate (HCO4-), which is most likely responsible for the enhanced inactivation. However, to account for the extent of enhancement, we propose that GAPDH must facilitate formation and/or targeting of HCO4- to promote its own inactivation. Inactivation of intracellular GAPDH was also strongly enhanced by bicarbonate: treatment of Jurkat cells with 20 µM H2O2 in 25 mM bicarbonate buffer for 5 min caused almost complete GAPDH inactivation, but no loss of activity when bicarbonate was not present. H2O2-dependent GAPDH inhibition in bicarbonate buffer was observed even in the presence of reduced peroxiredoxin 2 and there was a significant increase in cellular glyceraldehyde-3-phosphate/dihydroxyacetone phosphate. Our results identify an unrecognized role for bicarbonate in enabling H2O2 to influence inactivation of GAPDH and potentially reroute glucose metabolism from glycolysis to the pentose phosphate pathway and NAPDH production. They also demonstrate what could be wider interplay between CO2 and H2O2 in redox biology and the potential for variations in CO2 metabolism to influence oxidative responses and redox signaling.

Exploring functional and structural features of chemically related natural prenylated hydroquinone and benzoic acid from Piper crassinervium (Piperaceae) on bacterial peroxiredoxin inhibition

Multiple drug resistance (MDR) bacterial strains are responsible by 1.2 million of human deaths all over the world. The pathogens possess efficient enzymes which are able to mitigate the toxicity of reactive oxygen species (ROS) produced by some antibiotics and the host immune cells. Among them, the bacterial peroxiredoxin alkyl hydroperoxide reductase C (AhpC) is able to decompose efficiently several kinds of hydroperoxides. To decompose their substrates AhpC use a reactive cysteine residue (peroxidatic cysteine-CysP) that together with two other polar residues (Thr/Ser and Arg) comprise the catalytic triad of these enzymes and are involved in the substrate targeting/stabilization to allow a bimolecular nucleophilic substitution (SN2) reaction. Additionally to the high efficiency the AhpC is very abundant in the cells and present virulent properties in some bacterial species. Despite the importance of AhpC in bacteria, few studies aimed at using natural compounds as inhibitors of this class of enzymes. Some natural products were identified as human isoforms, presenting as common characteristics a bulk hydrophobic moiety and an α, β-unsaturated carbonylic system able to perform a thiol-Michael reaction. In this work, we evaluated two chemically related natural products: 1,4-dihydroxy-2-(3',7'-dimethyl-1'-oxo-2'E,6'-octadienyl) benzene (C1) and 4-hydroxy-2-(3',7'-dimethyl-1'-oxo-2'E,6'-octadienyl) benzoic acid (C2), both were isolated from branches Piper crassinervium (Piperaceae), over the peroxidase activity of AhpC from Pseudomonas aeruginosa (PaAhpC) and Staphylococcus epidermidis (SeAhpC). By biochemical assays we show that although both compounds can perform the Michael addition reaction, only compound C2 was able to inhibit the PaAhpC peroxidase activity but not SeAhpC, presenting IC50 = 20.3 μM. SDS-PAGE analysis revealed that the compound was not able to perform a thiol-Michael addition, suggesting another inhibition behavior. Using computer-assisted simulations, we also show that an acidic group present in the structure of compound C2 may be involved in the stabilization by polar interactions with the Thr and Arg residues from the catalytic triad and several apolar interactions with hydrophobic residues. Finally, C2 was not able to interfere in the peroxidase activity of the isoform Prx2 from humans or even the thiol proteins of the Trx reducing system from Escherichia coli (EcTrx and EcTrxR), indicating specificity for P. aeruginosa AhpC.

Interplay of carbon dioxide and peroxide metabolism in mammalian cells

The carbon dioxide/bicarbonate (CO₂/HCO₃^-) molecular pair is ubiquitous in mammalian cells and tissues, mainly as a result of oxidative decarboxylation reactions that occur during intermediary metabolism. CO₂ is in rapid equilibrium with HCO₃^-via the hydration reaction catalyzed by carbonic anhydrases. Far from being an inert compound in redox biology, CO₂ enhances or redirects the reactivity of peroxides, modulating the velocity, extent, and type of one- and two-electron oxidation reactions mediated by hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^-/ONOOH). Herein, we review the biochemical mechanisms by which CO₂ engages in peroxide-dependent reactions, free radical production, redox signaling, and oxidative damage. First, we cover the metabolic formation of CO₂ and its connection to peroxide formation and decomposition. Next, the reaction mechanisms, kinetics, and processes by which the CO₂/peroxide interplay modulates mammalian cell redox biology are scrutinized in-depth. Importantly, CO₂ also regulates gene expression related to redox and nitric oxide metabolism and as such influences oxidative and inflammatory processes. Accumulated biochemical evidence in vitro, in cellula, and in vivo unambiguously show that the CO₂ and peroxide metabolic pathways are intertwined and together participate in key redox events in mammalian cells.

The role of mitochondrial reactive oxygen species in insulin resistance

Insulin resistance is one of the earliest pathological features of a suite of diseases including type 2 diabetes collectively referred to as metabolic syndrome. There is a growing body of evidence from both pre-clinical studies and human cohorts indicating that reactive oxygen species, such as the superoxide radical anion and hydrogen peroxide are key players in the development of insulin resistance. Here we review the evidence linking mitochondrial reactive oxygen species generated within mitochondria with insulin resistance in adipose tissue and skeletal muscle, two major insulin sensitive tissues. We outline the relevant mitochondria-derived reactive species, how the mitochondrial redox state is regulated, and methodologies available to measure mitochondrial reactive oxygen species. Importantly, we highlight key experimental issues to be considered when studying the role of mitochondrial reactive oxygen species in insulin resistance. Evaluating the available literature on both mitochondrial reactive oxygen species/redox state and insulin resistance in a variety of biological systems, we conclude that the weight of evidence suggests a likely role for mitochondrial reactive oxygen species in the etiology of insulin resistance in adipose tissue and skeletal muscle. However, major limitations in the methods used to study reactive oxygen species in insulin resistance as well as the lack of data linking mitochondrial reactive oxygen species and cytosolic insulin signaling pathways are significant obstacles in proving the mechanistic link between these two processes. We provide a framework to guide future studies to provide stronger mechanistic information on the link between mitochondrial reactive oxygen species and insulin resistance as understanding the source, localization, nature, and quantity of mitochondrial reactive oxygen species, their targets and downstream signaling pathways may pave the way for important new therapeutic strategies.

Carbon dioxide redox metabolites in oxidative eustress and oxidative distress

High carbon dioxide tensions (hypercapnia) are toxic to mammals by both pH-dependent and pH-independent mechanisms that remain partially understood. Relevantly, carbon dioxide reacts with biologically ubiquitous oxygen metabolites such as peroxynitrite and hydrogen peroxide to produce carbonate radical and peroxymonocarbonate, respectively. These metabolites are redox active making it timely to discuss the potential role of carbon dioxide redox metabolites in oxidative eustress and oxidative distress conditions.

Can thiol-based redox systems be utilized as parts for synthetic biology applications?

OBJECTIVES

Synthetic biology has emerged from molecular biology and engineering approaches and aims to develop novel, biologically-inspired systems for industrial and basic research applications ranging from biocomputing to drug production. Surprisingly, redoxin (thioredoxin, glutaredoxin, peroxiredoxin) and other thiol-based redox systems have not been widely utilized in many of these synthetic biology applications.

METHODS

We reviewed thiol-based redox systems and the development of synthetic biology applications that have used thiol-dependent parts.

RESULTS

The development of circuits to facilitate cytoplasmic disulfide bonding, biocomputing and the treatment of intestinal bowel disease are amongst the applications that have used thiol-based parts. We propose that genetically encoded redox sensors, thiol-based biomaterials and intracellular hydrogen peroxide generators may also be valuable components for synthetic biology applications.

DISCUSSION

Thiol-based systems play multiple roles in cellular redox metabolism, antioxidant defense and signaling and could therefore offer a vast and diverse portfolio of components, parts and devices for synthetic biology applications. However, factors limiting the adoption of redoxin systems for synthetic biology applications include the orthogonality of thiol-based components, limitations in the methods to characterize thiol-based systems and an incomplete understanding of the design principles of these systems.

Hydrogen peroxide signaling via its transformation to a stereospecific alkyl hydroperoxide that escapes reductive inactivation

During systemic inflammation, indoleamine 2,3-dioxygenase 1 (IDO1) becomes expressed in endothelial cells where it uses hydrogen peroxide (H₂O₂) to oxidize L-tryptophan to the tricyclic hydroperoxide, cis-WOOH, that then relaxes arteries via oxidation of protein kinase G 1α. Here we show that arterial glutathione peroxidases and peroxiredoxins that rapidly eliminate H₂O₂, have little impact on relaxation of IDO1-expressing arteries, and that purified IDO1 forms cis-WOOH in the presence of peroxiredoxin 2. cis-WOOH oxidizes protein thiols in a selective and stereospecific manner. Compared with its epimer trans-WOOH and H₂O₂, cis-WOOH reacts slower with the major arterial forms of glutathione peroxidases and peroxiredoxins while it reacts more readily with its target, protein kinase G 1α. Our results indicate a paradigm of redox signaling by H₂O₂ via its enzymatic conversion to an amino acid-derived hydroperoxide that 'escapes' effective reductive inactivation to engage in selective oxidative activation of key target proteins.

Oxidative Modification of Proteins: From Damage to Catalysis, Signaling, and Beyond

Significance: The systematic investigation of oxidative modification of proteins by reactive oxygen species started in 1980. Later, it was shown that reactive nitrogen species could also modify proteins. Some protein oxidative modifications promote loss of protein function, cleavage or aggregation, and some result in proteo-toxicity and cellular homeostasis disruption. Recent Advances: Previously, protein oxidation was associated exclusively to damage. However, not all oxidative modifications are necessarily associated with damage, as with Met and Cys protein residue oxidation. In these cases, redox state changes can alter protein structure, catalytic function, and signaling processes in response to metabolic and/or environmental alterations. This review aims to integrate the present knowledge on redox modifications of proteins with their fate and role in redox signaling and human pathological conditions. Critical Issues: It is hypothesized that protein oxidation participates in the development and progression of many pathological conditions. However, no quantitative data have been correlated with specific oxidized proteins or the progression or severity of pathological conditions. Hence, the comprehension of the mechanisms underlying these modifications, their importance in human pathologies, and the fate of the modified proteins is of clinical relevance. Future Directions: We discuss new tools to cope with protein oxidation and suggest new approaches for integrating knowledge about protein oxidation and redox processes with human pathophysiological conditions. Antioxid. Redox Signal. 35, 1016-1080.

Effects of Serine or Threonine in the Active Site of Typical 2-Cys Prx on Hyperoxidation Susceptibility and on Chaperone Activity

Typical 2-Cys peroxiredoxins (2-Cys Prx) are ubiquitous Cys-based peroxidases, which are stable as decamers in the reduced state, and may dissociate into dimers upon disulfide bond formation. A peroxidatic Cys (CP) takes part of a catalytic triad, together with a Thr/Ser and an Arg. Previously, we described that the presence of Ser (instead of Thr) in the active site stabilizes yeast 2-Cys Prx as decamers. Here, we compared the hyperoxidation susceptibilities of yeast 2-Cys Prx. Notably, 2-Cys Prx containing Ser (named here Ser-Prx) were more resistant to hyperoxidation than enzymes containing Thr (Thr-Prx). In silico analysis revealed that Thr-Prx are more frequent in all domains of life, while Ser-Prx are more abundant in bacteria. As yeast 2-Cys Prx, bacterial Ser-Prx are more stable as decamers than Thr-Prx. However, bacterial Ser-Prx were only slightly more resistant to hyperoxidation than Thr-Prx. Furthermore, in all cases, organic hydroperoxide inhibited more the peroxidase activities of 2-Cys Prx than hydrogen peroxide. Moreover, bacterial Ser-Prx displayed increased thermal resistance and chaperone activity, which may be related with its enhanced stability as decamers compared to Thr-Prx. Therefore, the single substitution of Thr by Ser in the catalytic triad results in profound biochemical and structural differences in 2-Cys Prx.

Hydrogen peroxide reactivity and specificity in thiol-based cell signalling

Reversible oxidation of thiol proteins is an important cell signalling mechanism. In many cases, this involves generation or exposure of the cells to H2O2, and oxidation of proteins that are not particularly H2O2-reactive. There is a conundrum as to how these proteins are oxidised when other highly reactive proteins such as peroxiredoxins are present. This article discusses potential mechanisms, focussing on recent evidence for oxidation being localised within the cell, redox relays involving peroxiredoxins operating in some signalling pathways, and mechanisms for facilitated or directed oxidation of specific targets. These findings help define conditions that enable redox signalling but there is still much to learn regarding mechanisms.

Qualitative Differences in Protection of PTP1B Activity by the Reductive Trx1 or TRP14 Enzyme Systems upon Oxidative Challenges with Polysulfides or H2O2 Together with Bicarbonate

Protein tyrosine phosphatases (PTPs) can be regulated by several redox-dependent mechanisms and control growth factor-activated receptor tyrosine kinase phosphorylation cascades. Reversible oxidation of PTPs is counteracted by reductive enzymes, including thioredoxin (Trx) and Trx-related protein of 14 kDa (TRP14), keeping PTPs in their reduced active states. Different modes of oxidative inactivation of PTPs concomitant with assessment of activating reduction have been little studied in direct comparative analyses. Determining PTP1B activities, we here compared the potency of inactivation by bicarbonate-assisted oxidation using H₂O₂ with that of polysulfide-mediated inactivation. Inactivation of pure PTP1B was about three times more efficient with polysulfides as compared to the combination of bicarbonate and H₂O₂. Bicarbonate alone had no effect on PTP1B, neither with nor without a combination with polysulfides, thus strengthening the notion that bicarbonate-assisted H₂O₂-mediated inactivation of PTP1B involves formation of peroxymonocarbonate. Furthermore, PTP1B was potently protected from polysulfide-mediated inactivation by either TRP14 or Trx1, in contrast to the inactivation by bicarbonate and H₂O₂. Comparing reductive activation of polysulfide-inactivated PTP1B with that of bicarbonate- and H₂O₂-treated enzyme, we found Trx1 to be more potent in reactivation than TRP14. Altogether we conclude that inactivation of PTP1B by polysulfides displays striking qualitative differences compared to that by H₂O₂ together with bicarbonate, also with regard to maintenance of PTP1B activity by either Trx1 or TRP14.

Mechanisms and consequences of protein cysteine oxidation: the role of the initial short-lived intermediates

Thiol groups in protein cysteine (Cys) residues can undergo one- and two-electron oxidation reactions leading to the formation of thiyl radicals or sulfenic acids, respectively. In this mini-review we summarize the mechanisms and kinetics of the formation of these species by biologically relevant oxidants. Most of the latter react with the deprotonated form of the thiol. Since the pKa of the thiols in protein cysteines are usually close to physiological pH, the thermodynamics and the kinetics of their oxidation in vivo are affected by the acidity of the thiol. Moreover, the protein microenvironment has pronounced effects on cysteine residue reactivity, which in the case of the oxidation mediated by hydroperoxides, is known to confer specificity to particular protein cysteines. Despite their elusive nature, both thiyl radicals and sulfenic acids are involved in the catalytic mechanism of several enzymes and in the redox regulation of protein function and/or signaling pathways. They are usually short-lived species that undergo further reactions that converge in the formation of different stable products, resulting in several post-translational modifications of the protein. Some of these can be reversed through the action of specific cellular reduction systems. Others damage the proteins irreversibly, and can make them more prone to aggregation or degradation.

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents

'Reactive oxygen species' (ROS) is an umbrella term for an array of derivatives of molecular oxygen that occur as a normal attribute of aerobic life. Elevated formation of the different ROS leads to molecular damage, denoted as 'oxidative distress'. Here we focus on ROS at physiological levels and their central role in redox signalling via different post-translational modifications, denoted as 'oxidative eustress'. Two species, hydrogen peroxide (H₂O₂) and the superoxide anion radical (O₂^·-), are key redox signalling agents generated under the control of growth factors and cytokines by more than 40 enzymes, prominently including NADPH oxidases and the mitochondrial electron transport chain. At the low physiological levels in the nanomolar range, H₂O₂ is the major agent signalling through specific protein targets, which engage in metabolic regulation and stress responses to support cellular adaptation to a changing environment and stress. In addition, several other reactive species are involved in redox signalling, for instance nitric oxide, hydrogen sulfide and oxidized lipids. Recent methodological advances permit the assessment of molecular interactions of specific ROS molecules with specific targets in redox signalling pathways. Accordingly, major advances have occurred in understanding the role of these oxidants in physiology and disease, including the nervous, cardiovascular and immune systems, skeletal muscle and metabolic regulation as well as ageing and cancer. In the past, unspecific elimination of ROS by use of low molecular mass antioxidant compounds was not successful in counteracting disease initiation and progression in clinical trials. However, controlling specific ROS-mediated signalling pathways by selective targeting offers a perspective for a future of more refined redox medicine. This includes enzymatic defence systems such as those controlled by the stress-response transcription factors NRF2 and nuclear factor-κB, the role of trace elements such as selenium, the use of redox drugs and the modulation of environmental factors collectively known as the exposome (for example, nutrition, lifestyle and irradiation).

Tracking isotopically labeled oxidants using boronate-based redox probes

Reactive oxygen and nitrogen species have been implicated in many biological processes and diseases, including immune responses, cardiovascular dysfunction, neurodegeneration, and cancer. These chemical species are short-lived in biological settings, and detecting them in these conditions and diseases requires the use of molecular probes that form stable, easily detectable, products. The chemical mechanisms and limitations of many of the currently used probes are not well-understood, hampering their effective applications. Boronates have emerged as a class of probes for the detection of nucleophilic two-electron oxidants. Here, we report the results of an oxygen-18-labeling MS study to identify the origin of oxygen atoms in the oxidation products of phenylboronate targeted to mitochondria. We demonstrate that boronate oxidation by hydrogen peroxide, peroxymonocarbonate, hypochlorite, or peroxynitrite involves the incorporation of oxygen atoms from these oxidants. We therefore conclude that boronates can be used as probes to track isotopically labeled oxidants. This suggests that the detection of specific products formed from these redox probes could enable precise identification of oxidants formed in biological systems. We discuss the implications of these results for understanding the mechanism of conversion of the boronate-based redox probes to oxidant-specific products.

The Peroxidatic Thiol of Peroxiredoxin 1 is Nitrosated by Nitrosoglutathione but Coordinates to the Dinitrosyl Iron Complex of Glutathione

Protein S-nitrosation is an important consequence of NO^●·metabolism with implications in physiology and pathology. The mechanisms responsible for S-nitrosation in vivo remain debatable and kinetic data on protein S-nitrosation by different agents are limited. 2-Cys peroxiredoxins, in particular Prx1 and Prx2, were detected as being S-nitrosated in multiple mammalian cells under a variety of conditions. Here, we investigated the kinetics of Prx1 S-nitrosation by nitrosoglutathione (GSNO), a recognized biological nitrosating agent, and by the dinitrosyl-iron complex of glutathione (DNIC-GS; [Fe(NO)₂(GS)₂]⁻), a hypothetical nitrosating agent. Kinetics studies following the intrinsic fluorescence of Prx1 and its mutants (C83SC173S and C52S) were complemented by product analysis; all experiments were performed at pH 7.4 and 25 ℃. The results show GSNO-mediated nitrosation of Prx1 peroxidatic residue (k+NOCys52 = 15.4 ± 0.4 M⁻¹. s⁻¹) and of Prx1 Cys⁸³ residue (k+NOCys83 = 1.7 ± 0.4 M⁻¹. s⁻¹). The reaction of nitrosated Prx1 with GSH was also monitored and provided a second-order rate constant for Prx1Cys⁵²NO denitrosation of k−NOCys52 = 14.4 ± 0.3 M⁻¹. s⁻¹. In contrast, the reaction of DNIC-GS with Prx1 did not nitrosate the enzyme but formed DNIC-Prx1 complexes. The peroxidatic Prx1 Cys was identified as the residue that more rapidly replaces the GS ligand from DNIC-GS (kDNICCys52 = 7.0 ± 0.4 M⁻¹. s⁻¹) to produce DNIC-Prx1 ([Fe(NO)₂(GS)(Cys⁵²-Prx1)]⁻). Altogether, the data showed that in addition to S-nitrosation, the Prx1 peroxidatic residue can replace the GS ligand from DNIC-GS, forming stable DNIC-Prx1, and both modifications disrupt important redox switches.

Increased formation of reactive oxygen species during tumor growth: Ex vivo low-temperature EPR and in vivo bioluminescence analyses

Previous studies have shown that reactive oxygen species (ROS) such as superoxide or hydrogen peroxide generated at low levels can exert a tumor-promoting role via a redox-signaling mechanism. Reports also suggest that both tumorigenesis and tumor growth are associated with enhanced ROS formation. However, whether ROS levels or ROS-derived oxidative marker levels increase during tumor growth remains unknown. In this study, in vivo bioluminescence imaging with a boronate-based pro-luciferin probe was used to assess ROS formation. Additionally, probe-free cryogenic electron paramagnetic resonance was used to quantify a characteristic aconitase [3Fe4S]⁺ center that arises in the tumor tissue of mouse xenografts from the reaction of the native [4Fe4S]²⁺ cluster with superoxide. Results indicated that tumor growth is accompanied by increased ROS formation, and revealed differences in oxidant formation in the inner and outer sections of tumor tissue, respectively, demonstrating redox heterogeneity. Studies using luciferin and pro-luciferin probes enabled the assessment of tumor size, ROS formation, and bioenergetic status (e.g., ATP) in luciferase-transfected mice tumor xenografts. Probe-free ex vivo low-temperature electron paramagnetic resonance can also be translated to clinical studies.

AMP-Activated Protein Kinase (AMPK) at the Crossroads Between CO2 Retention and Skeletal Muscle Dysfunction in Chronic Obstructive Pulmonary Disease (COPD)

Skeletal muscle dysfunction is a major comorbidity in chronic obstructive pulmonary disease (COPD) and other pulmonary conditions. Chronic CO2 retention, or hypercapnia, also occur in some of these patients. Both muscle dysfunction and hypercapnia associate with higher mortality in these populations. Over the last years, we have established a mechanistic link between hypercapnia and skeletal muscle dysfunction, which is regulated by AMPK and causes depressed anabolism via reduced ribosomal biogenesis and accelerated catabolism via proteasomal degradation. In this review, we discuss the main findings linking AMPK with hypercapnic pulmonary disease both in the lungs and skeletal muscles, and also outline potential avenues for future research in the area based on knowledge gaps and opportunities to expand mechanistic research with translational implications.

Enhanced hyperoxidation of peroxiredoxin 2 and peroxiredoxin 3 in the presence of bicarbonate/CO2

Hydrogen peroxide undergoes an equilibrium reaction with bicarbonate/CO₂ to produce peroxymonocarbonate (HCO₄^-). Peroxymonocarbonate is more reactive with thiols than H₂O₂ but it makes up only a small fraction of the H₂O₂ in physiological bicarbonate buffers so the increase in rate of oxidation of low molecular weight thiols is modest. However, for some thiol proteins such as protein tyrosine phosphatases, the rate enhancement is very much greater. We have investigated the effect of bicarbonate/CO₂ on the oxidation of peroxiredoxins (Prdxs) 2 and 3. Using an assay in which reduced Prdx2 inhibits oxidation of horseradish peroxidase by H₂O₂, we saw no difference between phosphate and bicarbonate buffers (pH 7.4). However, hyperoxidation of both Prdxs in bicarbonate was considerably enhanced. Hyperoxidation involves the reaction of the sulfenic acid formed at the active site with a second H₂O₂, and prevents its condensation to a disulfide. Using LC/MS analysis, we determined that the presence of 25 mM bicarbonate/CO₂ increased the ratio of hyperoxidation compared with condensation 6-fold for Prdx2 and 11-fold for Prdx3. These results imply that Prdx hyperoxidation will occur more readily under physiological conditions than appreciated from in vitro experiments, which seldom use bicarbonate buffers. They also raise the possibility that variations in bicarbonate concentration could provide a mechanism for regulating the cellular level of active Prdxs.

Catalysis of Peroxide Reduction by Fast Reacting Protein Thiols

Life on Earth evolved in the presence of hydrogen peroxide, and other peroxides also emerged before and with the rise of aerobic metabolism. They were considered only as toxic byproducts for many years. Nowadays, peroxides are also regarded as metabolic products that play essential physiological cellular roles. Organisms have developed efficient mechanisms to metabolize peroxides, mostly based on two kinds of redox chemistry, catalases/peroxidases that depend on the heme prosthetic group to afford peroxide reduction and thiol-based peroxidases that support their redox activities on specialized fast reacting cysteine/selenocysteine (Cys/Sec) residues. Among the last group, glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) are the most widespread and abundant families, and they are the leitmotif of this review. After presenting the properties and roles of different peroxides in biology, we discuss the chemical mechanisms of peroxide reduction by low molecular weight thiols, Prxs, GPxs, and other thiol-based peroxidases. Special attention is paid to the catalytic properties of Prxs and also to the importance and comparative outlook of the properties of Sec and its role in GPxs. To finish, we describe and discuss the current views on the activities of thiol-based peroxidases in peroxide-mediated redox signaling processes.