Cysteine biosynthesis in bacteria proceeds primarily via the de novo pathway, involving serine acetyltransferase (CysE) and O-acetylserine sulfhydrylase (OASS). This pathway is absent in humans, and its inhibition impairs microbial fitness, virulence, and antibiotic resistance, making its enzymes attractive antimicrobial targets. Most bacteria encode two OASS isoforms: CysK, which forms the cysteine synthase complex (CSC) with CysE, and CysM, which typically acts independently. While conserved, their biochemical properties and regulatory roles vary across species. Here, we investigated CysK (PaCysK) and CysM (PaCysM) from Pseudomonas aeruginosa, an opportunistic pathogen of major concern due to intrinsic antibiotic resistance. We characterized their steady-state and pre-steady-state kinetics, structural features, and assessed substrate preferences through microbiological analyses of cysK and cysM deletion mutants. Our results revealed that PaCysK and PaCysM play redundant yet critical roles in cysteine biosynthesis in P. aeruginosa. PaCysK exhibits optimal activity with sulfide, supporting its primary function in sulfide-dependent cysteine biosynthesis. In contrast, PaCysM shows broader specificity, catalyzing cysteine formation from both sulfide and thiosulfate, suggesting a specialization for alternative sulfur sources. Structural modeling supports this, revealing PaCysM active-site features that facilitate thiosulfate binding and turnover. We also tested PaCysK binding to a synthetic peptide mimicking the C-terminal region of P. aeruginosa CysE1 and found no interaction, suggesting that the CSC does not form in P. aeruginosa. In contrast, PaCysK binds the equivalent peptide from Salmonella enterica serovar Typhimurium CysE, known to mediate CSC formation, confirming that the enzyme retains the capacity for protein-protein interactions through a canonical CSC-like mechanism.

Hydrogen sulfide (H2S) regulates multiple human physiological processes, its reactivity and range of action being tightly controlled through regulation of H2S-synthesizing and -detoxifying enzymes. H2S detoxification is mainly achieved by a mitochondrial sulfide detoxifying pathway including persulfide dioxygenase (PDO). Human PDO (known as ethylmalonic encephalopathy protein 1, ETHE1), a homodimeric enzyme with a mononuclear iron centre active site, catalyzes the conversion of glutathione persulfide (GSSH) and O2 to reduced glutathione (GSH) and sulfite. Here we report that ETHE1 is potently inhibited by authentic nitric oxide (NO) gas at physiological concentrations, as observed by high resolution respirometry. Inhibition is reversible, occurs via NO binding to the reduced mononuclear iron center and becomes more potent and persistent at lower O2 levels. Incubation with s-nitrosoglutathione (GSNO) also appears to partially and transiently inhibit ETHE1, this effect likely resulting from s-nitrosation of cysteine residues. While ETHE1 is devoid of NO reductase activity, in aerobic conditions it displays low NO degrading activity. These findings unravel a novel layer of cross-regulation between the H2S and NO gasotransmitters with possible implications on the regulation of numerous physiological and pathophysiological processes.

Infections by Pseudomonas aeruginosa are a major cause of severe morbidity and mortality in immunocompromised patients and people with cystic fibrosis, largely due to the pathogen's ability to resist antibiotic treatment and to deploy multiple virulence strategies that promote persistence in the host. We have recently uncovered that the signaling molecule diadenosine tetraphosphate (Ap4A) acts as a crucial regulator of P. aeruginosa virulence. Specifically, deletion of the Ap4A-degrading enzyme, diadenosine tetraphosphatase (ApaH), dramatically reduces the expression of key virulence factors. The structural and molecular properties of P. aeruginosa ApaH (PaApaH) remain uncharacterized. Here, we present an integrated biochemical, structural, computational, and phenotypic characterization of PaApaH. We define the molecular determinants of its manganese-dependent Ap4A hydrolytic mechanism and establish a direct functional link between PaApaH catalytic activity and virulence phenotypes in vivo. Together with the absence of ApaH homologs in eukaryotes, these findings place PaApaH as an attractive and selective target for antivirulence therapeutic strategies against P. aeruginosa infections.

Hydrogen sulfide is an important signaling molecule, beneficial at physiological concentrations but harmful at higher levels, due to which a tight control of its bioavailability is essential. Here, we investigated persulfide dioxygenase, an enzyme involved in H2S catabolism, from the pathogen Pseudomonas aeruginosa (PaPDO). Deletion of the gene pdo led to a 4-fold increase in H2S concentration, confirming its physiological role. The recombinant enzyme was structurally characterized at 2.06 Å resolution and assigned to the metallo-β-lactamase superfamily. Compared with its human homolog, PaPDO displayed a different dimerization area and a larger active site, suggesting different substrate preferences. Functionally, PaPDO catalyzed glutathione persulfide dioxygenation with a high turnover rate, and its activity was enhanced by reduced glutathione. Interestingly, the results show that PaPDO binds to nitric oxide, which reversibly inhibits its catalytic activity. These findings reveal a novel mechanism of crosstalk between hydrogen sulfide and nitric oxide signaling and provide insights into redox regulation in a multidrug-resistant pathogen.

Hydrogen sulfide (H2S) and nitric oxide (NO) are long-known inhibitors of terminal oxidases in the respiratory chain. Yet, they exert pivotal signaling roles in physiological processes, and in several bacterial pathogens have been reported to confer resistance against oxidative stress, host immune responses, and antibiotics. Pseudomonas aeruginosa, an opportunistic pathogen causing life-threatening infections that are difficult to eradicate, has a highly branched respiratory chain including four terminal oxidases of the haem-copper type (aa3, cbb3-1, cbb3-2, and bo3) and one oxidase of the bd-type (cyanide-insensitive oxidase, CIO). As Escherichia coli bd-type oxidases have been shown to be H2S-insensitive and to readily recover their activity from NO inhibition, here we tested the effect of H2S and NO on CIO by performing oxygraphic measurements on membrane preparations from P. aeruginosa PAO1 and isogenic mutants depleted of CIO only or all other terminal oxidases except CIO. We show that O2 consumption by CIO is unaltered even in the presence of high levels of H2S, and that CIO expression is enhanced and supports bacterial growth under such stressful conditions. In addition, we report that CIO is reversibly inhibited by NO, while activity recovery after NO exhaustion is full and fast, suggesting a protective role of CIO under NO stress conditions. As P. aeruginosa is exposed to H2S and NO during infection, the tolerance of CIO towards these stressors agrees with the proposed role of CIO in P. aeruginosa virulence.

Hydrogen sulfide (H2S) has been proposed to protect bacteria from antibiotics, pointing to H2S-producing enzymes as possible targets for the development of antibiotic adjuvants. Here, MIC assays performed with Pseudomonas aeruginosa mutants producing altered H2S levels demonstrate that H2S does not affect antibiotic resistance in this bacterium. Moreover, correlation analyses in a large collection of P. aeruginosa cystic fibrosis isolates argue against the protective role of H2S from antibiotic activity during chronic lung infection.

Merely considered as a toxic gas in the past, hydrogen sulfide (H2S) is now a recognized 'gasotransmitter'. Multiple studies have shown the involvement of H2S as a redox active multifaceted signaling molecule that regulates numerous physiological processes, that under different circumstances can act as an antioxidant or a pro-oxidant. The regulatory roles of H2S rely on its ability to interact with and/or modify target proteins. H2S homeostatic levels are kept under tight control by a fine regulation of its (enzymatic and non-enzymatic) biosynthesis and catabolism. Alteration of this balance, leading to non-physiological H2S levels, has been growingly associated with aging and numerous human pathologies, including cardiovascular and neurodegenerative diseases, cancer and diabetes. Functionally characterizing the human enzymes involved in H2S metabolism is of paramount importance to a deeper understanding of impactful human diseases. In particular, herein we focus on the post-translational regulatory mechanisms controlling human H2S metabolism enzymes. This knowledge can support devising new pharmacological/therapeutic interventions.

The 'gasotransmitters' hydrogen sulfide (H2S), nitric oxide (NO), and carbon monoxide (CO) act as second messengers in human physiology, mediating signal transduction via interaction with or chemical modification of protein targets, thereby regulating processes such as neurotransmission, blood flow, immunomodulation, or energy metabolism. Due to their broad reactivity and potential toxicity, the biosynthesis and breakdown of H2S, NO, and CO are tightly regulated. Growing evidence highlights the active role of gasotransmitters in their mutual cross-regulation. In human physiology, the transsulfuration enzymes cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are prominent H2S enzymatic sources. While CBS is known to be inhibited by NO and CO, little is known about CSE regulation by gasotransmitters. Herein, we investigated the effect of s-nitrosation on CSE catalytic activity. H2S production by recombinant human CSE was found to be inhibited by the physiological nitrosating agent s-nitrosoglutathione (GSNO), while reduced glutathione had no effect. GSNO-induced inhibition was partially reverted by ascorbate and accompanied by the disappearance of one solvent accessible protein thiol. By combining differential derivatization procedures and mass spectrometry-based analysis with functional assays, seven out of the ten protein cysteine residues, namely Cys84, Cys109, Cys137, Cys172, Cys229, Cys307, and Cys310, were identified as targets of s-nitrosation. By generating conservative Cys-to-Ser variants of the identified s-nitrosated cysteines, Cys137 was identified as most significantly contributing to the GSNO-mediated CSE inhibition. These results highlight a new mechanism of crosstalk between gasotransmitters.

Significance: Cytochrome bd is a ubiquinol:oxygen oxidoreductase of many prokaryotic respiratory chains with a unique structure and functional characteristics. Its primary role is to couple the reduction of molecular oxygen, even at submicromolar concentrations, to water with the generation of a proton motive force used for adenosine triphosphate production. Cytochrome bd is found in many bacterial pathogens and, surprisingly, in bacteria formally denoted as anaerobes. It endows bacteria with resistance to various stressors and is a potential drug target. Recent Advances: We summarize recent advances in the biochemistry, structure, and physiological functions of cytochrome bd in the light of exciting new three-dimensional structures of the oxidase. The newly discovered roles of cytochrome bd in contributing to bacterial protection against hydrogen peroxide, nitric oxide, peroxynitrite, and hydrogen sulfide are assessed. Critical Issues: Fundamental questions remain regarding the precise delineation of electron flow within this multihaem oxidase and how the extraordinarily high affinity for oxygen is accomplished, while endowing bacteria with resistance to other small ligands. Future Directions: It is clear that cytochrome bd is unique in its ability to confer resistance to toxic small molecules, a property that is significant for understanding the propensity of pathogens to possess this oxidase. Since cytochrome bd is a uniquely bacterial enzyme, future research should focus on harnessing fundamental knowledge of its structure and function to the development of novel and effective antibacterial agents.

Sulfane sulfur species comprise a variety of biologically relevant hydrogen sulfide (H2S)-derived species, including per- and poly-sulfidated low molecular weight compounds and proteins. A growing body of evidence suggests that H2S, currently recognized as a key signaling molecule in human physiology and pathophysiology, plays an important role in cancer biology by modulating cell bioenergetics and contributing to metabolic reprogramming. This is accomplished through functional modulation of target proteins via H2S binding to heme iron centers or H2S-mediated reversible per- or poly-sulfidation of specific cysteine residues. Since sulfane sulfur species are increasingly viewed not only as a major source of H2S but also as key mediators of some of the biological effects commonly attributed to H2S, the multifaceted role of these species in cancer biology is reviewed here with reference to H2S, focusing on their metabolism, signaling function, impact on cell bioenergetics and anti-tumoral properties.

Hydrogen sulfide (H2S), while historically perceived merely as a toxicant, has progressively emerged as a key regulator of numerous processes in mammalian physiology, exerting its signaling function essentially through interaction with and/or modification of proteins, targeting mainly cysteine residues and metal centers. As a gaseous signaling molecule that freely diffuses across aqueous and hydrophobic biological milieu, it has been designated the third 'gasotransmitter' in mammalian physiology. H2S is synthesized and detoxified by specialized endogenous enzymes that operate under a tight regulation, ensuring homeostatic levels of this otherwise toxic molecule. Indeed, imbalances in H2S levels associated with dysfunctional H2S metabolism have been growingly correlated with various human pathologies, from cardiovascular and neurodegenerative diseases to cancer. Several cancer cell lines and specimens have been shown to naturally overexpress one or more of the H2S-synthesizing enzymes. The resulting increased H2S levels have been proposed to promote cancer development through the regulation of various cancer-related processes, which led to the interest in pharmacological targeting of H2S metabolism. Herein are summarized some of the key observations that place H2S metabolism and signaling pathways at the forefront of the cellular mechanisms that support the establishment and development of a tumor within its complex and challenging microenvironment. Special emphasis is given to the mechanisms whereby H2S helps shaping cancer cell bioenergetic metabolism and affords resistance and adaptive mechanisms to hypoxia.

Nitric oxide (NO) is a well-known active site ligand and inhibitor of respiratory terminal oxidases. Here, we investigated the interaction of NO with a purified chimeric bcc-aa3 supercomplex composed of Mycobacterium tuberculosis cytochrome bcc and Mycobacterium smegmatisaa3-type terminal oxidase. Strikingly, we found that the enzyme in turnover with O2 and reductants is resistant to inhibition by the ligand, being able to metabolize NO at 25 °C with an apparent turnover number as high as ≈303 mol NO (mol enzyme)−1 min−1 at 30 µM NO. The rate of NO consumption proved to be proportional to that of O2 consumption, with 2.65 ± 0.19 molecules of NO being consumed per O2 molecule by the mycobacterial bcc-aa3. The enzyme was found to metabolize the ligand even under anaerobic reducing conditions with a turnover number of 2.8 ± 0.5 mol NO (mol enzyme)−1 min−1 at 25 °C and 8.4 µM NO. These results suggest a protective role of mycobacterial bcc-aa3 supercomplexes against NO stress.

Bacteria can not only encounter carbon monoxide (CO) in their habitats but also produce the gas endogenously. Bacterial respiratory oxidases, thus, represent possible targets for CO. Accordingly, host macrophages were proposed to produce CO and release it into the surrounding microenvironment to sense viable bacteria through a mechanism that in Escherichia (E.) coli was suggested to involve the targeting of a bd-type respiratory oxidase by CO. The aerobic respiratory chain of E. coli possesses three terminal quinol:O2-oxidoreductases: the heme-copper oxidase bo3 and two copper-lacking bd-type oxidases, bd-I and bd-II. Heme-copper and bd-type oxidases differ in the mechanism and efficiency of proton motive force generation and in resistance to oxidative and nitrosative stress, cyanide and hydrogen sulfide. Here, we investigated at varied O2 concentrations the effect of CO gas on the O2 reductase activity of the purified cytochromes bo3, bd-I and bd-II of E. coli. We found that CO, in competition with O2, reversibly inhibits the three enzymes. The inhibition constants Ki for the bo3, bd-I and bd-II oxidases are 2.4 ± 0.3, 0.04 ± 0.01 and 0.2 ± 0.1 μM CO, respectively. Thus, in E. coli, bd-type oxidases are more sensitive to CO inhibition than the heme-copper cytochrome bo3. The possible physiological consequences of this finding are discussed.

Hydrogen sulfide (H2S) is an endogenously produced signaling molecule. The enzymes 3-mercaptopyruvate sulfurtransferase (MST), partly localized in mitochondria, and the inner mitochondrial membrane-associated sulfide:quinone oxidoreductase (SQR), besides being respectively involved in the synthesis and catabolism of H2S, generate sulfane sulfur species such as persulfides and polysulfides, currently recognized as mediating some of the H2S biological effects. Reprogramming of H2S metabolism was reported to support cellular proliferation and energy metabolism in cancer cells. As oxidative stress is a cancer hallmark and N-acetylcysteine (NAC) was recently suggested to act as an antioxidant by increasing intracellular levels of sulfane sulfur species, here we evaluated the effect of prolonged exposure to NAC on the H2S metabolism of SW480 colon cancer cells. Cells exposed to NAC for 24 h displayed increased expression and activity of MST and SQR. Furthermore, NAC was shown to: (i) persist at detectable levels inside the cells exposed to the drug for up to 24 h and (ii) sustain H2S synthesis by human MST more effectively than cysteine, as shown working on the isolated recombinant enzyme. We conclude that prolonged exposure of colon cancer cells to NAC stimulates H2S metabolism and that NAC can serve as a substrate for human MST.

Biosynthesis of hydrogen sulfide (H2S), a key signalling molecule in human (patho)physiology, is mostly accomplished by the human enzymes cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (MST). Several lines of evidence have shown a close correlation between increased H2S production and human diseases, such as several cancer types and amyotrophic lateral sclerosis. Identifying compounds selectively and potently inhibiting the human H2S-synthesizing enzymes may therefore prove beneficial for pharmacological applications. Here, the human enzymes CBS, CSE and MST were expressed and purified from Escherichia coli, and thirty-one pyridine derivatives were synthesized and screened for their ability to bind and inhibit these enzymes. Using differential scanning fluorimetry (DSF), surface plasmon resonance (SPR), circular dichroism spectropolarimetry (CD), and activity assays based on fluorimetric and colorimetric H2S detection, two compounds (C30 and C31) sharing structural similarities were found to weakly inhibit both CBS and CSE: 1 mM C30 inhibited these enzymes by approx. 50% and 40%, respectively, while 0.5 mM C31 accounted for CBS and CSE inhibition by approx. 40% and 60%, respectively. This work, while presenting a robust methodological platform for screening putative inhibitors of the human H2S-synthesizing enzymes, highlights the importance of employing complementary methodologies in compound screenings.

Hydrogen sulfide (H2S), a known inhibitor of cytochrome c oxidase (CcOX), plays a key signaling role in human (patho)physiology. H2S is synthesized endogenously and mainly metabolized by a mitochondrial sulfide-oxidizing pathway including sulfide:quinone oxidoreductase (SQR), whereby H2S-derived electrons are injected into the respiratory chain stimulating O2 consumption and ATP synthesis. Under hypoxic conditions, H2S has higher stability and is synthesized at higher levels with protective effects for the cell. Herein, working on SW480 colon cancer cells, we evaluated the effect of hypoxia on the ability of cells to metabolize H2S. The sulfide-oxidizing activity was assessed by high-resolution respirometry, measuring the stimulatory effect of sulfide on rotenone-inhibited cell respiration in the absence or presence of antimycin A. Compared to cells grown under normoxic conditions (air O2), cells exposed for 24 h to hypoxia (1% O2) displayed a 1.3-fold reduction in maximal sulfide-oxidizing activity and 2.7-fold lower basal O2 respiration. Based on citrate synthase activity assays, mitochondria of hypoxia-treated cells were 1.8-fold less abundant and displayed 1.4-fold higher maximal sulfide-oxidizing activity and 2.6-fold enrichment in SQR as evaluated by immunoblotting. We speculate that under hypoxic conditions mitochondria undergo these adaptive changes to protect cell respiration from H2S poisoning.