REVERSE KREBS CYCLE

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.