Sulfur Reactivity: A New Perspective on Redox and Microbial Roles

Sulfur plays a crucial role in biological systems, influencing cellular function through redox reactions and metabolic pathways. Traditionally associated with structural roles in proteins and cofactors, sulfur is now recognized for its dynamic chemical reactivity, particularly in reactive sulfur species (RSS). These compounds contribute to signaling processes, oxidative stress responses, and microbial metabolism.

Understanding RSS formation and interactions within cells offers new insights into redox regulation and microbial physiology.

Formation Of Reactive Sulfur Species

RSS arise through biochemical and environmental processes, often from the metabolism of sulfur-containing compounds like cysteine, homocysteine, and glutathione. Enzymes such as cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS), and 3-mercaptopyruvate sulfurtransferase (3-MST) convert these amino acids into reactive intermediates, forming sulfane sulfur, persulfides, and polysulfides. Their activity is tightly regulated by cellular redox states to align RSS production with metabolic demands and oxidative stress conditions.

Non-enzymatic mechanisms also contribute to RSS formation, particularly through interactions with reactive oxygen species (ROS) and reactive nitrogen species (RNS). Hydrogen sulfide (H₂S), a key signaling molecule, undergoes oxidation in the presence of oxygen or metal catalysts, generating sulfane sulfur species like thiosulfate and persulfides. Additionally, H₂S reacts with superoxide or peroxynitrite, producing sulfite and sulfate. These transformations are influenced by pH, metal ion availability, and thiol-containing biomolecules, which shape RSS stability and reactivity in cells.

Mitochondria play a major role in RSS production due to their involvement in sulfur metabolism and redox homeostasis. The mitochondrial enzyme sulfide:quinone oxidoreductase (SQR) oxidizes H₂S, producing persulfides that can be further processed into thiosulfate or elemental sulfur. This pathway regulates intracellular sulfide levels and integrates RSS into broader metabolic networks, linking sulfur redox chemistry with energy production and antioxidant defense. The connection between mitochondrial RSS formation and electron transport chain activity highlights the role of sulfur metabolism in cellular adaptation to oxidative stress.

Types Of Reactive Sulfur Species

RSS include various sulfur-containing compounds with distinct chemical properties and biological roles. Among them, sulfane sulfur, sulfides, polysulfides, and persulfides are key categories.

Sulfane Sulfur

Sulfane sulfur consists of sulfur atoms in a zero or negative oxidation state, typically bound to another sulfur atom in persulfides (R-S-SH) or polysulfides (R-S-Sn-R). These highly reactive species act as intermediates in sulfur transfer reactions. A primary source of sulfane sulfur is the oxidation of H₂S by SQR in mitochondria, producing persulfides that can be further processed into thiosulfate or elemental sulfur.

Sulfane sulfur is involved in post-translational modifications, such as persulfidation, which alters protein function by modifying cysteine residues. This modification regulates enzymatic activity and redox homeostasis. Additionally, sulfane sulfur species contribute to biosynthetic pathways, aiding in the formation of iron-sulfur clusters and essential cofactors.

Sulfides

Sulfides, including H₂S and its anionic forms (HS⁻ and S²⁻), play key roles in cellular signaling and redox balance. Endogenously produced by enzymes like CSE and 3-MST, H₂S interacts with metal centers and thiol-containing biomolecules. Its reactivity depends on pH and oxidizing agents, leading to the formation of other RSS, such as polysulfides and persulfides.

In aqueous environments, H₂S exists in equilibrium with its deprotonated forms, with their relative abundance depending on physiological pH. Sulfides also participate in electron transfer reactions, particularly in mitochondria, where they serve as substrates for the electron transport chain, linking sulfur metabolism to cellular bioenergetics.

Polysulfides

Polysulfides (R-S-Sn-R) contain linear chains of sulfur atoms and exhibit unique redox properties that enable them to participate in electron transfer and thiol modification reactions. These compounds arise from H₂S oxidation or sulfurtransferase activity.

Polysulfides modulate protein function through persulfidation, converting cysteine thiols (-SH) into persulfides (-SSH), which alters protein reactivity and stability. The length of the sulfur chain influences their chemical behavior, with longer chains being more reactive. In biological systems, polysulfides serve as reservoirs of reactive sulfur, releasing sulfane sulfur under specific conditions. Their interactions with metal centers, such as iron-sulfur clusters, expand their role in enzymatic catalysis and electron transport.

Persulfides

Persulfides (R-S-SH) contain a sulfane sulfur (-SSH) moiety, making them highly reactive intermediates in sulfur metabolism. Formed through thiol oxidation or sulfurtransferase activity, persulfides modulate protein cysteine residues, protecting against oxidative damage by preventing irreversible oxidation. Their reversible nature allows dynamic regulation of protein activity, influencing enzyme function and transcriptional control.

Persulfides also act as precursors for other RSS, including polysulfides and thiosulfates, connecting them to broader sulfur redox networks. Their interactions with reactive oxygen and nitrogen species underscore their role in maintaining cellular redox balance.

Roles In Redox Regulation

RSS regulate redox homeostasis by modifying thiol-based redox switches and protecting against oxidative damage. Their ability to alter cysteine residues in proteins through persulfidation affects enzymatic activity, structural integrity, and signal transduction. Unlike reactive oxygen species (ROS), which often cause oxidative stress, RSS can act as both pro-oxidants and antioxidants, depending on their concentration and cellular context.

RSS interact closely with glutathione (GSH), a central thiol-based antioxidant. Persulfides and polysulfides enhance GSH-dependent detoxification pathways by increasing the thiol pool for redox buffering. This interaction extends to glutathione peroxidases, which neutralize hydrogen peroxide and lipid peroxides. Additionally, H₂S oxidation generates sulfane sulfur species that participate in reversible redox cycling, strengthening defenses against oxidative stress.

RSS also modulate mitochondrial function, particularly within the electron transport chain. Sulfide oxidation pathways, such as those catalyzed by SQR, integrate sulfur metabolism with ATP production, influencing cellular energy dynamics. Acting as alternative electron donors, RSS support bioenergetic efficiency under low oxygen conditions, highlighting their interconnected roles in sulfur metabolism and oxidative stress responses.

Interplay With Nitric Oxide And Other Signaling Molecules

RSS and nitric oxide (NO) share overlapping roles in cellular signaling, often interacting to regulate redox balance and enzymatic activity. Both modify cysteine residues in proteins—NO through S-nitrosation and RSS via persulfidation. This competition influences redox-sensitive enzymes and transcription factors, shaping cellular responses to oxidative stress and metabolic shifts.

One key interaction between NO and RSS involves nitrosopersulfide (SSNO⁻), a hybrid signaling molecule with prolonged bioactivity compared to NO alone. SSNO⁻ regulates vasodilation and mitochondrial respiration and influences cyclic GMP (cGMP) signaling, which is crucial for vascular homeostasis and neurotransmission. RSS also modulate NO bioavailability by interacting with nitric oxide synthase (NOS) or modifying NOS cofactors like tetrahydrobiopterin.

Relevance In Microbial Metabolism

Microorganisms integrate RSS into energy production, detoxification, and regulatory networks. Many bacteria and archaea use sulfur compounds as electron donors or acceptors in anaerobic respiration, linking RSS to bioenergetic processes. Sulfate-reducing bacteria (SRB) reduce sulfate to H₂S via dissimilatory sulfate reduction, playing a major role in global sulfur cycling. This allows them to thrive in anoxic environments such as deep-sea vents, sediments, and the human gut. Sulfur-oxidizing bacteria (SOB) complement this process by oxidizing H₂S and other reduced sulfur compounds into sulfate, maintaining ecological balance.

Beyond metabolism, RSS influence microbial stress responses and signaling. Many bacteria produce persulfides and polysulfides to counter oxidative stress, using their reactivity to neutralize ROS. In Escherichia coli, the sulfurtransferase enzyme TusA facilitates persulfidation of key proteins, enhancing resistance to oxidative damage. RSS also participate in bacterial quorum sensing, modulating gene expression based on population density. Some pathogens exploit RSS to evade immune defenses, as seen in Helicobacter pylori, which uses H₂S-derived modifications to protect its proteins from host oxidative attacks.

The diverse roles of RSS in microbial metabolism highlight their significance in environmental and host-associated microbial communities, shaping interactions that influence health, disease, and ecosystem stability.