Self-Sufficient Maturation and Catalysis of a Clade E CODH Encoded in a CooCTJ-Operon from Clostridium pasteurianum BC1

This study reports the heterologous production and characterization of a self-sufficient Clade E carbon monoxide dehydrogenase (CpBC1CODH-III) from *Clostridium pasteurianum* BC1, demonstrating that while its co-expression with the CooCTJ maturation machinery stabilizes the enzyme, its catalytic activity is primarily limited by nickel availability rather than maturation efficiency.

The Gist

Imagine a tiny, microscopic factory inside a bacterium. This factory's job is to take in Carbon Monoxide (CO)—a toxic gas—and turn it into useful energy or, conversely, take Carbon Dioxide (CO₂)—the gas we exhale—and turn it back into fuel. The machine that does this heavy lifting is called CODH (Carbon Monoxide Dehydrogenase).

Think of CODH as a high-performance race car engine. It's incredibly complex, made of metal parts (iron and nickel) that must be assembled perfectly for the engine to run.

The Mystery of the "Missing Mechanic"

Usually, building these engines requires a team of specialized mechanics (called maturases: CooC, CooT, and CooJ). These mechanics are like a pit crew that installs the spark plugs, tightens the bolts, and ensures the right amount of fuel (nickel) gets into the engine. Without them, the engine often just sits there, broken and useless.

Scientists have studied two main types of these engines:

• Clade F Engines: These always come with their own pit crew in the factory blueprint. They are picky; without the crew, they don't work.
• Clade E Engines: These are usually found in different factories and often don't have their pit crew listed in the blueprint. They are supposed to be self-sufficient, but scientists weren't sure how well they could build themselves.

The Surprise Discovery

The researchers in this paper found a weird case in a bacterium called Clostridium pasteurianum. They discovered a Clade E engine (which usually works alone) that was strangely packaged inside a Clade F blueprint (which usually includes the pit crew).

It was like finding a self-driving car that came with a manual transmission and a full set of tools in the trunk, even though the car was designed to drive itself.

The Big Question: Does this engine need the pit crew (the maturases) to work, or is it strong enough to build itself?

The Experiment: Building the Engine in a Lab

The scientists decided to build this engine in a test-tube lab (using E. coli bacteria) to see what happened. They tried two scenarios:

1. The Solo Build: They built the engine without the pit crew.
2. The Team Build: They built the engine with the pit crew.

The Results:

• The Solo Build: Surprisingly, the engine worked! It could still turn CO into energy, though it was a bit sluggish at first. It turned out the engine was "self-sufficient." It could assemble its own metal parts, but it needed a little help getting enough nickel (the fuel) to run at full speed.
• The Team Build: When they added the pit crew, the engine didn't get faster at its maximum speed, but it became much more stable. The crew acted like a safety net, making sure the engine was built consistently every time, regardless of how much nickel was available in the environment.

The "EPR" Crystal Ball

To see inside the engine while it was running, the scientists used a special tool called EPR spectroscopy. Think of this as a magical X-ray that can see the invisible "mood" of the metal parts.

• They saw that the engine's metal core was healthy and ready to work.
• They even saw the engine grabbing onto CO₂, proving it was doing its job correctly.

The "Pit Crew" Comparison

The team also compared this unique engine to a famous, well-studied engine from a different bacterium (Rhodospirillum rubrum).

• The Engine Block (CODH): The main body of the engine was almost identical between the two.
• The Mechanics (Maturases): Here's where it got interesting. The "mechanic" named CooJ looked very different between the two bacteria, even though they did the same job. It's like comparing a Swiss Army knife to a specialized wrench; they look different, but they both tighten the same bolt. The scientists used a computer AI (AlphaFold3) to predict their 3D shapes and found that despite looking different on paper, their "hands" (the parts that hold the metal) were shaped very similarly.

The Takeaway: Why This Matters

This discovery changes how we think about these biological machines.

1. Resilience: This specific engine (Clade E) is tougher than we thought. It doesn't need its pit crew to function; it just uses them to make sure things run smoothly and consistently.
2. Evolution: It suggests that nature might have added the "pit crew" to this engine later in history, not because the engine was broken, but to help it handle different environments (like low nickel levels).
3. Future Tech: Understanding that these engines can build themselves is huge for biotechnology. If we want to use these bacteria to clean up toxic gases or create green fuels, we don't always need to engineer the complex "pit crew" into our factories. We can just give the engine the nickel it needs, and it will do the rest.

In short: The scientists found a "self-driving" car that came with a mechanic. They discovered the car can drive itself just fine, but the mechanic helps it stay reliable when the road gets bumpy. This gives us a new blueprint for building better, more robust bio-machines for a greener future.

Technical Summary

1. Problem Statement

Carbon monoxide dehydrogenases (CODHs) are nickel-iron metalloenzymes essential for microbial CO metabolism and CO₂ fixation. While CODHs are generally classified into phylogenetic clades (A–G) with distinct genomic organizations, the relationship between enzyme activity and the presence of associated maturation proteins (maturases: CooC, CooT, CooJ) remains complex.

• The Gap: Most Clade E CODHs are encoded with maturases, yet some (like Carboxydothermus hydrogenoformans CODH-II) function without them. Conversely, Clade F CODHs (e.g., Rhodospirillum rubrum) typically require co-expression of CooCTJ for activity.
• The Specific Anomaly: The genome of Clostridium pasteurianum BC1 contains four CODHs, including a unique Clade E CODH (CpBC1CODH-III) encoded within an operon structure typical of Clade F (containing CooC, CooT, and CooJ).
• Research Question: Does this Clade E enzyme require its Clade F-associated maturases for assembly and catalytic activity, or does it possess intrinsic maturation capabilities? How do the maturases influence production stability and activity compared to the well-studied R. rubrum CODH (RrCODH)?

2. Methodology

The study employed a combination of heterologous expression, biochemical characterization, spectroscopic analysis, and comparative bioinformatics.

• Heterologous Expression:

  • CpBC1CODH-III was expressed in E. coli BL21(DE3) $\Delta$iscR.
  • Dual-Vector System: The CODH gene was cloned into pET-11a, while the maturase genes (cooCTJ) were cloned into pCDFDuet-1.
  • Conditions: Cultures were grown under anaerobic conditions with supplementation of NiCl₂, L-cysteine, and iron-sulfur precursors. Induction was performed with IPTG.
  • Controls: Experiments compared expression with and without maturases, and varying concentrations of NiCl₂ and IPTG.

• Purification and Reconstitution:

  • Proteins were purified via Strep-tag II affinity chromatography.
  • Fe-S Cluster Reconstitution: Purified enzymes (initially showing low iron content) were reconstituted in vitro using cysteine desulphurase (CsdA), L-cysteine, and ferrous ammonium citrate to restore [4Fe-4S] clusters.

• Activity Assays:

  • CO Oxidation: Measured via methyl viologen (MV) reduction at 604 nm.
  • CO₂ Reduction: Measured using hemoglobin as a reporter for CO formation, MV as an electron shuttle, and sodium dithionite as a donor, monitored at 429 nm.
  • High-Throughput Screening: A 96-well plate assay was used to screen crude lysates for CO₂ reduction activity under varying induction and nickel conditions.

• Spectroscopy:

  • Electron Paramagnetic Resonance (EPR): Performed at 5 K on samples treated with CO, CO₂, and sodium dithionite to identify redox states of metal clusters (B-clusters and C-clusters).

• Bioinformatics & Structural Modeling:

  • Comparative analysis of CpBC1CODH-III operon against RrCODH (Clade F) and Shewanella fodinae CODH (SfCODH).
  • AlphaFold3 (AF3): Used to generate structural models for CODH and maturases (CooC, CooT, CooJ).
  • Dot Plot Analysis: Used to identify conserved sequence regions and metal-binding motifs.

3. Key Results

• Catalytic Activity and Metal Content:

  • Initial State: Purified CpBC1CODH-III showed moderate activity (7.7 U/mg CO oxidation) and sub-stoichiometric metal content (0.7 Ni/monomer, 4.5 Fe/monomer).
  • After Reconstitution: Activity increased significantly (149.7 U/mg for CO oxidation; 0.568 U/mg for CO₂ reduction). Iron content reached 10 Fe/monomer, confirming successful Fe-S cluster reconstitution.
  • Maturase Impact: Co-expression of CooCTJ stabilized production but did not significantly increase the maximum specific activity compared to expression without maturases. The enzyme was catalytically active even without maturases.

• Role of Nickel and Maturases:

  • In crude lysate assays, nickel availability was the primary driver of activity, not the presence of maturases.
  • Maturases (CooCTJ) acted as stabilizers, ensuring consistent enzyme production levels across different induction strengths (IPTG concentrations), whereas expression without maturases was more variable.
  • This contrasts with RrCODH, which strictly requires maturases and high nickel for maximum activity.

• EPR Spectroscopy:

  • B-Clusters: Showed a characteristic rhombic signal at $g \approx 2.0$ (reduced state).
  • C-Clusters: Upon CO incubation, distinct signals appeared at $g \approx 1.75$ (Cred1, HO-bound) and $g \approx 1.72$ (Cred2, CO₂-bound), confirming the formation of a functional active site capable of substrate binding.
  • CO₂ incubation alone did not alter the signal significantly, suggesting a sluggish reaction rate without a strong driving force.

• Comparative Genomics and Structural Analysis:

  • CODH and CooC: High structural similarity (low RMSD) between CpBC1 and RrCODH/CooC.
  • CooT: Sequence variation was high, but the overall fold and the critical N-terminal Cys2 nickel-binding residue were conserved.
  • CooJ: Showed the lowest sequence similarity but high structural similarity (RMSD 0.96) when the disordered histidine-rich region (HRR) was excluded.
  • Conserved Motifs: A metal-binding motif (-HWXXHXXXH-) was identified in CooJ, consistent with known metal chaperone functions. The HRRs were found at different termini (N-terminal in CpBC1/Sf, C-terminal in Rr), suggesting functional convergence despite positional divergence.

4. Key Contributions

1. Discovery of a Hybrid Operon: Identification of a Clade E CODH (CpBC1CODH-III) encoded within a Clade F-type operon (CooCTJ), expanding the known genomic diversity of CODHs.
2. Intrinsic Robustness: Demonstration that CpBC1CODH-III is self-sufficient for maturation and catalysis. Unlike RrCODH, it does not strictly require maturases for activity, challenging the dogma that Clade E enzymes encoded with maturases are dependent on them.
3. Re-evaluation of Maturase Function: The study suggests that for CpBC1CODH-III, maturases primarily function to buffer nickel homeostasis and stabilize expression levels rather than being an absolute requirement for cofactor assembly.
4. Structural Insights: AlphaFold3 modeling and dot plot analysis revealed that while CooJ sequences diverge significantly, their core metal-binding structures and functional motifs remain conserved, highlighting the flexibility of maturase evolution.

5. Significance

• Evolutionary Insight: The findings suggest a unique evolutionary trajectory where a Clade E enzyme acquired Clade F maturases not for essential assembly, but likely to optimize nickel handling and expression stability in its native host (C. pasteurianum).
• Biotechnological Potential: CpBC1CODH-III represents a robust candidate for biotechnological applications (e.g., CO conversion or CO₂ fixation) because it can be produced heterologously without the complex requirement of co-expressing specific maturases, simplifying production protocols.
• Paradigm Shift: This work challenges the traditional view of maturase dependency, indicating that the presence of maturase genes in an operon does not automatically imply an absolute requirement for the enzyme's function, but rather a regulatory or stabilizing role.