Self-S-sulfonation in a bacterial persulfide dioxygenase mediates thiol persulfide detoxification

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This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a tiny, microscopic city inside a bacterium called Staphylococcus aureus. This city has a very specific problem: it lives in environments (like the human gut or wastewater) that are full of Hydrogen Sulfide (H₂S).

To us, H₂S smells like rotten eggs. To the bacterium, it's a double-edged sword. A little bit is helpful for energy and defense, but too much is a deadly poison that shuts down the city's power plants.

To survive, the bacteria need a specialized "waste management team." This paper introduces the star player of that team: an enzyme called CstB.

Here is the story of how CstB works, explained through a few creative analogies.

1. The Problem: Toxic "Rotten Egg" Waste

When the bacteria encounter too much H₂S, they convert it into a slightly less toxic but still dangerous form called a persulfide (think of it as a "sticky, sulfur-loaded garbage bag"). If these bags pile up, the city gets poisoned.

Most bacteria have a standard trash collector (an enzyme called PDO) that takes this garbage, chops off the sulfur, and dumps it out as sulfite (a liquid waste). But S. aureus is special. It doesn't just want to dump the waste; it wants to turn it into something useful and safe: Thiosulfate (a solid, stable salt).

2. The Machine: A Two-Part Robot

The CstB enzyme is like a two-part robot fused together:

The Head (PDO Domain): This part contains a metal center (Iron) that acts like a high-powered drill. It's designed to break chemical bonds using oxygen.
The Body (Rhodanese Domain): This part is a "transfer station" designed to catch things and move them around.

Usually, these two parts work separately in nature. But in S. aureus, they are welded together into one super-machine.

3. The Secret Weapon: The "Molecular Shuttle"

The most exciting discovery in this paper is how CstB moves the "garbage" from the Head to the Body without dropping it.

Imagine the Head of the robot has a magnetic arm (a flexible loop of protein) sticking out. At the end of this arm is a special hook (a cysteine amino acid, called C201).

Step 1: The Pickup. The "sticky garbage bag" (persulfide) from the environment doesn't go directly into the drill. Instead, it jumps onto the magnetic arm (C201). The arm acts like a glutathione mimic—basically, the enzyme tricks itself into thinking the arm is the garbage bag.
Step 2: The Transformation. Once the garbage is on the arm, the Iron "drill" spins up. It uses oxygen and water to chemically transform the garbage. Instead of chopping it off and releasing it as a liquid (sulfite), it turns the garbage into a charged, sticky note (an S-sulfonate) that stays firmly attached to the arm.
Step 3: The Shuttle. This is the magic part. The arm swings the sticky note 27 Angstroms (a tiny distance, but huge for a molecule) across the robot to the Body.
Step 4: The Drop-Off. The Body has a receiving dock (another hook called C408). Because the sticky note is negatively charged and the dock is positively charged, they snap together like magnets (electrostatic steering). The Body grabs the note, and poof! The two sulfur atoms combine to form Thiosulfate, which is safe and stable.

4. Why This is a Big Deal

In other bacteria (and even in humans), this type of enzyme usually acts like a guillotine: it chops the sulfur off and lets it fall away as sulfite.

But CstB is like a conveyor belt. It keeps the sulfur attached to the machine the whole time, shuttling it from the "processing plant" to the "packaging plant" inside the same enzyme. This prevents the toxic intermediate from leaking out and hurting the cell.

5. The "What If" Experiments

The scientists proved this by breaking the machine:

Cutting the arm (C201A mutant): The robot can't pick up the garbage. Nothing happens.
Blocking the dock (C408A mutant): The robot picks up the garbage and transforms it, but the sticky note gets stuck on the arm. The machine jams, and the process stops.
Removing the magnets (R370A/R413A mutants): The sticky note can't find the dock. It floats away, and the machine fails.

The Takeaway

This paper reveals a clever evolutionary trick. Staphylococcus aureus lives in sulfur-rich, dangerous environments. To survive, it evolved a "self-sulfonating" enzyme that acts like a molecular shuttle bus.

Instead of just dumping toxic waste, this bus picks up the danger, transforms it on board, and delivers it safely to a storage unit, turning a poison into a harmless salt. This allows the bacteria to not only survive high levels of sulfide but potentially use it to its advantage, which might even explain why these bacteria are so good at surviving in hospitals and wastewater systems where antibiotic resistance is a major problem.

In short: CstB is a master of "containment and conversion," turning a deadly gas into a safe solid by keeping the reaction ingredients glued to its own moving parts.

1. Problem Statement

Hydrogen sulfide ( $H_2S$ ) is a vital signaling molecule but is toxic at high concentrations, inhibiting respiratory chains. Bacteria like Staphylococcus aureus must detoxify reactive sulfur species (RSS) such as thiol persulfides (RSSH).

Canonical Mechanism: In humans and many other organisms, Persulfide Dioxygenases (PDOs) like ETHE1 oxidize glutathione persulfide (GSSH) to release sulfite ( $SO_3^{2-}$ ) and regenerate glutathione (GSH).
The Anomaly: S. aureus possesses a unique fusion protein, CstB, which combines a PDO domain with a sulfurtransferase (rhodanese) domain. Unlike canonical PDOs, CstB converts two equivalents of RSSH into thiosulfate ( $S_2O_3^{2-}$ ) without releasing free sulfite. The structural and mechanistic basis for this distinct reaction pathway, particularly how the enzyme handles the intermediate and shuttles it between domains separated by ~27 Å, was previously unknown.

2. Methodology

The authors employed an interdisciplinary approach combining structural biology, spectroscopy, mass spectrometry, and enzymatic kinetics:

Crystallography: Solved six crystal structures of wild-type (WT) and mutant CstB (C201A, C408A, C201S, C408S, C201S/C408S) to resolutions between 1.91 Å and 3.11 Å. Structures were solved with and without reductants to capture different oxidation states.
Spectroscopy: Used Fe K-edge X-ray Absorption Spectroscopy (XAS) and Extended X-ray Absorption Fine Structure (EXAFS) to determine the coordination environment of the Fe(II) center in the C408A mutant.
Enzymatic Assays: Measured $O_2$ consumption rates using an oxygraph with various persulfide substrates (GSSH, CSSH) and mutants.
Mass Spectrometry (MS/MS):
- Intact Mass: Analyzed C408A mutants under aerobic/anaerobic conditions with Fe(II) or Zn(II) to detect post-translational modifications (persulfidation, sulfonation).
- Peptide Mapping: Used trypsin digestion and LC-MS/MS to pinpoint the exact location of chemical modifications.
- Product Analysis: Utilized $^{18}O$ -labeled water and monobromobimane (mBBr) derivatization followed by LC-MS to track oxygen incorporation into the final thiosulfate product.
Mutagenesis: Generated point mutations (C201A, C408A, R370A, R413A, G198A) to probe the roles of specific residues in catalysis and inter-domain communication.

3. Key Contributions & Results

A. Structural Insights: The "Glutathione Mimic" Loop

CstB forms a dimer-of-dimers tetramer. The PDO domain contains a non-heme Fe(II) center coordinated by a His2Asp triad (H56, H119, D145).
A unique, dynamic loop (residues 192–213) harboring C201-G202 is visible in the crystal structures.
Key Finding: The C201 sulfur atom is positioned 3.4 Å from the Fe(II) center but is not directly coordinated to it. This loop acts as a "glutathione mimic," positioning C201 where canonical PDOs bind the substrate GSSH.
The C-terminal Rhod domain (containing C408) is ~27 Å away from the PDO active site. The interface between domains is dynamic, but a positively charged "wall" (R370, R413) surrounds C408, suggesting an electrostatic attraction for negatively charged intermediates.

B. Mechanism of Self-S-Sulfonation

Substrate Promiscuity: Unlike ETHE1, which strictly prefers GSSH, CstB shows no preference between GSSH and CSSH, confirming that the external RSSH does not bind the Fe site directly. Instead, RSSH transfers its persulfide group to C201.
The Reaction: Persulfidated C201 undergoes self-S-sulfonation in an Fe(II)- and $O_2$ $O_{2}$ -dependent manner.
- Unlike canonical PDOs that release sulfite, CstB retains the oxidized sulfur as an S-sulfonate adduct covalently bound to C201.
- MS/MS confirmed the presence of an S-sulfonate ( $-SO_3^-$ ) on C201 in the C408A mutant under aerobic conditions.
- The reaction requires two electrons; since Fe(II) cannot be reduced to Fe(0), the authors propose the formation of a two-electron reduced noncanonical sulfonate intermediate.

C. Inter-Domain Shuttling and Thiosulfate Formation

Electrostatic Steering: The negatively charged S-sulfonated C201 is "shuttled" to the Rhod domain. Mutations R370A and R413A (disrupting the positive charge patch near C408) abolished activity, confirming electrostatic steering is critical.
Nucleophilic Attack: The S-sulfonate on C201 is attacked by the persulfide on C408 (in the Rhod domain). This reaction releases thiosulfate ( $S_2O_3^{2-}$ ) as the sole product.
Coupling: The C201A and C408A mutants are completely inactive in $O_2$ consumption, proving that the two active sites are tightly coupled; the reaction stalls if the intermediate cannot be transferred to C408.
Loop Flexibility: The G198A mutant (disrupting a specific glycine angle in the loop) showed significantly reduced activity, indicating the loop's flexibility is essential for the "molecular shuttle" mechanism.

D. Oxygen Isotope Tracing

Product analysis using $^{18}O$ -water revealed that the oxygen atoms in the final thiosulfate product come from both water and molecular oxygen.
Specifically, the mechanism incorporates one oxygen from water and two from $O_2$ (consistent with the proposed Eq 1 in the paper), distinguishing it from sulfite-releasing PDOs.

4. Significance

Novel Catalytic Mechanism: This study identifies the first example of an enzyme performing self-S-sulfonation followed by inter-domain transfer. It challenges the canonical view of PDOs which release sulfite.
"Molecular Shuttle" Concept: The C201 loop functions as a dynamic "swinging arm" (similar to Fe-S cluster assembly proteins or CoA persulfide reductases) but uniquely transports a covalently bound sulfonate moiety rather than just a persulfide.
Biological Adaptation: By preventing the release of free sulfite (which can disrupt thiol homeostasis and antioxidant defenses), CstB allows S. aureus to safely detoxify diverse persulfides in sulfide-rich environments (like the gut). This mechanism likely contributes to the bacterium's resilience and the spread of antibiotic resistance in wastewater environments.
Therapeutic Potential: Understanding this unique detoxification pathway in S. aureus (especially in methicillin-resistant strains where the cst operon is duplicated) could inform new strategies to disrupt bacterial sulfur homeostasis and combat antibiotic resistance.

Conclusion

The paper elucidates a sophisticated, two-step detoxification mechanism in S. aureus CstB. The enzyme utilizes a dynamic loop to capture persulfides, oxidizes them into a stable S-sulfonate intermediate via a non-canonical Fe(II)/ $O_2$ chemistry, and electrostatically steers this intermediate to a distal active site for thiosulfate production. This "self-sulfonation and shuttling" mechanism represents a unique evolutionary solution to managing reactive sulfur species without generating toxic sulfite byproducts.