Ferredoxin hydrogenase

Ferredoxin hydrogenase catalyzes the following reversible reaction: :H2 + 2 oxidized ferredoxin \rightleftharpoons 2 reduced ferredoxin + 2 H+ The exact mechanism by which this reaction occurs is still not entirely known; however, several intermediates have been identified in steady-state conditions. Accessory clusters allow for the enzyme to retain full enzyme activity at potentials around and higher than the equilibrium potential. However, it is important to note that the accessory clusters are not as effective at extremely low potentials. == Enzyme structure ==

The reaction occurs in the [2Fe] moiety of the active H cluster site, which also includes a [4Fe4S] cluster covalently bonded via a cysteinyl thiolate. An additional accessory site known as the F-domain, which contains four Fe-S clusters, is known as the main gate for electron transfer to and from the catalytic site. The [2Fe] sub-site is coordinated by several CO and CN ligands and an azadithiolate bridge that allows for proton shuttling. The oxidized state of the sub-site, abbreviated as Hox, includes a paramagnetic, mixed-valence [Fe(I)Fe(II)] moiety and a diamagnetic, oxidized [4Fe4S] 2+ cluster. The single electron reduction of Hox yields two protomers that differ in the localization of the added electron and proton. The addition of the second electron results in the [Fe(I)Fe(II)]-[4Fe4S]+ configuration and a protonated azadithiolate bridge. == Biological function ==

Hydrogenases are found in prokaryotes and lower (unicellular) eukaryotes. This broad category of metalloenzymes can be divided into [NiFe], [FeFe], and [Fe] variants based on the transition metals found in their active sites. However, the hydrogen gas oxidation and proton reduction activities varies greatly among the variants and even within their own subcategories. The inter-conversion of protons and electrons with hydrogen gas allow organisms to modulate energy input and output, adjust organelle redox potential, and transduce chemical signals. == Industrial significance ==

Hydrogen gas is a potential candidate for the partial replacement of fossil fuels as a clean energy carrier in fuel cells. However, because of the gas's weight, it often escapes the Earth's atmosphere into space, making it a scarce resource on Earth. The hydrogen photo-production in green algae, catalyzed by ferredoxin hydrogenase, is a potential source of hydrogen gas. Unfortunately, the inefficiency of the reaction and enzyme as whole to produce hydrogen places a barrier on the viability of this reaction on an industrial scale. In addition, ferredoxin hydrogenase is sensitive to oxygen; the typical half life for the enzyme under aerobic conditions is on the scale of seconds, rendering it difficult to cultivate and manage commercially. == References ==