The [FeFe]-hydrogenase maturation protein HydF contains a H-cluster like [4Fe4S]-2Fe site.

Formation of the catalytic six-iron complex (H-cluster) of [FeFe]-hydrogenase (HydA) requires its interaction with a specific maturation protein, HydF. Comparison by X-ray absorption spectroscopy at the Fe K-edge of HydF from Clostridium acetobutylicum and HydA1 from Chlamydomonas reinhardtii revealed that the overall structure of the iron site in both proteins is highly similar, comprising a [4Fe4S] cluster (Fe-Fe distances of ∼2.7Å) and a di-iron unit (Fe-Fe distance of ∼2.

The [FeFe]-hydrogenase maturase HydF from Clostridium acetobutylicum contains a CO and CN- ligated iron cofactor.

Biosynthesis of the [FeFe] hydrogenases active site (H-cluster) requires three maturation factors whose respective roles are not understood yet. The clostridial maturation enzymes (CaHydE, CaHydF and CaHydG) were homologously overexpressed in their native host Clostridium acetobutylicum. CaHydF was able to activate Chlamydomonas reinhardtii [FeFe] hydrogenase apoprotein (CrHydA1(apo)) to almost 100% compared to the native specific hydrogen evolution activity.