REVERSE KREBS CYCLE
The reverse Krebs cycle (also known as the reverse
tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid
cycle) is a sequence of chemical reactions that are used by some bacteria to
produce carbon compounds from carbon dioxide and water.
The reaction is the citric acid cycle run in reverse: Where
the Krebs cycle takes complex carbon molecules in the form of sugars and
oxidizes them to CO2 and water, the reverse cycle takes CO2 and water to make
carbon compounds. This process is used by some bacteria to synthesise carbon
compounds, sometimes using hydrogen, sulfide, or thiosulfate as electron donors.[1][2]
In this process, it can be seen as an alternative to the fixation of inorganic
carbon in the reductive pentose phosphate cycle which occurs in a wide variety
of microbes and higher organisms.
The reaction is a possible candidate for prebiotic
early-earth conditions and, so, is of interest in the research of the origin of
life. It has been found that some of the steps can be catalysed by minerals.
Green photosynthetic bacteria (Chlorobium limicola), some
thermophillic bacteria that grow on hydrogen, (Hydrogenobacter thermophilus)
and certain bacteria that grow by reducing sulfate (Desulfobacter
hydrogenophilus ) have been shown to use rTCA.
"The reverse tricarboxylic acid (rTCA) cycle is an
alternative CO2 fixation pathway that generates a stable isotopic signature
more closely resembling "primary consumers from a number of hydrothermal
vent sites" (37, 44). To date, the rTCA cycle has been found in only a few
microorganisms, including Chlorobium species, a few members of the delta
subdivision of Proteobacteria (i.e., Desulfobacter hydrogenophilus), and some
members of the thermophilic Aquificales order and archaeal Thermoproteaceae
family (5, 13, 15, 19, 27, 42, 43). A reversal of the entire TCA cycle for
carbon dioxide fixation uses four or five ATP molecules and generates one
molecule of oxaloacetate from four molecules of CO2 (18). Key enzymes of the
rTCA cycle include ATP citrate lyase (encoded by the aclBA gene) and two of the
four carbon dioxide-fixing enzymes, 2-oxoglutarate:ferredoxin oxidoreductase
(encoded by the oorDABC gene), and pyruvate:ferredoxin oxidoreductase (encoded
by the porCDAB or nifJ gene). These enzymes catalyze the reversal of the TCA
cycle by making the energetically unfavorable reverse reactions possible. For
example, ATP citrate lyase catalyzes the cleavage of citrate into acetyl
coenzyme A (acetyl-CoA) and oxaloacetate in a CoA- and ATP-dependent matter (2).
Reversal of the TCA cycle requires the combined use of all three enzymes for
reductive carboxylation to occur.
Recent molecular and isotopic data indicate that the epsilon proteobacterial
community associated with Alvinella pompejana, which thrives on the sides of
the hotter black smoker chimneys at deep-sea hydrothermal vents, may utilize
the rTCA cycle for autotrophic growth. Two of the key genes were present and
expressed in the episymbiont community, which is dominated by members of the
epsilon subdivision of Proteobacteria (7). In addition, there is genetic
evidence of the rTCA cycle in at least two cultured autotrophic members of the
epsilon subdivision of Proteobacteria from hydrothermal vents and one
autotrophic member of the epsilon subdivision of Proteobacteria isolated from
marine sediments (6, 56). Interestingly, molecular studies indicate that
members of the epsilon subdivision of Proteobacteria dominate all free-living
deep-sea hydrothermal vent microbial environments studied so far (10, 23, 35, 39).
Enrichment of a diverse array of chemoautotrophic members of the epsilon
subdivision of Proteobacteria from hydrothermal vents further supports the
results of these molecular studies (1, 6, 33, 46). These chemoautotrophs
utilized H2 or reduced sulfur compounds as electron donors and O2, nitrate, or
elemental sulfur as electron acceptors." from:
Abundance of Reverse Tricarboxylic Acid Cycle Genes in Free-Living
Microorganisms at Deep-Sea Hydrothermal Vents.
Since the discovery of hydrothermal vents more than 25 years ago, the
Calvin-Bassham-Benson (Calvin) cycle has been considered the principal carbon
fixation pathway in this microbe-based ecosystem. However, on the basis of
recent molecular data of cultured free-living and noncultured episymbiotic
members of the epsilon subdivision of Proteobacteria and earlier carbon isotope
data of primary consumers, an alternative autotrophic pathway may predominate.
Here, genetic and culture-based approaches demonstrated the abundance of reverse
tricarboxylic acid cycle genes compared to the abundance of Calvin cycle genes
in microbial communities from two geographically distinct deep-sea hydrothermal
vents. PCR with degenerate primers for three key genes in the reverse
tricarboxylic acid cycle and form I and form II of ribulose 1,5-bisphosphate
carboxylase/oxygenase (Calvin cycle marker gene) were utilized to demonstrate
the abundance of the reverse tricarboxylic acid cycle genes in diverse vent
samples. These genes were also expressed in at least one chimney sample.
Diversity, similarity matrix, and phylogenetic analyses of cloned samples and
amplified gene products from autotrophic enrichment cultures suggest that the
majority of autotrophs that utilize the reverse tricarboxylic acid cycle are
members of the epsilon subdivision of Proteobacteria. These results parallel
the results of previously published molecular surveys of 16S rRNA genes,
demonstrating the dominance of members of the epsilon subdivision of Proteobacteria
in free-living hydrothermal vent communities. Members of the epsilon
subdivision of Proteobacteria are also ubiquitous in many other microaerophilic
to anaerobic sulfidic environments, such as the deep subsurface. Therefore, the
reverse tricarboxylic acid cycle may be a major autotrophic pathway in these
environments and significantly contribute to global autotrophic processes.
