Correspondence on "Fortification of FeS Clusters Reshapes Anaerobic CO Dehydrogenase into an Air-Viable Enzyme ThroughMultilayered Sealing of O2 Tunnels"

This correspondence challenges the findings of a recent study by reporting that Protein Film Electrochemistry reveals the A559W and A559W/V610H mutants of CO dehydrogenase II do not exhibit the claimed enhanced oxygen resistance compared to the wild-type enzyme.

The Gist

The Big Picture: A High-Performance Car That Breaks in the Rain

Imagine you have a super-fast, incredibly efficient car engine (the enzyme) that can turn a specific fuel (Carbon Monoxide) into energy. This engine is so good at its job that scientists want to put it into machines to help clean the air or generate electricity.

However, there is a huge problem: this engine is terrified of rain.

In the scientific world, the "rain" is Oxygen. As soon as a tiny bit of oxygen touches this enzyme, it stops working or gets damaged. This makes it useless for real-world devices that operate in normal air.

The Previous Claim: "We Built a Raincoat!"

Recently, a team of scientists (Kim and coworkers) published a paper claiming they had solved this problem. They said they took the enzyme and made tiny changes to its "air vents" (gas channels).

Think of the enzyme like a castle with a secret treasure room (the active site) deep inside. The treasure room is connected to the outside world by narrow tunnels.

• The Strategy: They blocked some of these tunnels with big boulders (mutating specific amino acids) to stop the "rain" (oxygen) from reaching the treasure.
• The Claim: They said these changes made the enzyme 300 times more resistant to oxygen. They claimed it was like giving the engine a waterproof raincoat, allowing it to run in the rain without breaking.

The New Investigation: "Let's Test the Raincoat"

The authors of this new paper (Opdam, Dobbek, and colleagues) decided to double-check those claims. They built the exact same "modified" enzymes and tested them using a very sensitive method called Protein Film Electrochemistry.

The Analogy of the Test:
Instead of just dipping the enzyme in a bucket of water (which is how the first team tested it), the new team used a high-speed, real-time stress test.

• They put the enzyme on a tiny electrode (like a test track).
• They ran it at full speed.
• Then, they blasted it with oxygen for a split second, like a sudden, heavy downpour.
• They watched exactly how fast the engine sputtered and died, and how long it took to recover.

The Shocking Result: The Raincoat Was a Fake

The new team found that the mutations did absolutely nothing to protect the enzyme from oxygen.

• The Claim: The modified enzymes should have survived the "rain" 300 times longer than the original.
• The Reality: The modified enzymes died just as fast as the original ones. In fact, they were almost identical in their sensitivity to oxygen.

It's as if the first team claimed to have built a submarine, but when the new team tested it, it sank just as fast as a regular boat.

Why Did This Happen? (The "Tunnel" Logic)

The authors explain why the first team's logic might have been flawed, using the castle analogy again:

1. The Tunnel System: The enzyme has a complex system of tunnels leading to the active site. It's not just one straight hallway; it's a maze with forks.
2. The Wrong Blockade: The first team blocked a tunnel that was too far away from the treasure room.
   • They blocked a spot 8.7 to 18.6 "steps" away from the center.
   • Imagine trying to stop water from flooding a basement by putting a brick in the driveway, while the water is actually pouring in through the front door.
3. The Result: Because they didn't block the final common path leading directly to the active site, the oxygen still found its way in easily. The "boulders" they placed were too far away to make a difference.

The Good News and The Bad News

• The Bad News: The specific mutations they tried (A559W and A559W/V610H) did not work. We still don't have an oxygen-proof version of this enzyme, and the "300x improvement" claim was incorrect.
• The Good News: The new team confirmed that the enzyme still works great at its main job (turning CO into CO2). The mutations didn't break the engine; they just failed to waterproof it.

The Takeaway

Science is a process of checking and re-checking.

• Team A said: "We fixed the oxygen problem!"
• Team B said: "We tested it with a more sensitive ruler, and you didn't actually fix it."

This paper serves as a crucial reality check. It tells us that simply blocking any part of the gas tunnel isn't enough. To make this enzyme air-resistant, we need to find the exact bottleneck right next to the active site and block that, rather than just putting up roadblocks further down the road. Until we find that perfect spot, this amazing enzyme will remain too fragile to use in open-air devices.

Technical Summary

1. Problem Statement

• Context: NiFe-containing Carbon Monoxide Dehydrogenases (CODHs) are highly efficient enzymes capable of reversibly oxidizing CO to CO₂. They are promising candidates for biotechnological applications (e.g., bioelectrolyzers, biosensors) due to their high catalytic rates and energetic efficiency.
• The Bottleneck: A major barrier to their practical application is their extreme sensitivity to molecular oxygen ($O_2$), which inactivates the enzyme.
• The Contested Claim: A recent study by Kim et al. (Angew. Chem.) claimed that introducing bulky residues (specifically A559W and the double mutant A559W/V610H) into the gas channels of Carboxydothermus hydrogenoformans CODH II (CODH-IICh) significantly increased oxygen resistance. They reported a >100-fold increase in the half-maximal inhibitory concentration ($IC_{50}$) for $O_2$ with minimal impact on CO affinity ($K_M$).
• The Discrepancy: The authors of this correspondence (Opdam et al.) sought to reproduce these findings to validate the mechanism of "multilayered sealing" of $O_2$ tunnels. Their preliminary data suggested a contradiction to Kim et al.'s claims.

2. Methodology

The authors employed a rigorous comparative approach combining structural biology and advanced electrochemistry:

• Protein Production & Validation: They expressed and purified the Wild Type (WT) CODH-IICh and the two mutants (A559W and A559W/V610H). They confirmed the successful introduction of mutations via X-ray crystallography (PDB codes 9TOX and 9TPO).
• Primary Technique: Protein Film Electrochemistry (PFE):
  • Unlike the solution assays used by Kim et al., the authors used PFE, where enzymes are immobilized on an electrode for direct electron transfer.
  • Advantages: PFE offers high time resolution (0.1 s), rapid homogenization of injected gases, and the ability to monitor activity continuously over time.
  • Protocol: The team performed transient exposure experiments. They injected $CO$ to establish baseline activity, followed by a transient injection of $O_2$ (30–50 s). They monitored the current decay (inactivation) and subsequent recovery upon $O_2$ removal and reductive poising.
• Kinetic Analysis:
  • Determined Michaelis constants ($K_M$) for CO by exposing enzyme films to varying CO concentrations.
  • Determined $IC_{50}$ values for $O_2$ inhibition at three distinct time points:
    1. $K_i^{imm}$: Immediately after $O_2$ injection.
    2. $K_i^{dep}$: After $O_2$ has departed the cell.
    3. $K_i^{red}$: After a reductive poise (to test for reactivation).
  • Used Nitratodesulfovibrio vulgaris (Nv) CODH as a positive control, known for its distinct reactivation behavior.

3. Key Contributions

• Direct Reproduction and Challenge: The authors successfully reproduced the mutants and provided a direct, high-resolution counter-analysis to the claims made by Kim et al.
• Methodological Superiority: They demonstrated that PFE is a more sensitive and insightful tool than solution assays for studying gas diffusion and enzyme inactivation kinetics, particularly for distinguishing between immediate inactivation and recovery mechanisms.
• Structural Contextualization: They integrated structural data to explain why the specific mutations (A559W, V610H) are unlikely to block $O_2$ diffusion effectively, given their distance from the active site and the branching nature of the gas channels.

4. Results

• CO Affinity ($K_M$): The mutations caused a moderate increase in $K_M$ for CO (WT: 4.5 µM; A559W: 7.7 µM; A559W/V610H: 11 µM). This trend aligns with Kim et al.'s data, confirming the mutations were successfully introduced and affect CO diffusion slightly.
• Oxygen Resistance ($IC_{50}$):
  • Contradiction: Contrary to Kim et al.'s report of a 50–300-fold increase in $O_2$ resistance, the authors found no significant difference in $IC_{50}$ values between the WT and the mutants.
  • Data: The $IC_{50}$ values for immediate inactivation ($K_i^{imm}$) were identical for all variants (~0.03 µM). Similarly, values for post-departure ($K_i^{dep}$) and post-reduction ($K_i^{red}$) showed no improvement in the mutants compared to WT.
  • Control: The positive control (Nv CODH) behaved as expected, showing significant reactivation after reduction, validating the experimental setup.
• Structural Analysis: Crystal structures revealed that residue A559 is ~8.7 Å from the active site Ni, and V610 is ~18.6 Å away (at the surface). Both are located before the final common branch of the gas channel leading to the active site. Therefore, blocking these positions does not effectively seal the final tunnel against $O_2$ diffusion.

5. Significance

• Correction of Literature: This correspondence serves as a critical correction to the field, indicating that the specific A559W/V610H mutations do not confer the claimed "air-viable" properties to CODH-IICh.
• Implications for Biotechnology: Researchers aiming to engineer oxygen-tolerant CODHs for industrial applications cannot rely on these specific mutations. The strategy of "sealing" gas channels must be re-evaluated, likely requiring mutations much closer to the active site to effectively block the final diffusion path.
• Validation of PFE: The study reinforces Protein Film Electrochemistry as the gold standard for characterizing the reactivity of redox enzymes with gaseous substrates and inhibitors, highlighting its ability to detect subtle kinetic differences that solution assays might miss or misinterpret.
• Mechanistic Insight: The results suggest that in CODH-IICh, $O_2$ diffusion is not rate-limiting at the mutation sites used, and the enzyme's sensitivity to $O_2$ is governed by factors other than simple steric blocking of the surface-accessible channel residues.