Iron-hydrogenase molecule. Computer model showing the molecular structure of an iron-hydrogenase ([FE]-hydrogenase) enzyme from the Methanocaldococcus jannaschii archaeon. Hydrogenase enzymes catalyse the reversible oxidation of hydrogen and play a vital role in anaerobic metabolism. [FE]-hydrogenases contain iron (Fe) at their active site. Archaea are a group of single-celled microorganisms that are similar to, but evolutionarily distinct from bacteria.