Hydrogen Sulfide Oxidation by Myoglobin

Inorganic Polysulfides in Solution: Structural Properties and Conformational Isomerism

We present a comprehensive theoretical examination of the structural properties of dianionic polysulfides [Sn]2- (n = 2-6), their conjugated monoacids [HSn]- (n = 2-6), and a selection of 1e--oxidized radical anions [Sn]•- (n = 2-4), in aqueous and dimethyl sulfoxide (DMSO) solutions. We investigated the structures and stabilities of various conformational isomers within these families of compounds by employing Quantum Mechanics-Molecular Mechanics (QM-MM) Molecular Dynamics (MD) simulations. The explicit inclusion of solvent molecules in the calculations revealed stable conformational structures that were previously unreported and might have appreciable concentrations in real systems. The interconversions between the isomeric structures proceed on the order of hundreds of picoseconds and are energetically similar to the isomerization processes in substituted cyclohexanes. We also conducted a detailed analysis of the stability of different isomers of the radical anion [S4]•- in solution. Our findings highlight the significant influence of the solvent on the isomerizations, a result that could be particularly relevant for enhancing the performance of metal-sulfur batteries.

Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Hierarchical Porous N-Doped Carbon Particles Derived from ZIF-8 as Highly Efficient H2S Selective Oxidation Catalysts

In the selective oxidation of H2S, the catalytic activity over N-doped carbon-based catalysts is significantly influenced by the accessibility of active sites and the mass transfer rates of reactant molecules (e.g., H2S and O2) as well as generated sulfur monomers. Therefore, it is crucial for enhancing the initial performance via the controlled synthesis of carbon-based catalysts with highly exposed active sites and unique porous structures. Herein, we reported on an efficient strategy to synthesize nanosized N-doped carbon particles with hierarchical porous structures by directly pyrolyzing an oversaturated NaCl-encapsulated ZIF-8 precursor mixture. The introduction of NaCl not only serves as a pollution-free template to promote the formation of graphitic carbon layers but also acts as an intercalating agent to guide the derivation of hierarchical porous structures, as well as enhances the amount of active nitrogen species in the catalysts. As a result, the as-prepared H-NC800 catalyst shows excellent H2S selective oxidation performance (sulfur formation rate is 794 gsulfur·kgcat-1·h-1), good stability (>80 h), and antiwater vapor properties. The characterization results and DFT calculations indicate the crucial role of pyridinic N in the adsorbing and activating reactant molecules (H2S, O2). Furthermore, nanoscale N-doped carbon particles accelerated the rapid transport of generated sulfur monomers under a hierarchical porous structure. This investigation introduces a distinctive strategy for synthesizing ZIF-8-derived N-doped carbon nanosized with a hierarchical porous structure, while its efficient and stable H2S selective oxidation performance highlights significant potential for practical implementation in the industrial desulfurization process.

H2S preconditioning induces long-lived perturbations in O2 metabolism

Hydrogen sulfide exposure in moderate doses can induce profound but reversible hypometabolism in mammals. At a cellular level, H2S inhibits the electron transport chain (ETC), augments aerobic glycolysis, and glutamine-dependent carbon utilization via reductive carboxylation; however, the durability of these changes is unknown. We report that despite its volatility, H2S preconditioning increases P50(O2), the O2 pressure for half-maximal cellular respiration, and has pleiotropic effects on oxidative metabolism that persist up to 24 to 48 h later. Notably, cyanide, another complex IV inhibitor, does not induce this type of metabolic memory. Sulfide-mediated prolonged fractional inhibition of complex IV by H2S is modulated by sulfide quinone oxidoreductase, which commits sulfide to oxidative catabolism. Since induced hypometabolism can be beneficial in disease settings that involve insufficient or interrupted blood flow, our study has important implications for attenuating reperfusion-induced ischemic injury and/or prolonging the shelf life of biologics like platelets.

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Manganese Porphyrin-Containing Polymeric Micelles: A Novel Approach for Intracellular Catalytic Formation of Per/Polysulfide Species from a Hydrogen Sulfide Donor

Per/polysulfide species that are generated from endogenously produced hydrogen sulfide have critical regulatory roles in a wide range of cellular processes. However, the lack of delivery systems that enable controlled and sustained release of these unstable species in biological systems hinders the advancement of sulfide biology research, as well as the translation of knowledge to therapeutic applications. Here, a novel approach is developed to generate per/polysulfide species in cells by combining an H2 S donor and manganese porphyrin-containing polymeric micelles (MnPMCs) that catalyze oxidization of H2 S to per/polysulfide species. MnPMCs serve as a catalyst for H2 S oxidation in aerobic phosphate buffer. HPLC-MS/MS analysis reveals that H2 S oxidation by MnPMCs in the presence of glutathione results in the formation of glutathione-SnH (n = 2 and 3). Furthermore, co-treatment of human umbilical vein endothelial cells with the H2 S donor anethole dithiolethione and MnPMCs increases intracellular per/polysulfide levels and induces a proangiogenic response. Co-delivery of MnPMCs and an H2 S donor is a promising approach for controlled delivery of polysulfides for therapeutic applications.

Hydrogen Sulfide (H2S)/Polysulfides (H2Sn) Signalling and TRPA1 Channels Modification on Sulfur Metabolism

Hydrogen sulfide (H2S) and polysulfides (H2Sn, n ≥ 2) produced by enzymes play a role as signalling molecules regulating neurotransmission, vascular tone, cytoprotection, inflammation, oxygen sensing, and energy formation. H2Sn, which have additional sulfur atoms to H2S, and other S-sulfurated molecules such as cysteine persulfide and S-sulfurated cysteine residues of proteins, are produced by enzymes including 3-mercaptopyruvate sulfurtransferase (3MST). H2Sn are also generated by the chemical interaction of H2S with NO, or to a lesser extent with H2O2. S-sulfuration (S-sulfhydration) has been proposed as a mode of action of H2S and H2Sn to regulate the activity of target molecules. Recently, we found that H2S/H2S2 regulate the release of neurotransmitters, such as GABA, glutamate, and D-serine, a co-agonist of N-methyl-D-aspartate (NMDA) receptors. H2S facilitates the induction of hippocampal long-term potentiation, a synaptic model of memory formation, by enhancing the activity of NMDA receptors, while H2S2 achieves this by activating transient receptor potential ankyrin 1 (TRPA1) channels in astrocytes, potentially leading to the activation of nearby neurons. The recent findings show the other aspects of TRPA1 channels-that is, the regulation of the levels of sulfur-containing molecules and their metabolizing enzymes. Disturbance of the signalling by H2S/H2Sn has been demonstrated to be involved in various diseases, including cognitive and psychiatric diseases. The physiological and pathophysiological roles of these molecules will be discussed.

H 2 S preconditioning induces long-lived perturbations in O 2 metabolism

Hydrogen sulfide exposure in moderate doses can induce profound but reversible hypometabolism in mammals. At a cellular level, H 2 S inhibits the electron transport chain (ETC), augments aerobic glycolysis, and glutamine-dependent carbon utilization via reductive carboxylation; however, the durability of these changes is unknown. We report that despite its volatility, H 2 S preconditioning increases P 50(O2) , the O 2 pressure for half maximal cellular respiration, and has pleiotropic effects on oxidative metabolism that persist up to 24-48 h later. Notably, cyanide, another complex IV inhibitor, does not induce this type of metabolic memory. Sulfide-mediated prolonged fractional inhibition of complex IV by H 2 S is modulated by sulfide quinone oxidoreductase, which commits sulfide to oxidative catabolism. Since induced hypometabolism can be beneficial in disease settings that involve insufficient or interrupted blood flow, our study has important implications for attenuating reperfusion-induced ischemic injury, and/or prolonging shelf life of biologics like platelets.

Hydrogen Sulfide Ligation in Hemoglobin I of Lucina pectinata─A QM/MM and Local Mode Study

In this study, we investigated the interaction between the H2S ligand and the heme pocket of hemoglobin I (HbI) of Lucina pectinata for the wild-type protein; three known mutations where distal glutamine is replaced by hydrophobic valine (Gln64Val) and hydrophilic histidine in both protonation forms (Gln64Hisϵ and Gln64Hisδ); five known mutations of the so-called phenyl cage, replacing the hydrophobic phenylalanines Phe29 and Phe43 with tyrosine (Tyr), valine (Val), or leucine (Leu); and two additional mutations, Phe68Tyr and Phe68Val, in order to complement previous studies with new insights about the binding mechanism at the molecular level. A particular focus was on the intrinsic strengths of the chemical bonds involved, utilizing local vibrational force constants based on combined quantum mechanical-molecular mechanical calculations. Wild-type protein and mutations clustered into two distinct groups: Group 1 protein systems with a proton acceptor in the distal protein pocket, close to one of the H2S bonds, and Group 2 protein systems without a hydrogen acceptor close by in the active site of the protein. According to our results, the interactions between H2S and HbI of Lucina pectinata involve two important elements, namely, binding of H2S to Fe of the heme group, followed by the proton transfer from the HS bond to the distal residue. The distal residue is additionally stabilized by a second proton transfer from the distal residue to COO- of the propionate group in heme. We could identify the FeS bond as a key player and discovered that the strength of this bond depends on two mutual factors, namely, the strength of the HS bond involved in the proton transfer and the electrostatic field of the protein pocket qualifying the FeS bond as a sensitive probe for monitoring changes in H2S ligation upon protein mutations. We hope our study will inspire and guide future experimental studies, targeting new promising mutations such as Phe68Tyr, Phe68Val, or Phe43Tyr/Phe68Val.

Binding mechanism of disulfide species to ferric hemeproteins: The case of metmyoglobin

The interactions of the heme iron of hemeproteins with sulfide and disulfide compounds are of potential interest as physiological signaling processes. While the interaction with hydrogen sulfide has been described computationally and experimentally, the reaction with disulfide, and specifically the molecular mechanism for ligand binding has not been studied in detail. In this work, we study the association process for disulfane and its conjugate base disulfanide at different pH conditions. Additionally, by means of advanced sampling techniques based on multiple steered molecular dynamics, we provide free energy profiles for ligand migration for both acid/base species, showing a similar behavior to the previously reported for the related H2S/HS¯ pair. Finally, we studied the ligand interchange reaction (H2O/H2S, HS¯ and H2O/HSSH, HSS¯) by means of hybrid quantum mechanics-molecular mechanics calculations. We show that the anionic species are able to displace more efficiently the H2O bound to the iron, and that the H-bond network in the distal cavity can help the neutral species to perform the reaction. Altogether, we provide a molecular explanation for the experimental information and show that the global association process depends on a fine balance between the migration towards the active site and the ligand interchange reaction.

A Review of the Current State of Magnetic Force Microscopy to Unravel the Magnetic Properties of Nanomaterials Applied in Biological Systems and Future Directions for Quantum Technologies

Magnetism plays a pivotal role in many biological systems. However, the intensity of the magnetic forces exerted between magnetic bodies is usually low, which demands the development of ultra-sensitivity tools for proper sensing. In this framework, magnetic force microscopy (MFM) offers excellent lateral resolution and the possibility of conducting single-molecule studies like other single-probe microscopy (SPM) techniques. This comprehensive review attempts to describe the paramount importance of magnetic forces for biological applications by highlighting MFM's main advantages but also intrinsic limitations. While the working principles are described in depth, the article also focuses on novel micro- and nanofabrication procedures for MFM tips, which enhance the magnetic response signal of tested biomaterials compared to commercial nanoprobes. This work also depicts some relevant examples where MFM can quantitatively assess the magnetic performance of nanomaterials involved in biological systems, including magnetotactic bacteria, cryptochrome flavoproteins, and magnetic nanoparticles that can interact with animal tissues. Additionally, the most promising perspectives in this field are highlighted to make the reader aware of upcoming challenges when aiming toward quantum technologies.

Reduction of metmyoglobin by inorganic disulfide species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by inorganic disulfide species has been studied by combined spectroscopic and kinetic analyses, under argon atmosphere. The process is kinetically characterized by biexponential time traces, for variable ratios of excess disulfide to protein, in the pH interval 6.6-8.0. Using UV-vis and resonance Raman spectroscopies, we observed that MbFeIII is converted into a low spin hexacoordinated ferric complex, tentatively assigned as MbFeIII(HSS-)/MbFeIII(SS2-), in an initial fast step. The complex is slowly converted into a pentacoordinated ferrous form, assigned as MbFeII according to the resonance Raman records. The reduction is a pH-dependent process, but independent of the initial disulfide concentration, suggesting the unimolecular decomposition of the intermediate complex following a reductive homolysis. We estimated the rate of the fast formation of the complex at pH 7.4 (kon = 3.7 × 103 M-1 s-1), and a pKa2 = 7.5 for the equilibrium MbFeIII(HSS-)/MbFeIII(SS2-). Also, we estimated the rate for the slow reduction at the same pH (kred = 10-2 s-1). A reaction mechanism compliant with the experimental results is proposed. This mechanistic study provides a differential kinetic signature for the reactions of disulfide compared to sulfide species on metmyoglobin, which may be considered in other hemeprotein systems.

Noncovalent π Interactions in Mutated Aquomet-Myoglobin Proteins: A QM/MM and Local Vibrational Mode Study

Protein dynamics and function is strongly connected to the energy flow taking place. Myoglobin (Mb) and its mutations are ideal systems to study the process of vibrational energy transfer (VET) at the molecular level. Anti-Stokes ultraviolet resonance Raman studies using a tryptophan (Trp) probe, introduced at different Mb positions by amino acid replacement, have suggested that the amount of VET depends on the position of the Trp probe relative to the heme group. Inspired by this experimental work, we explored the strength of noncovalent π interactions, as well as covalent interactions for both the axial and distal ligands bound to iron in aquomet-Mb with the local vibrational mode analysis (LMA), originally developed by Konkoli and Cremer. Two sets of noncovalent interactions were investigated: (1) the interaction between the water ligand and Trp rings and (2) the interaction between the Trp and the porphyrin rings of the heme group. We assessed the strength of these noncovalent interactions via a special local mode force constant. Various Trp-modified water-bound ferric Mb proteins in the ground state were studied (6 in total) using gas-phase and QM/MM calculations followed by LMA. Our results disclose that VET is indeed dependent on the position of the Trp probe relative to the heme group but also on the tautomeric nature of distal histidine. They provide new guidelines on how to assess noncovalent π interactions in proteins utilizing LMA and how to use these data to explore VET, and more generally protein dynamics and function.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by sulfide species (H2S/HS-) under an argon atmosphere has been studied by a combination of spectroscopic, kinetic, and computational methods. Asymmetric S-shaped time-traces for the formation of MbFeII at varying ratios of excess sulfide were observed at pH 5.3 < pH < 8.0 and 25 °C, suggesting an autocatalytic reaction mechanism. An increased rate at more alkaline pHs points to HS- as relevant reactive species for the reduction. The formation of the sulfanyl radical (HS•) in the slow initial phase was assessed using the spin-trap phenyl N-tert-butyl nitrone. This radical initiates the formation of S-S reactive species as disulfanuidyl/ disulfanudi-idyl radical anions and disulfide (HSSH•-/HSS•2- and HSS-, respectively). The autocatalysis has been ascribed to HSS-, formed after HSSH•-/HSS•2- disproportionation, which behaves as a fast reductant toward the intermediate complex MbFeIII(HS-). We propose a reaction mechanism for the sulfide-mediated reduction of metmyoglobin where only ferric heme iron initiates the oxidation of sulfide species. Beside the chemical interest, this insight into the MbFeIII/sulfide reaction under an argon atmosphere is relevant for the interpretation of biochemical aspects of ectopic myoglobins found on hypoxic tissues toward reactive sulfur species.

Emerging Chemical Biology of Protein Persulfidation

Significance: Protein persulfidation (the formation of RSSH), an evolutionarily conserved oxidative posttranslational modification in which thiol groups in cysteine residues are converted into persulfides, has emerged as one of the main mechanisms through which hydrogen sulfide (H2S) conveys its signaling. Recent Advances: New methodological advances in persulfide labeling started unraveling the chemical biology of this modification and its role in (patho)physiology. Some of the key metabolic enzymes are regulated by persulfidation. RSSH levels are important for the cellular defense against oxidative injury, and they decrease with aging, leaving proteins vulnerable to oxidative damage. Persulfidation is dysregulated in many diseases. Critical Issues: A relatively new field of signaling by protein persulfidation still has many unanswered questions: the mechanism(s) of persulfide formation and transpersulfidation and the identification of "protein persulfidases," the improvement of methods to monitor RSSH changes and identify protein targets, and understanding the mechanisms through which this modification controls important (patho)physiological functions. Future Directions: Deep mechanistic studies using more selective and sensitive RSSH labeling techniques will provide high-resolution structural, functional, quantitative, and spatiotemporal information on RSSH dynamics and help with better understanding how H2S-derived protein persulfidation affects protein structure and function in health and disease. This knowledge could pave the way for targeted drug design for a wide variety of pathologies. Antioxid. Redox Signal. 39, 19-39.

A portable electrochemical microsensor for in-site measurement of dissolved oxygen and hydrogen sulfide in natural water

Dissolved oxygen (O₂) and hydrogen sulfide (H₂S) are two important indicators of water quality, their levels are of intimate dependence and varying over time. It is of great significance to monitoring of dissolved O₂ and H₂S simultaneously in natural water, yet has not been reported because of lack of effective approaches. In this work, a portable electrochemical microsensor was developed for simultaneously quantifying dissolved O₂ and H₂S. Here, Pd@Ni nanoparticles (NPs) were self-assembled onto the microelectrode by MXene titanium carbide (Ti₃C₂T_x), which were of responsibility towards O₂ and H₂S detection within single electrochemical reduction process. On this regard, Pd NPs facilitated catalyzing the electrochemical reduction of O₂, while Ni NPs were employed as recognition element for H₂S detection. With the electrochemical reduction sweep, the initial application of a positive voltage rendered the Ni to be oxidized to be Ni ions, contributing to their following capture of surrounding S^2- to form nickel sulfide. Nickel sulfide with highly electrochemical activity were capable of generating detecting reduction current. In consequence, the as-designed microsensor can simultaneously determine O₂ concentrations ranging from 36 to 318 μM and H₂S levels ranging from 0.1 to 2.5 μM with high selectivity. Finally, the portable microsensor was successfully applied to simultaneous detection dissolved O₂ and H₂S in natural water in-site, the results of which were comparable to the classical methods.

Hemoglobin Nanocrystals for Drugs Free, Synergistic Theranostics of Colon Tumor

The conventional approach in cancer nanomedicine involves advanced drug nanocarriers delivering preloaded therapeutics to targeted tumor sites to maximize drug efficiency. However, both cancer drugs and nanocarriers inevitably produce side effects and systemic toxicity. Herein, hemoglobin nanocrystals (HbC) as drug-free theranostic nanoformulations with the tumor microenvironment (TME) activated diagnostic and therapeutic abilities towards colon tumors are introduced. HbC can release Fe²⁺ oxidized to Fe³⁺ in the Fenton reaction with tumor endogenous H₂ O₂ , concurrently with the generation of cytotoxic hydroxyl radicals (•OH) that allow for chemodynamic therapy (CDT). Furthermore, in situ-produced Fe³⁺ reacts with colon tumor-abundant H₂ S, resulting in the production of Fe_{1- x} S, which provides magnetic resonance imaging (MRI) contrast and allows for NIR light-inducible photothermal therapy (PTT). In vitro and in vivo studies revealed that HbC produced CDT towards 4T1 tumors, and MRI-guided, synergistically enhanced combination of CDT and PTT against H₂ S abundant colon tumors (CT26), with negligible toxicity towards normal tissues, enlightening HbC as highly efficient and biocompatible TME activated theranostic nanoplatform specific against colon cancer without any traditional drugs and drug carriers.

Interactions of reactive sulfur species with metalloproteins

Reactive sulfur species (RSS) entail a diverse family of sulfur derivatives that have emerged as important effector molecules in H₂S-mediated biological events. RSS (including H₂S) can exert their biological roles via widespread interactions with metalloproteins. Metalloproteins are essential components along the metabolic route of oxygen in the body, from the transport and storage of O₂, through cellular respiration, to the maintenance of redox homeostasis by elimination of reactive oxygen species (ROS). Moreover, heme peroxidases contribute to immune defense by killing pathogens using oxygen-derived H₂O₂ as a precursor for stronger oxidants. Coordination and redox reactions with metal centers are primary means of RSS to alter fundamental cellular functions. In addition to RSS-mediated metalloprotein functions, the reduction of high-valent metal centers by RSS results in radical formation and opens the way for subsequent per- and polysulfide formation, which may have implications in cellular protection against oxidative stress and in redox signaling. Furthermore, recent findings pointed out the potential role of RSS as substrates for mitochondrial energy production and their cytoprotective capacity, with the involvement of metalloproteins. The current review summarizes the interactions of RSS with protein metal centers and their biological implications with special emphasis on mechanistic aspects, sulfide-mediated signaling, and pathophysiological consequences. A deeper understanding of the biological actions of reactive sulfur species on a molecular level is primordial in H₂S-related drug development and the advancement of redox medicine.

A Metabolic Paradigm for Hydrogen Sulfide Signaling via Electron Transport Chain Plasticity

Significance: A burgeoning literature has attributed varied physiological effects to hydrogen sulfide (H2S), which is a product of eukaryotic sulfur amino acid metabolism. Protein persulfidation represents a major focus of studies elucidating the mechanism underlying H2S signaling. On the contrary, the capacity of H2S to induce reductive stress by targeting the electron transport chain (ETC) and signal by reprogramming redox metabolism has only recently begun to be elucidated. Recent Advances: In contrast to the nonspecific reaction of H2S with oxidized cysteines to form protein persulfides, its inhibition of complex IV represents a specific mechanism of action. Studies on the dual impact of H2S as an ETC substrate and an inhibitor have led to the exciting discovery of ETC plasticity and the use of fumarate as a terminal electron acceptor. H2S oxidation combined with complex IV targeting generates mitochondrial reductive stress, which is signaled through the metabolic network, leading to increased aerobic glycolysis, glutamine-dependent reductive carboxylation, and lipogenesis. Critical Issues: Insights into H2S-induced metabolic reprogramming are ushering in a paradigm shift for understanding the mechanism of its cellular action. It will be critical to reevaluate the physiological effects of H2S, for example, cytoprotection against ischemia-reperfusion injury, through the framework of metabolic reprogramming and ETC remodeling by H2S. Future Directions: The metabolic ramifications of H2S in other cellular compartments, for example, the endoplasmic reticulum and the nucleus, as well as the intersections between hypoxia and H2S signaling are important future directions that merit elucidation. Antioxid. Redox Signal. 38, 57-67.

GLB-3: A resilient, cysteine-rich, membrane-tethered globin expressed in the reproductive and nervous system of Caenorhabditis elegans

The popular genetic model organism Caenorhabditis elegans (C. elegans) encodes 34 globins, whereby the few that are well-characterized show divergent properties besides the typical oxygen carrier function. Here, we present a biophysical characterization and expression analysis of C. elegans globin-3 (GLB-3). GLB-3 is predicted to exist in two isoforms and is expressed in the reproductive and nervous system. Knockout of this globin causes a 99% reduction in fertility and reduced motility. Spectroscopic analysis reveals that GLB-3 exists as a bis-histidyl-ligated low-spin form in both the ferrous and ferric heme form. A function in binding of diatomic gases is excluded on the basis of the slow CO-binding kinetics. Unlike other globins, GLB-3 is also not capable of reacting with H₂O₂, H₂S, and nitrite. Intriguingly, not only does GLB-3 contain a high number of cysteine residues, it is also highly stable under harsh conditions (pH = 2 and high concentrations of H₂O₂). The resilience diminishes when the N- and C-terminal extensions are removed. Redox potentiometric measurements reveal a slightly positive redox potential (+8 ± 19 mV vs. SHE), suggesting that the heme iron may be able to oxidize cysteines. Electron paramagnetic resonance shows that formation of an intramolecular disulphide bridge, involving Cys70, affects the heme-pocket region. The results suggest an involvement of the globin in (cysteine) redox chemistry.

Hydrogen Sulfide and Its Donors: Keys to Unlock the Chains of Nonalcoholic Fatty Liver Disease

Hydrogen sulfide (H2S) has emerged as the third “gasotransmitters” and has a crucial function in the diversity of physiological functions in mammals. In particular, H2S is considered indispensable in preventing the development of liver inflammation in the case of excessive caloric ingestion. Note that the concentration of endogenous H2S was usually low, making it difficult to discern the precise biological functions. Therefore, exogenous delivery of H2S is conducive to probe the physiological and pathological roles of this gas in cellular and animal studies. In this review, the production and metabolic pathways of H2S in vivo, the types of donors currently used for H2S release, and study evidence of H2S improvement effects on nonalcoholic fatty liver disease are systematically introduced.

Supranutritional Supplementation of Vitamin E Influences Myoglobin Post-Translational Modifications in Postmortem Beef Longissimus Lumborum Muscle

Post-translational modifications (PTM) in myoglobin (Mb) can influence fresh meat color stability. Dietary supplementation of vitamin E improves beef color stability by delaying lipid oxidation–induced Mb oxidation and influences proteome profile of postmortem beef skeletal muscles. Nonetheless, the influence of vitamin E on Mb PTM in postmortem beef skeletal muscles has yet to be investigated. Therefore, the objective of the current study was to examine the effect of dietary vitamin E on Mb PTM in postmortem beef longissimus lumborum muscle. Beef longissimus lumborum muscle samples (24 h postmortem) were obtained from the carcasses of 9 vitamin E–supplemented (VITE; 1,000 IU vitamin E diet/heifer·d−1for 89 d) and 9 control (CONT; no supplemental vitamin E) heifers. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to separate Mb from other sarcoplasmic proteins of beef longissimus lumborum muscle. Tandem mass spectrometry identified multiple PTM (phosphorylation, acetylation, 4-hydroxynonenalalkylation, methylation, dimethylation, trimethylation, and carboxymethylation) in the protein bands (17 kDa) representing Mb. The amino acids susceptible to phosphorylation were threonine (T) and tyrosine (Y), whereas lysine (K) residues were prone to other PTM. The same sites of phosphorylation (T34, T67, Y103), carboxymethylation (K77, K78), and 4-hydroxynonenal alkylation (K77, K78, K79) were identified in Mb from CONT and VITE samples, indicating that these PTM were not influenced by the vitamin E supplementation in cattle. Nonetheless, differential occurrence of acetylation, methylation, dimethylation, and trimethylation were identified in Mb from CONT and VITE samples. Overall, a greater number of amino acids were modified in CONT than VITE, suggesting that the supplementation of vitamin E decreased thenumbers of post-translationally modified residues in Mb. Additionally, PTM at K87, K96, K98, and K102 were unique to CONT, whereas PTM at K118 were unique to VITE. These findings suggested that dietary supplementation of vitamin E in beef cattle might protect amino acid residues in Mb—especially those located spatially close to proximal histidine—from undergoing PTM, thereby improving Mb redox stability.

Rapid and selective generation of H2S within mitochondria protects against cardiac ischemia-reperfusion injury

Mitochondria-targeted H2S donors are thought to protect against acute ischemia-reperfusion (IR) injury by releasing H2S that decreases oxidative damage. However, the rate of H2S release by current donors is too slow to be effective upon administration following reperfusion. To overcome this limitation here we develop a mitochondria-targeted agent, MitoPerSulf that very rapidly releases H2S within mitochondria. MitoPerSulf is quickly taken up by mitochondria, where it reacts with endogenous thiols to generate a persulfide intermediate that releases H2S. MitoPerSulf is acutely protective against cardiac IR injury in mice, due to the acute generation of H2S that inhibits respiration at cytochrome c oxidase thereby preventing mitochondrial superoxide production by lowering the membrane potential. Mitochondria-targeted agents that rapidly generate H2S are a new class of therapy for the acute treatment of IR injury.

Decomposition kinetics and postmortem production of hydrogen sulfide and its metabolites

BACKGROUND

Hydrogen sulfide (H₂S), an endogenous gas, can also be generated from organics putrefaction. It is difficult for suspected cases of H₂S poisoning to determine whether H₂S in specimens is ingested by antemortem poisoning or generated from organics putrefaction. The aim of this study was to find the biomarkers of acute H₂S poisoning via comparing the concentrations of H₂S and its metabolites over time in specimens.

METHODS

The H₂S-spiked blood and blank blood group were established. The decomposition kinetics and the postmortem production of H₂S were studied due to organics putrefaction. The specimens were placed under 4 conditions of 37, 20, 4 and - 20 ℃. The content of H₂S in specimens was quantified by gas chromatography-mass spectrometry, and the contents of its metabolites (thiosulfate and trimethylsulfonium) were measured by liquid chromatography-mass spectrometry, and the variation of its concentration was evaluated.

RESULTS

In H₂S-spiked blood, H₂S decreased sharply in the initial stage at 37, 20 and 4 °C, and increased first and then decreased later; but it was relatively stable at - 20 °C. In spiked blood, thiosulfate was 9-fold higher than endogenous concentrations, which increased at first and then decreased during storage. Except for thiosulfate at 37 °C, H₂S and thiosulfate in blank blood both increased at first and then decreased in storage; but trimethylsulfonium (TMS) gradually decreased over time in both groups.

CONCLUSIONS

Thiosulfate is a reliable biomarker of acute H₂S poisoning at - 20℃ within 7 days. But H₂S, because of instability and volatility, is not an ideal poisoning marker. TMS is not an appropriate biomarker due to extremely low concentration in blood.

Thionitrite (SNO − ) and Perthionitrite (SSNO − ) are Simple Synthons for Nitrosylated Iron Sulfur Clusters

S/N crosstalk species derived from the interconnected reactivity of H₂ S and NO facilitate the transport of reactive sulfur and nitrogen species in signaling, transport, and regulatory processes. We report here that simple Fe²⁺ ions, such as those that are bioavailable in the labile iron pool (LIP), react with thionitrite (SNO^- ) and perthionitrite (SSNO^- ) to yield the dinitrosyl iron complex [Fe(NO)₂ (S₅ )]^- . In the reaction of FeCl₂ with SNO^- we were able to isolate the unstable intermediate hydrosulfido mononitrosyl iron complex [FeCl₂ (NO)(SH)]^- , which was characterized by X-ray crystallography. We also show that [Fe(NO)₂ (S₅ )]^- is a simple synthon for nitrosylated Fe-S clusters via its reduction with PPh₃ to yield Roussin's Red Salt ([Fe₂ S₂ (NO)₄ ]^2- ), which highlights the role of S/N crosstalk species in the assembly of fundamental Fe-S motifs.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Role of hydrogen sulfide in subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a common acute and severe disease worldwide, which imposes a heavy burden on families and society. However, the current therapeutic strategies for SAH are unsatisfactory. Hydrogen sulfide (H₂ S), as the third gas signaling molecule after carbon monoxide and nitric oxide, has been widely studied recently. There is growing evidence that H₂ S has a promising future in the treatment of central nervous system diseases. In this review, we focus on the effects of H₂ S in experimental SAH and elucidate the underlying mechanisms. We demonstrate that H₂ S has neuroprotective effects and significantly reduces secondary damage caused by SAH via antioxidant, antiinflammatory, and antiapoptosis mechanisms, and by alleviating cerebral edema and vasospasm. Based on these findings, we believe that H₂ S has great potential in the treatment of SAH and warrants further study to promote its early clinical application.

Mycobacterium tuberculosis DosS binds H2S through its Fe3+ heme iron to regulate the DosR dormancy regulon

Mycobacterium tuberculosis (Mtb) senses and responds to host-derived gasotransmitters NO and CO via heme-containing sensor kinases DosS and DosT and the response regulator DosR. Hydrogen sulfide (H2S) is an important signaling molecule in mammals, but its role in Mtb physiology is unclear. We have previously shown that exogenous H2S can modulate expression of genes in the Dos dormancy regulon via an unknown mechanism(s). Here, we test the hypothesis that Mtb senses and responds to H2S via the DosS/T/R system. Using UV-Vis and EPR spectroscopy, we show that H2S binds directly to the ferric (Fe3+) heme of DosS (KDapp = 5.30 μM) but not the ferrous (Fe2+) form. No interaction with DosT(Fe2+-O2) was detected. We found that the binding of sulfide can slowly reduce the DosS heme iron to the ferrous form. Steered Molecular Dynamics simulations show that H2S, and not the charged HS- species, can enter the DosS heme pocket. We also show that H2S increases DosS autokinase activity and subsequent phosphorylation of DosR, and H2S-mediated increases in Dos regulon gene expression is lost in Mtb lacking DosS. Finally, we demonstrate that physiological levels of H2S in macrophages can induce DosR regulon genes via DosS. Overall, these data reveal a novel mechanism whereby Mtb senses and responds to a third host gasotransmitter, H2S, via DosS(Fe3+). These findings highlight the remarkable plasticity of DosS and establish a new paradigm for how bacteria can sense multiple gasotransmitters through a single heme sensor kinase.

Hydrogen sulfide and metal interaction: the pathophysiological implications

Hydrogen sulfide (H₂S), previously recognized as a toxic gas, has emerged as an important gaseous signaling molecule along with nitric oxide, carbon monoxide and also hydrogen. H₂S can be endogenously produced in the mammalian body at a very low level for various pathophysiological processes. Notably, H₂S can interact with several essential metals in the body such as iron, copper, nickel, and zinc to carry out specific functions. The interactions of H₂S with metal-binding proteins have been shown to aid in its signal transduction and cellular metabolism. In addition, H₂S is capable of providing a cytoprotective role against metal toxicity. As the research in the field of H₂S signaling in biology and medicine increases, much progresses have been developed for detecting H₂S via interaction with metals. In this review, the interaction of H₂S with metals, specifically in regard to metal-driven metabolism of H₂S, the protection against metal toxicity by H₂S and the detection of H₂S using metals will be discussed. Discovering the interactions of this gasotransmitter with metals is important for determining the mechanisms underlying the cellular functions of H₂S as well as developing novel therapeutic avenues.

Gas regulation of complex II reversal via electron shunting to fumarate in the mammalian ETC

The electron transport chain (ETC) is a major currency converter that exchanges the chemical energy of fuel oxidation to proton motive force and, subsequently, ATP generation, using O₂ as a terminal electron acceptor. Discussed herein, two new studies reveal that the mammalian ETC is forked. Hypoxia or H₂S exposure promotes the use of fumarate as an alternate terminal electron acceptor. The fumarate/succinate and CoQH₂/CoQ redox couples are nearly iso-potential, revealing that complex II is poised for facile reverse electron transfer, which is sensitive to CoQH₂ and fumarate concentrations. The gas regulators, H₂S and ^•NO, modulate O₂ affinity and/or inhibit the electron transfer rate at complex IV. Their induction under hypoxia suggests a mechanism for how traffic at the ETC fork can be regulated.

A sensitive colorimetric hydrogen sulfide detection approach based on copper-metal-organic frameworks and a smartphone

In this study, we demonstrate a colorimetric approach for the detection of hydrogen sulfide (H₂S) in water samples with high sensitivity. Firstly, copper-metal-organic frameworks (Cu-MOFs) were synthesized by ultrasonic-assisted hydrothermal method, presenting a maximum absorption peak at 700 nm. It was found that Cu-MOFs could react with H₂S to form a copper-sulfur complex along with a decrease of the absorption peak at 700 nm and a visible color change from blue to tan. Under the optimal reaction conditions, the absorption intensity at 700 nm was linear with H₂S concentration in a range of 0.05-2 mM (R² = 0.9928), providing a detection limit of 22 μM. Furthermore, the method was successfully applied to the detection of H₂S in lake water samples with a recovery rate between 94.4% and 112.6%. In addition, a practical and portable device for on-site H₂S detection was designed by using agarose hydrogels, and a simple colorimetric detection method based on a smartphone was developed. This analytical method showed good selectivity for H₂S compared to other interfering substances, and the feasibility of the agarose hydrogel-based device was proved by the determination of H₂S in real lake water samples.

C-H⋯S hydrogen bonding interactions

The short C-H⋯S contacts found in available structural data for both small molecules and larger biomolecular systems suggest that such contacts are an often overlooked yet important stabilizing interaction. Moreover, many of these short C-H⋯S contacts meet the definition of a hydrogen bonding interaction. Using available structural data from the Cambridge Structural Database (CSD), as well as selected examples from the literature in which important C-H⋯S contacts may have been overlooked, we highlight the generality of C-H⋯S hydrogen bonding as an important stabilizing interaction. To uncover and establish the generality of these interactions, we compare C-H⋯S contacts with other traditional hydrogen bond donors and acceptors as well as investigate how coordination number and metal bonding affect the preferred geometry of interactions in the solid state. This work establishes that the C-H⋯S bond meets the definition of a hydrogen bond and serves as a guide to identify C-H⋯S hydrogen bonds in diverse systems.

Methods for Suppressing Hydrogen Sulfide in Biological Systems

Significance: Hydrogen sulfide (H₂S) plays critical roles in redox biology, and its regulatory effects are tightly controlled by its cellular location and concentration. The imbalance of H₂S is believed to contribute to some pathological processes. Recent Advances: Downregulation of H₂S requires chemical tools such as inhibitors of H₂S-producing enzymes and H₂S scavengers. Recent efforts have discovered some promising inhibitors and scavengers. These advances pave the road toward better understanding of the functions of H₂S. Critical Issues: Precise H₂S downregulation is challenging. The potency and specificity of current inhibitors are still far from ideal. H₂S-producing enzymes are involved in complex sulfur metabolic pathways and ubiquitously present in biological matrices. The inhibition of these enzymes can cause unwanted side effects. H₂S scavengers allow targeted H₂S clearance, but their options are still limited. In addition, the scavenging process often results in biologically active by-products. Future Directions: Further development of potent and specific inhibitors for H₂S-producing enzymes is needed. Scavengers that can rapidly and selectively remove H₂S while generating biocompatible by-products are needed. Potential therapeutic applications of scavengers and inhibitors are worth exploring. Antioxid. Redox Signal. 36, 294-308.

Capability of Au nano-rhombic dodecahedra in a label-free colorimetric assay: application in the determination of S2− and Hg2+

Au nano-rhombic dodecahedra  with high sensitivity to the environmental refractive index afford sensitive detection of S2- and Hg2+.

Sulfide and transition metals - A partnership for life

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron‑sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron‑sulfur, heme and zinc proteins is also addressed.

A redox cycle with complex II prioritizes sulfide quinone oxidoreductase-dependent H2S oxidation

The dual roles of H₂S as an endogenously synthesized respiratory substrate and as a toxin raise questions as to how it is cleared when the electron transport chain is inhibited. Sulfide quinone oxidoreductase (SQOR) catalyzes the first step in the mitochondrial H₂S oxidation pathway, using CoQ as an electron acceptor, and connects to the electron transport chain at the level of complex III. We have discovered that at high H₂S concentrations, which are known to inhibit complex IV, a new redox cycle is established between SQOR and complex II, operating in reverse. Under these conditions, the purine nucleotide cycle and the malate aspartate shuttle furnish fumarate, which supports complex II reversal and leads to succinate accumulation. Complex II knockdown in colonocytes decreases the efficiency of H₂S clearance while targeted knockout of complex II in intestinal epithelial cells significantly decreases the levels of thiosulfate, a biomarker of H₂S oxidation, to approximately one-third of the values seen in serum and urine samples from control mice. These data establish the physiological relevance of this newly discovered redox circuitry between SQOR and complex II for prioritizing H₂S oxidation and reveal the quantitatively significant contribution of intestinal epithelial cells to systemic H₂S metabolism.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H₂S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O₂ sensing mechanism. This hypothesis is based on observations that H₂S and RSS metabolism is inversely correlated with O₂ tension, exogenous H₂S elicits physiological responses identical to those produced by hypoxia, factors that affect H₂S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O₂. H₂S-mediated O₂ sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O₂ sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.

Hydrogen sulfide in ageing, longevity and disease

Hydrogen sulfide (H₂S) modulates many biological processes, including ageing. Initially considered a hazardous toxic gas, it is now recognised that H₂S is produced endogenously across taxa and is a key mediator of processes that promote longevity and improve late-life health. In this review, we consider the key developments in our understanding of this gaseous signalling molecule in the context of health and disease, discuss potential mechanisms through which H₂S can influence processes central to ageing and highlight the emergence of novel H₂S-based therapeutics. We also consider the major challenges that may potentially hinder the development of such therapies.

Reactivity of inorganic sulfide species towards a pentacoordinated heme model system

The reactivity of inorganic sulfide towards ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate has been explored by means of visible absorption and resonance Raman spectroscopies. The reaction has been previously studied in buffered solutions at neutral pH and in the presence of excess sulfide, revealing the formation of a moderately stable hexacoordinated low spin ferric sulfide complex that yields the ferrous form in the hour's timescale. In the surfactant solution, instead, the ferrous form is rapidly formed. The spectroscopic characterization of the heme structure in the surfactant milieu revealed the stabilization of a major ferric mono-histidyl high spin heme, which may be ascribed to out of plane distortions prompting the detachment of the axially ligated water molecule, thus leading to a differential reactivity. The ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate provides a model for pentacoordinated heme platforms with an imidazole-based ligand.

The contribution of metalloporphyrin complexes in molecular sensing and in sustainable polymerization processes: a new and unique perspective

This review highlights the recent developments in the field of metalloporphyrins as optical probes for biologically relevant molecules, such as nitric oxide (NO) and hydrogen sulfide (H2S), and as catalysts for the preparation of sustainable polymers such as polyesters, by the ring-opening polymerization (ROP) of cyclic esters and the ring-opening co-polymerization (ROCOP) of epoxides and anhydrides, and polycarbonates by the chemical fixation of carbon dioxide (CO2). The great potential of porphyrins is mainly due to the possibility of making various synthetic modifications to the porphyrin ring, such as modifying the coordinated metal, peripheral substituents, or even the molecular skeleton. Due to the strict structure-property relationships, one can use porphyrinoids in several different applications such as, for instance, activation of molecular oxygen or catalysis of photosynthetic processes. These possibilities broaden the application of porphyrins in several different fields of research, further mimicking what nature does. In this context, here, we want to provide evidence for the great flexibility of metalloporphyrins by presenting an overview of results obtained by us and others in the research fields we are currently involved in. More specifically, we report a survey of our most significant achievements regarding their use as optical probes in the context of the results reported in the literature from other research groups, and of the use of porphyrin metal(iii) complexes as catalysts for sustainable polymerization processes. As for the optical probe section, in addition to the metalloporphyrins synthesized ad hoc in the laboratory, the present work also covers the natural proteins containing a porphyrin core.

Diet and Hydrogen Sulfide Production in Mammals

Significance: In recent times, it has emerged that some dietary sulfur compounds can act on mammalian cell signaling systems via their propensity to release hydrogen sulfide (H₂S). H₂S plays important biochemical and physiological roles in the heart, gastrointestinal tract, brain, kidney, and immune systems of mammals. Reduced levels of H₂S in cells and tissues correlate with a spectrum of pathophysiological conditions, including heart disease, diabetes, obesity, and altered immune function. Recent Advances: In the last decade, researchers have now begun to explore the mechanisms by which dietary-derived sulfur compounds, in addition to cysteine, can act as sources of H₂S. This research has led to the identified several compounds, organic sulfides, isothiocyanates, and inorganic sulfur species including sulfate that can act as potential sources of H₂S in mammalian cells and tissues. Critical Issues: We have summarised progress made in the identification of dietary factors that can impact on endogenous H₂S levels in mammals. We also describe current research focused on how some sulfur molecules present in dietary plants, and associated chemical analogues, act as sources of H₂S, and discuss the biological properties of these molecules as studied in a range of in vitro and in vivo systems. Future Directions: The identification of sulfur compounds in edible plants that can act as novel H₂S releasing molecules is intriguing. Research in this area could inform future studies exploring the impact of diet on H₂S levels in mammalian systems. Despite recent progress, additional work is needed to determine the mechanisms by which H₂S is released from these molecules following ingestions of dietary plants in humans, whether the amounts of H₂S produced is of physiological significance following the metabolism of these compounds in vivo, and if diet could be used to manipulated H₂S levels in humans. Importantly, this will lead to a better understanding of the biological significance of H₂S generated from dietary sources, and this information could be used in the development of plant breeding initiatives to increase the levels of H₂S releasing sulfur compounds in crops, or inform dietary intervention strategies that could be used to alter the levels of H₂S in humans.

Abiological catalysis by myoglobin mutant with a genetically incorporated unnatural amino acid

To inculcate biocatalytic activity in the oxygen-storage protein myoglobin (Mb), a genetically engineered myoglobin mutant H64DOPA (DOPA = L-3,4-dihydroxyphenylalanine) has been created. Incorporation of unnatural amino acids has already demonstrated their ability to accomplish many non-natural functions in proteins efficiently. Herein, the presence of redox-active DOPA residue in the active site of mutant Mb presumably stabilizes the compound I in the catalytic oxidation process by participating in an additional hydrogen bonding (H-bonding) as compared to the WT Mb. Specifically, a general acid-base catalytic pathway was achieved due to the availability of the hydroxyl moieties of DOPA. The reduction potential values of WT (E° = -260 mV) and mutant Mb (E° = -300 mV), w.r.t. Ag/AgCl reference electrode, in the presence of hydrogen peroxide, indicated an additional H-bonding in the mutant protein, which is responsible for the peroxidase activity of the mutant Mb. We observed that in the presence of 5 mM H2O2, H64DOPA Mb oxidizes thioanisole and benzaldehyde with a 10 and 54 folds higher rate, respectively, as opposed to WT Mb. Based on spectroscopic, kinetic, and electrochemical studies, we deduce that DOPA residue, when present within the distal pocket of mutant Mb, alone serves the role of His/Arg-pair of peroxidases.

Myoglobin Post-Translational Modifications Influence Color Stability of Beef Longissimus Lumborum

Post-translational modifications (PTM) of proteins play critical roles in biological processes. PTM of muscle proteins influence meat quality. Nonetheless, myoglobin (Mb) PTM and their impact on fresh beef color stability have not been characterized yet. Therefore, our objectives were to identify Mb PTM in beef longissimus lumborum muscle during postmortem aging and to characterize their influence on color stability. The longissimus lumborum muscles from 9 (n = 9) beef carcasses (24 h postmortem) were subjected to wet aging for 0, 7, 14, and 21 d. At the end of each wet-aging period, steaks were fabricated. One steak for analyses of PTM was immediately frozen at −80°C, whereas other steaks were assigned to refrigerated storage in the darkness under aerobic packaging. Instrumental color and biochemical attributes were evaluated on day 0, 3, or 6 of storage. Mb PTM were analyzed using two-dimensional electrophoresis and tandem mass spectrometry. Surface redness (a* value), color stability, and Mb concentration decreased (P &lt; 0.05) upon aging. Gel image analyses identified 6 Mb spots with similar molecular weight (17 kDa) but different isoelectric pH. Tandem mass spectrometry identified multiple PTM (phosphorylation, methylation, carboxymethylation, acetylation, and 4-hydroxynonenal alkylation) in these 6 isoforms. The amino acids susceptible to phosphorylation were serine (S), threonine (T), and tyrosine, whereas other PTM were detected in lysine (K), arginine (R), and histidine residues. Additionally, distal histidine (position 64), critical to heme stability, was found to be alkylated. Overall, Mb PTM increased with aging. The aging-induced PTM, especially those occurring close to hydrophobic heme pocket, could disrupt Mb tertiary structure, influence heme affinity, and compromise oxygen binding capacity, leading to decreased color stability of fresh beef. Furthermore, PTM at K45, K47, and K87 were unique to Mb from non-aged beef, whereas PTM at R31, T51, K96, K98, S121, R139, and K147 were unique to Mb from aged counterparts, indicating that these Mb PTM could be used as novel biomarkers for fresh beef color stability.

Regulation of the redox metabolome and thiol proteome by hydrogen sulfide

Overproduction of reactive oxygen species and compromised antioxidant defenses perturb intracellular redox homeostasis and is associated with a myriad of human diseases as well as with the natural process of aging. Hydrogen sulfide (H₂S), which is biosynthesized by organisms ranging from bacteria to man, influences a broad range of physiological functions. A highly touted molecular mechanism by which H₂S exerts its cellular effects is via post-translational modification of the thiol redox proteome, converting cysteine thiols to persulfides, in a process referred to as protein persulfidation. The physiological relevance of this modification in the context of specific signal transmission pathways remains to be rigorously established, while a general protective role for protein persulfidation against hyper-oxidation of the cysteine proteome is better supported. A second mechanism by which H₂S modulates redox homeostasis is via remodeling the redox metabolome, targeting the electron transfer chain and perturbing the major redox nodes i.e. CoQ/CoQH₂, NAD⁺/NADH and FAD/FADH₂. The metabolic changes that result from H₂S-induced redox changes fan out from the mitochondrion to other compartments. In this review, we discuss recent developments in elucidating the roles of H₂S and its oxidation products on redox homeostasis and its role in protecting the thiol proteome.

Hydrogen Sulfide-Clues from Evolution and Implication for Neonatal Respiratory Diseases

Reactive oxygen species (ROS) have been the focus of redox research in the realm of oxidative neonatal respiratory diseases such as bronchopulmonary dysplasia (BPD). Over the years, nitric oxide (NO) and carbon monoxide (CO) have been identified as important gaseous signaling molecules involved in modulating the redox homeostasis in the developing lung. While animal data targeting aspects of these redox pathways have been promising in treating and/or preventing experimental models of neonatal lung disease, none are particularly effective in human neonatal clinical trials. In recent years, hydrogen sulfide (H2S) has emerged as a novel gasotransmitter involved in a magnitude of cellular signaling pathways and functions. The importance of H2S signaling may lie in the fact that early life-forms evolved in a nearly anoxic, sulfur-rich environment and were dependent on H2S for energy. Recent studies have demonstrated an important role of H2S and its synthesizing enzymes in lung development, which normally takes place in a relatively hypoxic intrauterine environment. In this review, we look at clues from evolution and explore the important role that the H2S signaling pathway may play in oxidative neonatal respiratory diseases and discuss future opportunities to explore this phenomenon in the context of neonatal chronic lung disease.