Cupriavidus necator is a Gram-negative soil bacterium of the class Betaproteobacteria.
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 chemolithotrophic organisms were clustered into the group Hydrogenomonas. C. necator was originally named Hydrogenomonas eutrophus because it fell under the Hydrogenomonas classification and was "well nourished and robust". Some of the original H. eutrophus cultures isolated were by Bovell and Wilde. After characterizing cell morphology, metabolism and GC content, the Hydrogenomonas nomenclature was disbanded because it comprised many species of microorganisms. H. eutrophus was then renamed Alcaligenes eutropha because it was a micro-organism with degenerated peritrichous flagellation. Investigating phenotype, lipid composition, fatty acid composition and 16S rRNA analysis, A. eutropha was found to belong to the genus Ralstonia and named Ralstonia eutropha. Upon further study of the genus, Ralstonia was found to comprise two phenotypically distinct clusters. The new genus Wautersia was created from one of these clusters which included R. eutropha. In turn R. eutropha was renamed Wautersia eutropha. Looking at DNA–DNA hybridization and phenotype comparison with Cupriavidus necator, W. eutropha was found to be the same species as previously described C. necator. Because C. necator was named in 1987 far before the name change to R. eutropha and W. eutropha, the name C. necator was assigned to R. eutropha according to Rule 23a of the International Code of Nomenclature of Bacteria.
Metabolism
Cupriavidus necator is a hydrogen-oxidizing bacterium ("knallgas" bacterium) capable of growing at the interface of anaerobic and aerobic environments. It can easily adapt between heterotrophic and autotrophic lifestyles. Both organic compounds and hydrogen can be used as a source of energy C. necator can perform aerobic or anaerobic respiration by denitrification of nitrate and/or nitrite to nitrogen gas. When growing under autotrophic conditions, C. necator fixes carbon through the reductive pentose phosphate pathway. It is known to produce and sequester polyhydroxyalkanoate (PHA) plastics when exposed to excess amounts of sugar substrate. PHA can accumulate to levels around 90% of the cell's dry weight. To better characterize the lifestyle of C. necator, the genomes of two strains have been sequenced.
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: 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. 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. 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. 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. 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.