Back جسيم كربوكسيلي Arabic Carboxisoma Catalan Karboxyzom Czech Carboxysom German Carboxisoma Spanish Carboxysome French Carboxisoma Galician Կարբօքսիսոմներ Armenian Carbossisoma Italian カルボキシソーム Japanese

Carboxysome

Electron micrographs showing alpha-carboxysomes from the chemoautotrophic bacterium Halothiobacillus neapolitanus: (A) arranged within the cell, and (B) intact upon isolation. Scale bars indicate 100 nm.[1]

Carboxysomes are bacterial microcompartments (BMCs) consisting of polyhedral protein shells filled with the enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)—the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin cycle—and carbonic anhydrase.[2]

Carboxysomes are thought to have evolved as a consequence of the increase in oxygen concentration in the ancient atmosphere; this is because oxygen is a competing substrate to carbon dioxide in the RuBisCO reaction.[3] To overcome the inefficiency of RuBisCO, carboxysomes concentrate carbon dioxide inside the shell by means of co-localized carbonic anhydrase activity, which produces carbon dioxide from the bicarbonate that diffuses into the carboxysome. The resulting concentration of carbon dioxide near RuBisCO decreases the proportion of ribulose-1,5-bisphosphate oxygenation and thereby avoids costly photorespiratory reactions. The surrounding shell provides a barrier to carbon dioxide loss, helping to increase its concentration around RuBisCO.[4][5][6]

Carboxysomes are an essential part of the broader metabolic network called the Carbon dioxide-Concentrating Mechanism (CCM), which functions in two parts:[7] (1) Membrane transporters concentrate inorganic carbon (Ci) in the cell cytosol which is devoid of carbonic anhydrases. Carbon is primarily stored in the form of HCO3- which cannot re-cross the lipid membrane, as opposed to neutral CO2 which can easily escape the cell. This stockpiles carbon in the cell, creating a disequilibrium between the intracellular and extracellular environments of about 30x the Ci concentration in water.[8] (2) Cytosolic HCO3- diffuses into the carboxysome, where carboxysomal carbonic anhydrases dehydrate it back to CO2 in the vicinity of Rubisco, allowing Rubisco to operate at its maximal rate.

Carboxysomes are the best studied example of bacterial microcompartments, the term for functionally diverse organelles that are alike in having a protein shell.[9][10]

  1. ^ Tsai Y, Sawaya MR, Cannon GC, Cai F, Williams EB, Heinhorst S, et al. (June 2007). "Structural analysis of CsoS1A and the protein shell of the Halothiobacillus neapolitanus carboxysome". PLOS Biology. 5 (6): e144. doi:10.1371/journal.pbio.0050144. PMC 1872035. PMID 17518518.
  2. ^ Cite error: The named reference YeatesKerfeld2008 was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference Badger2003 was invoked but never defined (see the help page).
  4. ^ Cite error: The named reference CaiMenon2009 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference DouHeinhorst2008 was invoked but never defined (see the help page).
  6. ^ Mangan NM, Flamholz A, Hood RD, Milo R, Savage DF (September 2016). "pH determines the energetic efficiency of the cyanobacterial CO2 concentrating mechanism". Proceedings of the National Academy of Sciences of the United States of America. 113 (36): E5354–E5362. Bibcode:2016PNAS..113E5354M. doi:10.1073/pnas.1525145113. PMC 5018799. PMID 27551079.
  7. ^ Badger MR, Hanson D, Price GD (April 2002). "Evolution and diversity of CO2 concentrating mechanisms in cyanobacteria". Functional Plant Biology. 29 (3): 161–173. doi:10.1071/PP01213. PMID 32689463.
  8. ^ Price GD (September 2011). "Inorganic carbon transporters of the cyanobacterial CO2 concentrating mechanism". Photosynthesis Research. 109 (1–3): 47–57. doi:10.1007/s11120-010-9608-y. PMID 21359551. S2CID 25867128.
  9. ^ Cite error: The named reference KerfeldErbilgin2015 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference AxenErbilgin2014 was invoked but never defined (see the help page).
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