Probing the Role of Accessory Domains in Oxygen Stability of [FeFe]-Hydrogenases

[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.