Engineering the Electron Relay in [FeFe]-Hydrogenase Enhances Electrocatalytic H ₂ Evolution

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 <i>Clostridium pasteurianum</i> and <i>Clostridium acetobutylicum</i> 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 <i>C. acetobutylicum</i> 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.