Reductive citric acid cycle

The citric acid cycle [TCA cycle] (oxidative) is one of the main cycles used by all aerobic organisms. The rTCA reverses the reactions of the oxidative citric acid cycle. It is used to generate energy through oxidation of acetate which is derived from different substances like fats, carbohydrates and proteins with the products carbon dioxide and ATP. The reductive citric acid cycle runs in reverse. That means it uses two molecules carbon dioxide and ATP to generate carbohydrates, fats and proteins from acetyl-CoA. It is used for autotrophic growth (Schauder et al., 1987).
The green sulfur bacterium Chlorobium thiosulfatophilum is the first organism where this cycle could be observed by Evans, Buchanan and Arnon 1966 (Arnon-Buchanan Cycle) (Evans et al., 1966). It has also been found in anaerobic and microaerobic bacteria.
There are three reactions of the oxidative citric acid cycle which are known irreversible (Buchanan et al., 1990). The establishment of the reverse cycle requires three additional enzymes: succinate dehydrogenase by fumarate reductase, NAD⁺-dependent 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarat synthase and citrate synthase by ATP citrate lyase. The product, Acetyl-CoA, is further carboxylated to pyruvate which is used by the cell. As electron donors ferredoxin and NAD(P)H are used. The cycle uses two ATP to form one pyruvate molecule.
We decided not to work with this cycle because we would have we had to use anaerobic cultivation conditions which we tried to avoid.

Reductive acetyl CoA pathway

The reductive acetyl CoA pathway also called Wood-Ljungdahl pathway (1965) uses carbon dioxide as an electron acceptor and hydrogen as an electron donor for biosynthesis. The product of this pathway is acetyl-CoA which is used for several biological reactions. The key enzyme is called CO dehydrogenase / acetyl-CoA synthase and represents a substantial part of the cell protein. Acetyl-CoA is assimilated to pyruvate by pyruvate synthase. The pathway works in both directions depending on the environment condition (Schauder et al., 1988).
The pathway has been found in Clostridium thermoacetium which is a strictly anaerobic bacteria (acetogens) (Ragsdale et al., 2008). It is preferred by bacteria living close to the thermodynamic limit (Acetogens, methanogens). There the cycle is also used for energy generation.
Because this pathway also only occurs under strict anoxic conditions we decided to not use it for our project. Besides the cycle depends strongly on metals (Mo or W, Co, Ni and Fe).

3-Hydroxypropionate bicycle

The 3-hydroxypropionate bicycle produces 3-hydroxypropionate by consuming carbon dioxide. The bi-cycle allows coassimilation of different compounds, e.g. propionate, acetate and succinate. These substances are metabolized via acetyl-CoA. The energy costs of the cycle are high with seven molecules ATP for one pyruvate and three additional for triose phosphates (Strauss et al., 1993).The enzymes of this cycle are not especially oxygen sensitive. Additionally the enzymes are multifunctional which means there are only 13 enzymes for 17 reactions.
The cycle was discovered first in Chloroflexus aurantiacus by Helge Holo (Holo et al., 1986). It occurs in bacteria living under neutrophilic and alakliphilic conditions.
We decided to do some research about this cycle but our main focus lies on the Calvin cycle.

The 3-hydroxypropionate (Fuchs-Holo) bicycle. 1. acetyl-CoA carboxylase, 2. malonyl-CoA reductase, 3. propionyl-CoA synthase, 4. propionyl-CoA carboxylase, 5. methylmalonyl-CoA epimerase, 6. methylmalonyl-CoA mutase, 7. succinyl-CoA:(S)-malate-CoA transferase, 8. succinate dehydrogenase, 9. fumarate hydratase, 10. (a,b,c) trifunctional (S)-malonyl-CoA a/(β)-methylmalonyl-CoA (b)/(S)-citramalyl-CoA lyase, 11. mesaconyl-C1-CoA hydratase, 12. mesaconyl-CoA C1-C4 CoA transferase, 13. mesaconyl-C4-CoA hydratase (Berg et al., 2011)

Calvin cycle

The Calvin cycle is the light independent reaction of the photosynthesis. Photosynthesis is done by all plants and some sulfurbacteria. The products of the photosynthesis ATP and NADPH are used for the Calvin cycle. By using ATP and NADPH carbohydrates were produced. In the cycle carbon dioxide is taken up and higher sugars were produced. We decided to work with the Calvin cycle and did further research on this theme because the cyclic and aerobic conditions as well as the number of missing enzymes fit our project idea.

The reductive pentose phosphate (Calvin-Benson-Bassham) cycle. 1. ribulose 1,5-bisphosphate carboxylase/oxygenase, 2. 3-phosphoglycerate kinase, 3. glyceraldehyde 3-phosphate dehydrogenase, 4. triose-phosphate isomerase, 5. fructose bisphosphate aldolase, 6. fructose-bisphosphate phosphatase, 7. transketolase, 8. sedoheptulose bisphosphate aldolase, 9. sedoheptulose bisphosphate phosphatase, 10. ribose-phosphate isomerase, 11. ribulose-phosphate epimerase, 12. phosphoribulokinase (Berg et al., 2011)

References

• Berg (2011) Ecological Aspects of the Distribution of Different Autotrophic CO₂ Fixation Pathways Applied and Environmental Microbiology, vol. 77, no. 6, pp. 1925-1936
• Evans et al., 1966. A new ferredoxin dependent carbon reduction cycle in a photosynthetic bacterium. Proc. Natl. Acad. Sci. U. S. A., vol. 55, pp.928-934
• Schauder et al., 1987. Carbon assimilation pathways in sulfate-reducing bacteria II. Enzymes of a reductive citric acid cycle in the autotrophic Desulfobacter hydrogenophilus. Arch Microbiol., vol. 148, pp.218-225
• Buchanan et al., 1990. A reverse KREBS cycle in photosynthesis: consensus at last . Photosynthesis Research, vol. 24, pp.47-53
• Schauder et al., 1988. Oxidative and reductive acetyl CoA/carbon monoxide dehydrogenase pathway in Desulfobacterium autotrophicum . Archives of Microbiology, vol. 151, pp.84-89
• Ragsdale et al., 2008. Acetogenesis and the Wood-Ljungdahl pathway of CO₂ fixation. . Biochim. Biophys. Acta, vol. 1784, pp.1873-1898
• Holo et al., 1986. Autotrophic growth and CO₂ fixation of Chloroflexus aurantiacus. . Arch Microbiol., vol. 100, pp.5-24
• Strauss et al., 1993. Enzymes of a novel autotrophic CO₂ fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus, the 3-hydroxypropionate cycle . Arch Microbiol., vol. 100, pp.5-24