Simone Morra

[FeFe]-hydrogenases are the fastest enzymes for hydrogen evolution, yet their irreversible inhibition by oxygen has thwarted their industrial use. CbA5H is an exception as its inhibition by oxygen is reversible. Protein scaffold rearrangement near the active site, allowing a ligand to coordinate the metal center in addition to the formation of a highly oxidized state of the metal center named H_inact, is the current hypothesis for CbA5H oxygen stability. However, the ligand identity has been disputed and there is no evidence to suggest that protein scaffold rearrangement is the sole reason for oxygen stability. Here, we investigate CbA5H oxygen stability by providing a high-resolution (1.96 Å) X-ray structure that shows that the protective ligand is a conserved cysteine thiol group, which directly coordinates the metal center. The local rearrangement also encompasses structural water molecules and the side chain of E341, associated with proton transfer. In addition, we illustrate that C236 and H245, located close to accessory iron sulfur clusters in the Fd domain, influence oxygen stability. We show that mutating these residues significantly decreases oxygen stability but not the ability to form H_inact. Variant C236A displays a slower inactivation rate, which we suggest is due to tuning the redox properties of one of the accessory iron sulfur clusters. We also show that the soluble ligand-binding β-grasp domain (SLBB) may not be required for oxygen stability by comparing CbA5H to a novel homolog lacking this domain. Collectively, these findings expand our understanding of oxygen stability in [FeFe]-hydrogenases.

The production of biofuels by bacterial fermentation receives sustained attention due to the need to develop novel circular and sustainable technologies. Clostridium beijerinckii produces both hydrogen (H2) and carbon-based biofuels acetone, butanol and ethanol (ABE solvents). H2 metabolism in C. beijerinckii is complex and mostly unexplored. Seven hydrogenase genes are contained in the genome, but their exact physiological role is unknown. Here, we report on the characterisation of a novel heterotetrameric soluble enzyme complex composed of an [FeFe]-hydrogenase component stably bound to a formate dehydrogenase subunit, which we name CbFdh/Hyd. We show that the four subunits form a stable complex that can be conveniently overexpressed and purified recombinantly. CbFdh/Hyd is highly sensitive to atmospheric oxygen and displays reversible catalytic features, including H2 evolution, H2 uptake, formate oxidation and the ability to split formate into H2 and CO2 (formate hydrogen lyase activity, FHL) as well as the opposite reaction, H2-driven CO2 reduction (HDCR). CbFdh/Hyd displays functional and spectroscopic features very similar to Fdh/Hyd complexes previously described in acetogens, suggesting that this enzyme is at the basis of the previously reported unconventional ability of C. beijerinckii to fix CO2 into acetate and butyrate. CbFdh/Hyd could also represent a key player in H2 production metabolism by degrading formate produced from the decarboxylation of pyruvate.

H₂ is an ideal energy vector, but catalysts for its clean production from water are inefficient or expensive. [FeFe]-hydrogenases are the most active H₂-converting catalysts in nature, using a unique organometallic active site finely tuned by the protein matrix. M3 type [FeFe]-hydrogenases from Clostridium pasteurianum and Clostridium acetobutylicum are exceptionally active for H₂ production, and less O₂ sensitive than most other types of [FeFe]-hydrogenases, making them attractive targets for biotechnology. However, they are more challenging to work with because of their large size and the number of iron–sulfur clusters. Here, the [FeFe]-hydrogenase from C. acetobutylicum was systematically engineered to truncate each iron–sulfur-containing region of the F-domain, yielding smaller and easier-to-produce catalytic systems. Detailed characterization revealed that these variants retain high electrocatalytic performance and other essential properties of the natural enzyme.

[FeFe]-hydrogenases are highly-active hydrogen-conversion biocatalysts using Earth-abundant metals in their active-site. Understanding their mechanism may enable design of catalysts for renewable energy storage. Here, observation of the crucial Fe-hydride-containing (Hhyd) intermediate in a PDT-variant of [FeFe]-hydrogenase reveals deeper insight into the role of the dithiolate bridgehead in the catalytic mechanism.

The enzyme CbA5H, a [FeFe]‐hydrogenase from Clostridium beijerinckii, has previously been shown to survive exposure to oxygen, making it a promising candidate for biotechnological applications. Thus far [NiFe]‐hydrogenases are typically considered for such applications, due to the superior O₂‐tolerance and therefore simplified enzyme handling. However, methods for production of [FeFe]‐hydrogenases are generally more successful than for other classes of hydrogenases, therefore in this work we focus on demonstrating scalable CbA5H production, and report results with active enzyme prepared in bioreactors (up to 10 L) with >20‐fold improvement in purified enzyme yield. We then go on to confirm excellent H₂/H⁺‐cycling activity of the air‐purified protein, highlighting that CbA5H can be prepared and isolated without the need for complex and expensive infrastructure. Next, we demonstrate good stability of the air‐purified CbA5H both in solution assays, and as a heterogenous catalyst system when immobilized on a carbon support. Finally, we successfully implement this enzyme within previously demonstrated biotechnologies for flavin and NADH recycling, highlighting its relevance in chemical synthesis, and we demonstrate production of an important API precursor, 3‐quinuclidinol at >0.4 g scale in standard benchtop hydrogenation infrastructure, with >100,000 CbA5H turnovers over 18 operational hours.