[FeFe]-hydrogenases as biocatalysts in bio-hydrogen production

[FeFe]-hydrogenases catalyse H-2 production at exceptionally high turnover numbers (up to 10(4) s(-1)). They are found in a variety of strict or facultative anaerobic microorganisms, such as bacteria of the genus Clostridium, Desulfovibrio, Thermotoga, and eukaryotes ranging from unicellular and coenobial green algae to anaerobic fungi, ciliates and trichomonads. Key to their activity is an organometallic centre, the H-cluster that cooperates tightly with the protein framework to reduce two protons into molecular hydrogen. The assembly of the catalytic site requires a specialised cellular mechanism based on the action of three other enzymes, called maturases: HydE, HydF and HydG. Recent advancements in the recombinant production of [FeFe]-hydrogenases have provided leaps forward in their exploitation in H-2 production for clean energy storage. [FeFe]-hydrogenases have been used in several fermentative approaches where microorganisms are engineered to overexpress specific [FeFe]-hydrogenases to convert low-cost materials (e.g. wastes) into H-2. [FeFe]-hydrogenases have also been proven to be excellent catalysts in different in vitro devices that can produce hydrogen directly from water, either via water electrolysis or via light-driven mechanisms, thus allowing the direct storage of solar energy into H-2.