Abstract
[FeFe]-hydrogenases catalyse H2 production at exceptionally high turnover numbers (up to 104 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 H2 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 H2. [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 H2.
Highlights
The issues raised by the massive use of fossil fuels are promoting the research of new renewable energy technologies (Luque et al, 2008, Jacobson, 2009)
A site saturation mutagenesis study has shown that replacement of C298 in C. acetobutylicum [FeFe]-hydrogenase HydA (CaHydA) with any other residue strongly affects the enzyme activity, with the only exception of C298D; considering that this mutant showed a shift in the pH activity profile, these results demonstrated that this cysteine is the key residue in the process of proton transfer to the H-cluster during catalysis (Morra et al, 2012)
[FeFe]-hydrogenases are excellent natural catalysts that have evolved for efficient H2 production
Summary
The issues raised by the massive use of fossil fuels (i.e. pollution, climate changes, resource depletion) are promoting the research of new renewable energy technologies (Luque et al, 2008, Jacobson, 2009). Hoxair) is an inactive but oxygen-stable state observed only in the hydrogenases from Desulfovibrio vulgaris and D. desulfuricans, characterized by a diamagnetic [4Fe4S]2+ subcluster and a diamagnetic Fe(II)–Fe(II) It can be irreversibly converted in the active Hox form by anaerobic reduction, through the intermediate Htrans. Since the active site of [FeFe]-hydrogenases is a complex organometallic cluster that is not present in any other protein, it requires to be assembled by a specific cellular machinery This process is a socalled maturation process that involves at least three maturases: HydE, HydF and HydG (Posewitz et al, 2004, Nicolet and Fontecilla-Camps 2012, Peters et al, 2015). The recombinant systems that have been developed are either cell-hosted or cell-free (Tab. 1)
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