The anaerobic fungus Caecomyces churrovis produces H2 via a non-bifurcating NADH-dependent enzyme complex

This study reveals that the anaerobic fungus *Caecomyces churrovis* produces hydrogen via a newly identified non-bifurcating NADH-dependent enzyme complex formed by [FeFe] hydrogenase and NADH dehydrogenase subunits, a mechanism distinct from the ferredoxin-dependent pathways previously assumed and likely shared across anaerobic fungi.

The Gist

The Big Picture: The Fungal Power Plant

Imagine a tiny, single-celled fungus living inside a cow's stomach. This fungus, called Caecomyces churrovis, lives in a world with absolutely no oxygen. To survive, it needs to break down food (like grass) to get energy.

In our own bodies, when we eat, our cells use oxygen to burn fuel and make energy. But since this fungus has no oxygen, it has to use a different strategy. It has a special "power plant" inside its cell called a hydrogenosome. Think of this organelle as a tiny, anaerobic battery factory.

The main job of this factory is to turn food into energy (ATP) and, as a byproduct, release Hydrogen gas (H₂). This hydrogen gas is crucial because it helps the fungus get rid of excess electrons (like taking out the trash) so the factory can keep running.

The Mystery: How is the Hydrogen Made?

For over 40 years, scientists have been puzzled by how these fungi make that hydrogen gas.

The Old Theory (The Relay Race):
Scientists thought the fungus used a "relay race" system.

1. First, an enzyme would take electrons from food and pass them to a small carrier molecule called Ferredoxin (let's call this the "Red Ball").
2. Then, a hydrogen-making machine (Hydrogenase) would grab that "Red Ball" and use its energy to create Hydrogen gas.
   Analogy: Imagine a runner passing a baton to a second runner, who then crosses the finish line.

The New Discovery (The Direct Line):
This paper reveals that Caecomyces churrovis doesn't use a relay race at all. Instead, it uses a Direct Line.
The fungus takes electrons directly from a molecule called NADH (the fuel) and sends them straight to the Hydrogen machine to make gas. It skips the "Red Ball" (Ferredoxin) entirely.

The Detective Work: How They Solved It

The researchers acted like detectives, using three different tools to solve the case:

1. The Lab Test (Enzyme Assays):
   They took the "power plants" out of the fungus and tested them.

   • The Test: They tried to see if the power plant could make hydrogen using the "Red Ball" (Ferredoxin). Result: Nothing happened.
   • The Test: They tried to see if it could make hydrogen using NADH directly. Result: Boom! Hydrogen gas was produced.
   • Conclusion: The factory only works with the direct line, not the relay race.

2. The Genetic Search (Genomics & Proteomics):
   They looked at the fungus's instruction manual (DNA) and the actual workers on the assembly line (Proteins).

   • They found the blueprints for the Hydrogen machine and a specific part of the NADH machine (called NuoE and NuoF).
   • Crucially, they found that the "Red Ball" (Ferredoxin) was barely there, or missing entirely in the power plant. This confirmed that the factory wasn't built to use the relay race.

3. The Rebuild (Reconstitution):
   To be 100% sure, they built the machine from scratch in a test tube using pure proteins they made in a lab.

   • They mixed the Hydrogen machine with the NADH machine.
   • They added NADH. Result: Hydrogen gas!
   • They added the "Red Ball" (Ferredoxin) instead. Result: Nothing.
   • The "Aha!" Moment: They proved that these two specific proteins stick together to form a new, unique machine that works like a direct line.

The "Non-Bifurcating" Concept

The paper uses a fancy word: "Non-bifurcating."

• Bifurcating means "splitting in two." Some bacteria use a complex trick where they split energy to do two things at once.
• Non-bifurcating means "straightforward." This fungus uses a simple, direct path. It's like taking a highway (Direct Line) instead of a winding, confusing country road with many turns (The Relay Race/Splitting).

Why Does This Matter?

1. Rewriting the Textbook: For decades, scientists thought all these tiny anaerobic power plants worked the same way (the relay race). This paper shows that fungi have a completely different, simpler strategy. It's like discovering that while most cars use a V8 engine, this specific model uses a high-tech electric motor.
2. Bioenergy: These fungi are great at breaking down plant waste (like wood chips or grass) into useful things. If we understand exactly how they make hydrogen, we can potentially engineer them to make more hydrogen gas. This could help us create clean, renewable fuel from waste.
3. Evolution: It shows that nature is full of surprises. Even though these fungi are related to other organisms that use the "relay race," they evolved a unique "direct line" solution to survive in the dark, oxygen-free world of a cow's stomach.

Summary in One Sentence

Scientists discovered that a tiny fungus living in a cow's stomach makes hydrogen gas not by passing electrons through a middleman, but by using a unique, direct machine that turns fuel straight into gas, a strategy that changes how we understand energy production in the microscopic world.

Technical Summary

1. Problem Statement

Anaerobic fungi (AF) are critical decomposers of lignocellulose in herbivore guts and play a significant role in global hydrogen ($H_2$) flux. They possess mitochondria-derived organelles called hydrogenosomes that generate ATP and $H_2$ to maintain redox balance during fermentation.

• The Knowledge Gap: While the mechanism of $H_2$ production in trichomonads (via a ferredoxin-dependent pathway involving Pyruvate:Ferredoxin Oxidoreductase, PFOR) is well-established, the mechanism in AF has remained unclear for over 40 years.
• The Debate: It was unknown whether AF hydrogenosomes use:
  1. A ferredoxin-dependent pathway (similar to trichomonads).
  2. A non-bifurcating NADH-dependent pathway (direct electron transfer from NADH to $H_2$).
  3. An electron-bifurcating/confurcating pathway (coupling exergonic and endergonic reactions).
• The Challenge: Previous genomic data suggested the presence of [FeFe]-hydrogenases and Complex I subunits (NuoE, NuoF) in AF, but biochemical evidence was lacking due to the difficulty of culturing strict anaerobes and reconstituting their oxygen-sensitive enzymes.

2. Methodology

The authors employed a multi-disciplinary approach combining genomics, proteomics, enzymology, and protein engineering using the model anaerobic fungus Caecomyces churrovis.

• Metabolic Phenotyping: Cultured C. churrovis under anaerobic conditions to measure $H_2$ production and fermentation byproducts (formate, ethanol, acetate, etc.) via Gas Chromatography (GC) and HPLC.
• Organelle Isolation & Enzyme Assays:
  • Isolated the Large Organelle Fraction (LOF) and enriched hydrogenosome fractions via differential centrifugation and OptiPrep density gradient centrifugation.
  • Measured specific enzyme activities in these fractions, including Malic Enzyme (marker), Pyruvate:Ferredoxin Oxidoreductase (PFOR), $H_2$:NAD$^+$ oxidoreductase, and NADH dehydrogenase.
• Genomic & Proteomic Analysis:
  • Searched the C. churrovis genome for genes encoding [FeFe]-hydrogenase (Hyd), NuoE, and NuoF.
  • Performed nanoPOTS proteomics on enriched hydrogenosome fractions to confirm protein expression and localization.
• Recombinant Protein Production:
  • Cloned and expressed C. churrovis Hyd, NuoE, and NuoF in E. coli (using an iscR deletion strain to support Fe-S cluster assembly).
  • Co-expressed with Shewanella oneidensis maturases to ensure proper [FeFe]-hydrogenase assembly.
  • Purified individual components (Hyd-Strep, NuoEF-Strep) and a ternary complex (Strep-Hyd-NuoE-NuoF).
• Reconstituted Enzyme Assays:
  • Tested purified enzymes individually and in mixtures to determine electron donors (NADH vs. NADPH vs. Ferredoxin) and acceptors.
  • Measured $H_2$ production rates and reverse reactions ($H_2$ oxidation) under strict anaerobic conditions.
• Bioinformatics: Used AlphaFold to predict protein structures and interactions; searched homologs across 12 other AF genomes to assess evolutionary conservation.

3. Key Results

A. Enzymatic Activity in Native Fractions

• $H_2$ Production: C. churrovis produced ~106 µmol $H_2$ from 767 µmol glucose over 7 days.
• Pathway Identification: The LOF and enriched hydrogenosome fractions exhibited high $H_2$:NAD$^+$ oxidoreductase activity (reducing NAD$^+$ using $H_2$).
• Absence of Ferredoxin Pathway:
  • No PFOR activity was detected in the LOF, despite the presence of a PFOR homolog in the genome.
  • No reduction of ferredoxin by $H_2$ was observed.
  • Proteomic analysis detected Hyd, NuoE, and NuoF but found no detectable hydrogenosomal ferredoxin in the enriched fractions.

B. Recombinant Enzyme Characterization

• NuoEF Complex: The purified NuoE-NuoF complex showed high NADH dehydrogenase activity but could not reduce ferredoxin.
• Hydrogenase: The purified Hyd could reduce methyl viologen (MV) with $H_2$ but could not reduce NAD$^+$ or ferredoxin using $H_2$ alone.
• The Hybrid Complex (The Discovery):
  • When Hyd and NuoEF were mixed, the system catalyzed the direct reduction of NAD$^+$ to NADH using $H_2$ (reverse reaction) independently of ferredoxin.
  • Crucially, the mixture catalyzed $H_2$ production from NADH at a rate of ~98 nmol/min/mg.
  • Specificity: The reaction was strictly NADH-dependent (no activity with NADPH) and did not require reduced ferredoxin. Adding ferredoxin did not enhance the rate, nor did reduced ferredoxin serve as an electron donor in the absence of NADH.
• Complex Formation: Co-expression of Hyd, NuoE, and NuoF resulted in a stable protein complex confirmed by native PAGE and LC-MS/MS. However, the co-expressed complex showed low maturation efficiency (low $H_2$:MV activity), suggesting that individual purification and mixing is necessary for full activity.

C. Evolutionary Conservation

• Homologs of Hyd, NuoE, and NuoF were identified in 11 out of 12 other anaerobic fungal genomes tested (spanning 5 genera: Anaeromyces, Caecomyces, Neocallimastix, Pecoramyces, Piromyces).
• Sequence identity was high (86–93%), indicating this non-bifurcating NADH-dependent mechanism is a conserved trait across the phylum Neocallimastigomycota.

4. Key Contributions

1. Mechanistic Resolution: The study definitively identifies the mechanism of $H_2$ production in anaerobic fungi as a non-bifurcating NADH-dependent enzyme complex composed of [FeFe]-hydrogenase and Complex I subunits (NuoE/NuoF).
2. Exclusion of Alternative Pathways: It provides biochemical evidence ruling out the ferredoxin-dependent pathway (common in trichomonads) and the electron-bifurcating mechanism for C. churrovis.
3. Thermodynamic Insight: The authors explain how this reaction, which is thermodynamically unfavorable under standard conditions ($\Delta G > 0$), proceeds in vivo by maintaining low dissolved $H_2$ partial pressures via syntrophic interactions with methanogens and rumen bacteria.
4. Methodological Advance: Successfully reconstituted an active eukaryotic [FeFe]-hydrogenase system by overcoming maturation challenges through individual protein purification and in vitro mixing.

5. Significance

• Fundamental Biology: This discovery expands the understanding of eukaryotic energy metabolism, revealing a unique adaptation where mitochondrial Complex I subunits have been repurposed to support anaerobic fermentation directly linked to hydrogenase activity.
• Ecological Impact: It clarifies the role of AF in the global carbon and hydrogen cycles, specifically how they maintain redox balance and interact with methanogens in the rumen and deep biosphere.
• Biotechnological Applications:
  • Metabolic Engineering: Identifying Hyd and NuoEF as the specific targets allows for the rational engineering of AF to redirect carbon flux. For example, modulating this pathway could increase yields of biofuels (ethanol, medium-chain fatty acids) or optimize $H_2$ production.
  • Modeling: These findings provide the necessary kinetic and mechanistic data to build accurate genome-scale metabolic models (GEMs) for anaerobic fungi, which are currently limited by the lack of this pathway definition.
• Broader Implications: The study suggests that similar non-bifurcating mechanisms may exist in other anaerobic eukaryotes (e.g., Nyctotherus ovalis), challenging the dogma that ferredoxin is the universal electron carrier for hydrogenosomal $H_2$ production.