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Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below:
Hydrogen oxidation (1) is coupled to the reduction of electron acceptors such as oxygen, nitrate, ferric ion, sulfate, carbon dioxide (CO2), and fumarate. On the other hand, proton reduction (2) is coupled to the oxidation of electron donors such as ferredoxin (FNR), and serves to dispose excess electrons in cells (essential in pyruvate fermentation). Both low-molecular weight compounds and proteins such as FNRs, cytochrome c3, and cytochrome c6 can act as physiological electron donors or acceptors for hydrogenases.
It has been estimated that 99% of all organisms utilize hydrogen, H2. Most of these species are microbes and their ability to use H2 as a metabolite arises from the expression of metalloenzymes known as hydrogenases. Hydrogenases are sub-classified into three different types based on the active site metal content: nickel-iron hydrogenase, iron-iron hydrogenase, and iron hydrogenase.
Hydrogenases catalyze, sometimes reversibly, H2 uptake. The [FeFe] and [NiFe] hydrogenases are true redox catalysts, driving H2 oxidation and proton (H+) reduction (equation 3), the [Fe] hydrogenases catalyze the reversible heterolytic cleavage of H2 shown by reaction (4).
Although originally believed to be "metal-free", the [Fe]-only hydrogenases contain Fe at the active site and no iron-sulfur clusters. [NiFe] and [FeFe] hydrogenases have some common features in their structures: Each enzyme has an active site and a few Fe-S clusters that are buried in protein. The active site, which is believed to be the place where catalysis takes place, is also a metallocluster, and each iron is coordinated by carbon monoxide (CO) and cyanide (CN−) ligands.
The [NiFe] hydrogenases are heterodimeric proteins consisting of small (S) and large (L) subunits. The small subunit contains three iron-sulfur clusters while the large subunit contains the active site, a nickel-iron centre which is connected to the solvent by a molecular tunnel. In some [NiFe] hydrogenases, one of the Ni-bound cysteine residues is replaced by selenocysteine. On the basis of sequence similarity, however, the [NiFe] and [NiFeSe] hydrogenases should be considered a single superfamily. To date, periplasmic, cytoplasmic, and cytoplasmic membrane-bound hydrogenases have been found. The [NiFe] hydrogenases, when isolated, are found to catalyse both H2 evolution and uptake, with low-potential multihaem cytochromes such as cytochrome c3 acting as either electron donors or acceptors, depending on their oxidation state. Generally speaking, however, [NiFe] hydrogenases are more active in oxidizing H2. A wide spectrum of H2 affinities have also been observed in H2-oxidizing hydrogenases.
Like [FeFe] hydrogenases, [NiFe] hydrogenases are known to be usually deactivated by molecular oxygen (O2). Hydrogenase from Ralstonia eutropha, and several other so-called Knallgas-bacteria, were found to be oxygen-tolerant. The soluble [NiFe] hydrogenase from Ralstonia eutropha H16 can be conveniently produced on heterotrophic growth media. This finding increased hope that hydrogenases can be used in photosynthetic production of molecular hydrogen via splitting water. Another [NiFe], called Huc or Hyd1 or cyanobacterial-type uptake hydrogenase, has been found to be oxygen insensitive while having a very high affinity for hydrogen. Hydrogen is able to penetrate narrow channels in the enzyme that oxygen molecules cannot enter. This allows bacteria such as Mycobacterium smegmatis to utilize the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking.
The hydrogenases containing a di-iron center with a bridging dithiolate cofactor are called [FeFe] hydrogenases. Three families of [FeFe] hydrogenases are recognized:
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Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below:
Hydrogen oxidation (1) is coupled to the reduction of electron acceptors such as oxygen, nitrate, ferric ion, sulfate, carbon dioxide (CO2), and fumarate. On the other hand, proton reduction (2) is coupled to the oxidation of electron donors such as ferredoxin (FNR), and serves to dispose excess electrons in cells (essential in pyruvate fermentation). Both low-molecular weight compounds and proteins such as FNRs, cytochrome c3, and cytochrome c6 can act as physiological electron donors or acceptors for hydrogenases.
It has been estimated that 99% of all organisms utilize hydrogen, H2. Most of these species are microbes and their ability to use H2 as a metabolite arises from the expression of metalloenzymes known as hydrogenases. Hydrogenases are sub-classified into three different types based on the active site metal content: nickel-iron hydrogenase, iron-iron hydrogenase, and iron hydrogenase.
Hydrogenases catalyze, sometimes reversibly, H2 uptake. The [FeFe] and [NiFe] hydrogenases are true redox catalysts, driving H2 oxidation and proton (H+) reduction (equation 3), the [Fe] hydrogenases catalyze the reversible heterolytic cleavage of H2 shown by reaction (4).
Although originally believed to be "metal-free", the [Fe]-only hydrogenases contain Fe at the active site and no iron-sulfur clusters. [NiFe] and [FeFe] hydrogenases have some common features in their structures: Each enzyme has an active site and a few Fe-S clusters that are buried in protein. The active site, which is believed to be the place where catalysis takes place, is also a metallocluster, and each iron is coordinated by carbon monoxide (CO) and cyanide (CN−) ligands.
The [NiFe] hydrogenases are heterodimeric proteins consisting of small (S) and large (L) subunits. The small subunit contains three iron-sulfur clusters while the large subunit contains the active site, a nickel-iron centre which is connected to the solvent by a molecular tunnel. In some [NiFe] hydrogenases, one of the Ni-bound cysteine residues is replaced by selenocysteine. On the basis of sequence similarity, however, the [NiFe] and [NiFeSe] hydrogenases should be considered a single superfamily. To date, periplasmic, cytoplasmic, and cytoplasmic membrane-bound hydrogenases have been found. The [NiFe] hydrogenases, when isolated, are found to catalyse both H2 evolution and uptake, with low-potential multihaem cytochromes such as cytochrome c3 acting as either electron donors or acceptors, depending on their oxidation state. Generally speaking, however, [NiFe] hydrogenases are more active in oxidizing H2. A wide spectrum of H2 affinities have also been observed in H2-oxidizing hydrogenases.
Like [FeFe] hydrogenases, [NiFe] hydrogenases are known to be usually deactivated by molecular oxygen (O2). Hydrogenase from Ralstonia eutropha, and several other so-called Knallgas-bacteria, were found to be oxygen-tolerant. The soluble [NiFe] hydrogenase from Ralstonia eutropha H16 can be conveniently produced on heterotrophic growth media. This finding increased hope that hydrogenases can be used in photosynthetic production of molecular hydrogen via splitting water. Another [NiFe], called Huc or Hyd1 or cyanobacterial-type uptake hydrogenase, has been found to be oxygen insensitive while having a very high affinity for hydrogen. Hydrogen is able to penetrate narrow channels in the enzyme that oxygen molecules cannot enter. This allows bacteria such as Mycobacterium smegmatis to utilize the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking.
The hydrogenases containing a di-iron center with a bridging dithiolate cofactor are called [FeFe] hydrogenases. Three families of [FeFe] hydrogenases are recognized: