enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

, ferredoxin hydrogenase (), also referred to as e-Fe'' hydrogenase, H2 oxidizing hydrogenase, H2 producing hydrogenase, bidirectional hydrogenase, hydrogenase (ferredoxin), hydrogenlyase, and uptake hydrogenase, is found in ''Clostridium pasteurianum, Clostridium acetobutylicum,'' ''Chlamydomonas reinhardtii'', and other organisms. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme is hydrogen:ferredoxin oxidoreductase Ferredoxin hydrogenase belongs to the family of

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

s, specifically those acting on hydrogen as donor with an iron-sulfur protein as acceptor. Ferredoxin hydrogenase has an active metallocluster site referred to as an "H-cluster" or "H domain" that is involved in the inter-conversion of protons and electrons with hydrogen gas.

Enzyme Reaction and Mechanism

: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. A proposed mechanism for the catalyzed reaction by ferredoxin hydrogenase is:
Ferredoxin_hydrogenase_mechanism

The two substrates of this enzyme are H₂ and oxidized ferredoxin, whereas its two

products Product may refer to: Business * Product (business), an item that serves as a solution to a specific consumer problem. * Product (project management), a deliverable or set of deliverables that contribute to a business solution Mathematics * Produ ...

are reduced ferredoxin and H⁺. During the hydrogen gas turnover, the H-cluster undergoes a series of redox transitions as protons are translocated. The reaction rate is dependent on the environment pH and sees an activity increase in pH environments between 7-9. 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 Femoiety of the active H cluster site, which also includes a Fe4Scluster 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 Fesub-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 e(I)Fe(II)moiety and a diamagnetic, oxidized Fe4S2+ 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 e(I)Fe(II) Fe4S configuration and a protonated azadithiolate bridge.

Biological Function

Hydrogenases are found in prokaryotes, lower eukaryotes, and archaea. This broad category of metalloenzymes can be divided into iFe

eFe Agencia EFE, S.A. () is a Spanish international news agency, the major multimedia news agency in Spanish language and the world's fourth largest wire service after the Associated Press, Reuters, and Agence France-Presse. EFE was created in 1939 ...

and evariants 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. Ferredoxin hydrogenase participates in

glyoxylate and dicarboxylate metabolism Glyoxylate and dicarboxylate metabolism describes a variety of reactions involving glyoxylate or dicarboxylates. Glyoxylate is the conjugate base of glyoxylic acid, and within a buffered environment of known pH such as the cell cytoplasm these t ...

and methane metabolism. It has 3

cofactors Cofactor may also refer to: * Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed * A domain parameter in elliptic curve cryptography, defined as the ratio between the order ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

Sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

, and

Nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...

. Ferredoxin hydrogenase found in the green algae ''Chlamydomonas reinhardtii'' use supplied electrons from photosystem I to reduce protons into hydrogen gas. This electron supply transfer is possible through photosystem I interactions with photosynthetic electron transfer ferredoxin (PetF). 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

{{Portal bar, Biology, border=no EC 1.12.7 Iron enzymes Sulfur enzymes Nickel enzymes Enzymes of unknown structure