How to feed an enzyme ethane: When released from ocean floor seeps, small hydrocarbons are rapidly consumed by micro-organisms. Methane is highly abundant and is both produced and consumed by microbes through well understood biochemical pathways. Less well understood is how ethane, also a major natural component of gaseous hydrocarbons, is metabolized. To understand how microbes take advantage of this energy and carbon source, Hahn et al. solved the x-ray crystal structures of an enzyme they call ethyl coenzyme-M reductase, which converts ethane into the thioether ethyl-coenzyme M as the entry point for catabolism. They found an expanded active site and, using a xenon gas derivatization experiment, a distinctive tunnel through the protein that is proposed to permit access of the gaseous substrate. Science , abg1765, this issue p. 118

In the ancient microbial Wood-Ljungdahl pathway, carbon dioxide (CO ₂ ) is fixed in a multistep process that ends with acetyl–coenzyme A (acetyl-CoA) synthesis at the bifunctional carbon monoxide dehydrogenase/acetyl-CoA synthase complex (CODH/ACS). In this work, we present structural snapshots of the CODH/ACS from the gas-converting acetogen Clostridium autoethanogenum , characterizing the molecular choreography of the overall reaction, including electron transfer to the CODH for CO ₂ reduction, methyl transfer from the corrinoid iron-sulfur protein (CoFeSP) partner to the ACS active site, and acetyl-CoA production. Unlike CODH, the multidomain ACS undergoes large conformational changes to form an internal connection to the CODH active site, accommodate the CoFeSP for methyl transfer, and protect the reaction intermediates. Altogether, the structures allow us to draw a detailed reaction mechanism of this enzyme, which is crucial for CO ₂ fixation in anaerobic organisms.

Massive efforts are invested in developing innovative CO₂‐sequestration strategies to counter climate change and transform CO₂ into higher‐value products. CO₂‐capture by reduction is a chemical challenge, and attention is turned toward biological systems that selectively and efficiently catalyse this reaction under mild conditions and in aqueous solvents. While a few reports have evaluated the effectiveness of isolated bacterial formate dehydrogenases as catalysts for the reversible electrochemical reduction of CO₂, it is imperative to explore other enzymes among the natural reservoir of potential models that might exhibit higher turnover rates or preferential directionality for the reductive reaction. Here, we present electroenzymatic catalysis of formylmethanofuran dehydrogenase, a CO₂‐reducing‐and‐fixing biomachinery isolated from a thermophilic methanogen, which was deposited on a graphite rod electrode to enable direct electron transfer for electroenzymatic CO₂ reduction. The gas is reduced with a high Faradaic efficiency (109±1 %), where a low affinity for formate prevents its electrochemical reoxidation and favours formate accumulation. These properties make the enzyme an excellent tool for electroenzymatic CO₂‐fixation and inspiration for protein engineering that would be beneficial for biotechnological purposes to convert the greenhouse gas into stable formate that can subsequently be safely stored, transported, and used for power generation without energy loss.

Microbial alcohol production from waste gases is a game changer for sustainable carbon cycling and remediation. While the biotechnological process using Clostridium autoethanogenum to transform syngas (H ₂ , CO ₂ and CO) is blooming, scientific debates remain on the ethanol biosynthesis pathway. Here, we experimentally validated that ethanol production is initiated through a tungsten-dependent aldehyde:ferredoxin oxidoreductase (AFOR), which reduces acetate to acetaldehyde. The reaction, thermodynamically unfavorable under standard conditions, has been considered by many as unsuitable in vivo but is rather approved by metabolic modeling. To answer this riddle, we demonstrated that the thermodynamic coupling of CO oxidation and ethanol synthesis allows acetate reduction. The experiments, performed with native CO dehydrogenase and AFOR, highlighted the key role of ferredoxin in stimulating the activity of both metalloenzymes and electron shuttling. The crystal structure of holo AFOR, refined to 1.59-Å resolution, and its biochemical characterization provide new insights into the cofactor chemistry and the specificities of this enzyme, fundamental to sustainable biofuel production.

Ketol-acid reductoisomerase (KARI) orchestrates the biosynthesis of branched-chain amino acids, an elementary reaction in prototrophic organisms as well as a valuable process in biotechnology. Bacterial KARIs belonging to class I organise as dimers or dodecamers and were intensively studied to understand their remarkable specificity towards NADH or NADPH, but also to develop antibiotics. Here, we present the first structural study on a KARI natively isolated from a methanogenic archaea. The dodecameric structure of 0.44-MDa was obtained in two different conformations, an open and close state refined to a resolution of 2.2-Å and 2.1-Å, respectively. These structures illustrate the conformational movement required for substrate and coenzyme binding. While the close state presents the complete NADP bound in front of a partially occupied Mg²⁺-site, the Mg²⁺-free open state contains a tartrate at the nicotinamide location and a bound NADP with the adenine-nicotinamide protruding out of the active site. Structural comparisons show a very high conservation of the active site environment and detailed analyses point towards few specific residues required for the dodecamerisation. These residues are not conserved in other dodecameric KARIs that stabilise their trimeric interface differently, suggesting that dodecamerisation, the cellular role of which is still unknown, might have occurred several times in the evolution of KARIs.

Massive efforts are invested in developing innovative CO₂‐sequestration strategies to counter climate change and transform CO₂ into higher‐value products. CO₂‐capture by reduction is a chemical challenge, and attention is turned toward biological systems that selectively and efficiently catalyse this reaction under mild conditions and in aqueous solvents. While a few reports have evaluated the effectiveness of isolated bacterial formate dehydrogenases as catalysts for the reversible electrochemical reduction of CO₂, it is imperative to explore other enzymes among the natural reservoir of potential models that might exhibit higher turnover rates or preferential directionality for the reductive reaction. Here, we present electroenzymatic catalysis of formylmethanofuran dehydrogenase, a CO₂‐reducing‐and‐fixing biomachinery isolated from a thermophilic methanogen, which was deposited on a graphite rod electrode to enable direct electron transfer for electroenzymatic CO₂ reduction. The gas is reduced with a high Faradaic efficiency (109±1 %), where a low affinity for formate prevents its electrochemical reoxidation and favours formate accumulation. These properties make the enzyme an excellent tool for electroenzymatic CO₂‐fixation and inspiration for protein engineering that would be beneficial for biotechnological purposes to convert the greenhouse gas into stable formate that can subsequently be safely stored, transported, and used for power generation without energy loss.