The 3-Hydroxypropionate/4-hydroxybutyrate cycle, also known as the 3HP/4HB cycle, is a specialized carbon fixation process used by some archaea, including Thermoproteota.[1] For these organisms to survive and grow autotrophically in hostile settings, such as hydrothermal vents, this cycle is essential.[2] Carbon dioxide (CO2) is effectively transformed by the process into organic chemicals like acetyl-CoA, which can then be utilized for growth and energy production.[3] This route is specific to organisms that fix CO2 in high-temperature, low-oxygen settings, in contrast to the more well-known Calvin cycle which does not perform as well at fixing CO2 under these conditions.[2]
Steps
editThe 3-hydroxypropionate/4-hydroxybutyrate cycle is one of six known carbon fixation mechanisms.[1] One of the most energy-efficient processes for transforming inorganic carbon (CO2) into organic compounds, especially in thermal and acidic environments, was initially discovered in the archaea Thermoproteota.
There are two primary sections to this pathway: The carboxylation of acetyl-CoA yields malonyl-CoA, which is then transformed into intermediates that feed into the subsequent cycle in the 3-hydroxypropionate cycle.
Acetyl-CoA is created from intermediates such as succinyl-CoA and 4-hydroxybutyrate in the 4-Hydroxybutyrate cycle, which eventually regenerates the cycle by synthesizing organic molecules for energy and cellular development.
Key intermediates and enzymatic reactions
editThe cycle starts when the enzyme acetyl-CoA carboxylase catalyzes the carboxylation of acetyl-CoA to malonyl-CoA.[4] Subsequently, this intermediate is converted into propionyl-CoA and other organic molecules. The cycle is completed in the second portion by regenerating acetyl-CoA using intermediates like succinyl-CoA and 4-hydroxybutyrate.[4] Acetyl-CoA and pyruvate, the cycle's end products, are essential for a number of metabolic processes, such as the citric acid cycle and fatty acid synthesis. The 3-HP/4-HB cycle is very effective for autotrophic carbon fixation under harsh circumstances because of the cyclical regeneration of acetyl-CoA.[5]
Adaptation to extreme environments: The 3-HP/4-HB cycle-dependent species are usually found in settings where more traditional carbon fixation routes, including the Calvin cycle, would not function well.[4] Among these are hydrothermal vents, which have high temperatures, low oxygen concentrations, and copious amounts of CO2.[4] By reducing adenosine triphosphate (ATP) and energy consumption during carbon fixation, the 3-HP/4-HB cycle helps organisms flourish and allows these archaea to maintain existence in some of the most hostile environments on Earth.[5]
Comparison to other carbon fixation pathways
editIn high-temperature and low-oxygen environments, the 3-HP/4-HB cycle uses less energy than the Calvin cycle, which is common in plants and algae. The 3-HP/4-HB cycle is a perfect method for energy conservation in archaea compared to the Calvin cycle since it fixes carbon with fewer ATP molecules. The Calvin cycle requires 9 ATP and 6 NADPH to fix three molecules of CO2 into a triose phosphate, which is eventually converted to glucose.[6] However, the 3-HP/4-HB cycle only requires 5 ATP and % NADPH to fix three molecules of CO2 into glucose.[5] The 3-HP/4-HB cycle's intermediates also directly feed into other critical metabolic pathways, making it a flexible tool for surviving in harsh environments.
Metabolic engineering
editCO2 is a greenhouse gas that contributes to global warming. The use of metabolic pathways to fix atmospheric CO2 is being evaluated to address climate change. The 3-hydroxypropionate/4-hydroxybutyrate cycle is a proposed pathway to fix excess CO2 in the atmosphere.[1]
References
edit- ^ a b c Loder, Andrew J.; Han, Yejun; Hawkins, Aaron B.; Lian, Hong; Lipscomb, Gina L.; Schut, Gerrit J.; Keller, Matthew W.; Adams, Michael W. W.; Kelly, Robert M. (2016-11-01). "Reaction kinetic analysis of the 3-hydroxypropionate/4-hydroxybutyrate CO2 fixation cycle in extremely thermoacidophilic archaea". Metabolic Engineering. 38: 446–463. doi:10.1016/j.ymben.2016.10.009. ISSN 1096-7176. PMC 5433351. PMID 27771364.
- ^ a b Minic, Zoran; Thongbam, Premila D. (2011-04-28). "The Biological Deep Sea Hydrothermal Vent as a Model to Study Carbon Dioxide Capturing Enzymes". Marine Drugs. 9 (5): 719–738. doi:10.3390/md9050719. ISSN 1660-3397. PMC 3111178. PMID 21673885.
- ^ Nisar, Ayesha; Khan, Sawar; Hameed, Muddassar; Nisar, Alisha; Ahmad, Habib; Mehmood, Sardar Azhar (2021-10-01). "Bio-conversion of CO2 into biofuels and other value-added chemicals via metabolic engineering". Microbiological Research. 251: 126813. doi:10.1016/j.micres.2021.126813. ISSN 0944-5013. PMID 34274880.
- ^ a b c d Berg, Ivan A.; Kockelkorn, Daniel; Buckel, Wolfgang; Fuchs, Georg (2007-12-14). "A 3-Hydroxypropionate/4-Hydroxybutyrate Autotrophic Carbon Dioxide Assimilation Pathway in Archaea". Science. 318 (5857): 1782–1786. Bibcode:2007Sci...318.1782B. doi:10.1126/science.1149976. ISSN 0036-8075. PMID 18079405.
- ^ a b c Fuchs, Georg (2011-10-13). "Alternative Pathways of Carbon Dioxide Fixation: Insights into the Early Evolution of Life?". Annual Review of Microbiology. 65 (1): 631–658. doi:10.1146/annurev-micro-090110-102801. ISSN 0066-4227. PMID 21740227.
- ^ Johnson, Matthew P. (2016-10-31). "Photosynthesis". Essays in Biochemistry. 60 (3): 255–273. doi:10.1042/EBC20160016. ISSN 0071-1365. PMC 5264509. PMID 27784776.