Cupriavidus necator
Cupriavidus necator | |
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Scientific classification | |
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Phylum: | |
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Species: | C. necator
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Binomial name | |
Cupriavidus necator (Davis 1969) Yabuuchi et al. 1996
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Synonyms | |
Ralstonia eutropha |
Cupriavidus necator is a
Taxonomy
Cupriavidus necator has gone through a series of name changes. In the first half of the 20th century, many micro-organisms were isolated for their ability to use hydrogen. Hydrogen-metabolizing
Metabolism
Cupriavidus necator is a
Hydrogenases
Cupriavidus necator can use hydrogen gas as a source of energy when growing under autotrophic conditions. It contains four different hydrogenases that have [Ni-Fe] active sites and all perform this reaction:[14][15]
- H2 2H+ + 2e−
The hydrogenases of C. necator are like other typical [Ni-Fe] hydrogenases because they are made up of a large and a small subunit. The large subunit is where the [Ni-Fe] active site resides and the small subunit is composed of [Fe-S] clusters.[16] However, the hydrogenases of C. necator are different from typical [Ni-Fe] hydrogenases because they are tolerant to oxygen and are not inhibited by CO.[14] While the four hydrogenases perform the same reaction in the cell, each hydrogenase is linked to a different cellular process. The differences between the regulatory hydrogenase, membrane-bound hydrogenase, soluble hydrogenase and actinobacterial hydrogenase in C. necator are described below.
Regulatory hydrogenase
The first hydrogenase is a regulatory hydrogenase (RH) that signals to the cell hydrogen is present. The RH is a protein containing large and small [Ni-Fe] hydrogenase subunits attached to a histidine protein kinase subunit.[17] The hydrogen gas is oxidized at the [Ni-Fe] center in the large subunit and in turn reduces the [Fe-S] clusters in the small subunit. It is unknown whether the electrons are transferred from the [Fe-S] clusters to the protein kinase domain.[14] The histidine protein kinase activates a response regulator. The response regulator is active in the dephosphorylated form. The dephosphorylated response regulator promotes the transcription of the membrane bound hydrogenase and soluble hydrogenase.[18]
Membrane-bound hydrogenase
The membrane-bound hydrogenase (MBH) is linked to the respiratory chain through a specific cytochrome b-related protein in C. necator.[19] Hydrogen gas is oxidized at the [Ni-Fe] active site in the large subunit and the electrons are shuttled through the [Fe-S] clusters in the small subunit to the cytochrome b-like protein.[14] The MBH is located on the outer cytoplasmic membrane. It recovers energy for the cell by funneling electrons into the respiratory chain and by increasing the proton gradient.[19] The MBH in C. necator is not inhibited by CO and is tolerant to oxygen.[20]
NAD+-reducing hydrogenase
The NAD+-reducing hydrogenase (soluble hydrogenase, SH) creates a NADH-reducing equivalence by oxidizing hydrogen gas. The SH is a heterohexameric protein[21] with two subunits making up the large and small subunits of the [Ni-Fe] hydrogenase and the other two subunits comprising a reductase module similar to the one of Complex I.[22] The [Ni-Fe] active site oxidized hydrogen gas which transfers electrons to a FMN-a cofactor, then to a [Fe-S] cluster relay of the small hydrogenase subunit and the reductase module, then to another FMN-b cofactor and finally to NAD+.[14] The reducing equivalences are then used for fixing carbon dioxide when C. necator is growing autotrophically.
The active site of the SH of C. necator H16 has been extensively studied because C. necator H16 can be produced in large amounts, can be genetically manipulated, and can be analyzed with spectrographic techniques. However, no crystal structure is currently available for the C. necator H16 soluble hydrogenase in the presence of oxygen to determine the interactions of the active site with the rest of the protein.[14]
Typical anaerobic [Ni-Fe] hydrogenases
The [Ni-Fe] hydrogenase from
C. necator oxygen-tolerant SH
The SH in C. necator are unique for other organisms because it is oxygen tolerant.[27] The active site of the SH has been studied to learn why this protein is tolerant to oxygen. A recent study showed that oxygen tolerance as implemented in the SH is based on a continuous catalytically driven detoxification of O2 [Ref missing]. The genes encoding this SH can be up-regulated under heterotrophic growth condition using glycerol in the growth media [28] and this enables aerobic production and purification of the same enzyme.[29]
Applications
The oxygen-tolerant hydrogenases of C. necator have been studied for diverse purposes. C. necator was studied as an attractive organism to help support life in space. It can fix carbon dioxide as a carbon source, use the urea in urine as a nitrogen source, and use hydrogen as an energy source to create dense cultures that could be used as a source of protein.[30][31]
Electrolysis of water is one way of creating oxygenic atmosphere in space and C. necator was investigated to recycle the hydrogen produced during this process.[32]
Oxygen-tolerant hydrogenases are being used to investigate biofuels. Hydrogenases from C. necator have been used to coat electrode surfaces to create hydrogen fuel cells tolerant to oxygen and carbon monoxide[20] and to design hydrogen-producing light complexes.[33] In addition, the hydrogenases from C. necator have been used to create hydrogen sensors.[34] Genetically modified C. necator can produce isobutanol from CO
2 that can directly substitute or blend with gasoline. The organism emits the isobutanol without having to be destroyed to obtain it.[35]
Industrial uses
Researchers at UCLA have genetically modified a strain of the species C. necator (formerly known as R. eutropha H16) to produce isobutanol from CO2 feedstock using electricity produced by a solar cell. The project, funded by the U.S. Dept. of Energy, is a potential high energy-density electrofuel that could use existing infrastructure to replace oil as a transportation fuel.[36]
Chemical and biomolecular engineers at
References
- ^ PMID 8657018.
- ^ .
- ^ PMID 6271040.
- PMID 14491520.
- S2CID 25824711.
- S2CID 24798412.
- PMID 15023939.
- PMID 15545472.
- ^ PMID 16964242.
- PMID 18957861.
- S2CID 26360677.
- S2CID 206894168.
- PMID 20339589.
- ^ S2CID 8030367.
- PMID 23793632.
- ISBN 978-0-387-25492-0.
- PMID 9770510.
- PMID 15667276.
- ^ PMID 9310376.
- ^ PMID 16260746.
- PMID 186126.
- PMID 2188945.
- ^ PMID 9438867.
- ^ .
- S2CID 4335445.
- PMID 8990114.
- PMID 226163.
- PMID 25880663.
- PMID 27077052.
- PMID 10840.
- PMID 4963807.
- S2CID 84358305.
- S2CID 37919998.
- PMID 16053311.
- ^ "Teaching a microbe to make fuel - MIT News Office". Web.mit.edu. Retrieved 2012-08-22.
- S2CID 24328552.
- .