Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. [cite journal |author=Badger MR, Price GD |title=CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution |journal=J. Exp. Bot. |volume=54 |issue=383 |pages=609–22 |year=2003 |month=February |pmid=12554704 |url=] Carboxysomes are made of polyhedral protein shells about 80 to 140 nanometres in diameter. These compartments are thought to concentrate carbon dioxide to overcome the inefficiency of RuBisCo - the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin cycle. These organelles are found in all cyanobacteria and many chemotrophic bacteria that fix carbon dioxide.

Carboxysomes are an example of a wider group of protein micro-compartments that have dissimilar functions, but similar structures, based on homology of the two shell protein families. [cite journal |author=Cannon GC, Bradburne CE, Aldrich HC, Baker SH, Heinhorst S, Shively JM |title=Microcompartments in prokaryotes: carboxysomes and related polyhedra |journal=Appl. Environ. Microbiol. |volume=67 |issue=12 |pages=5351–61 |year=2001 |pmid=11722879 |doi=10.1128/AEM.67.12.5351-5361.2001 |url=]


Using electron microscopy the first carboxysomes were seen in 1956, in the cyanobacterium "Phormidium uncinatum" [cite journal |author=Drews G, Niklowitz W |title= [Cytology of Cyanophycea. II. Centroplasm and granular inclusions of Phormidium uncinatum.] |language=German |journal=Arch Mikrobiol |volume=24 |issue=2 |pages=147–62 |year=1956 |pmid=13327992] cite journal |author=Yeates TO, Kerfeld CA, Heinhorst S, Cannon GC, Shively JM |title=Protein-based organelles in bacteria: carboxysomes and related microcompartments |journal=Nat. Rev. Microbiol. |volume=6 |pages=681-691 |year=2008 |month=August |pmid=18679172 |doi=10.1038/nrmicro1913] in the early 1960's similar polyhedral objects were observed in other cyanobacteria. [cite journal |author=Gantt E, Conti SF |title=Ultrastructure of blue-green algae |journal=J. Bacteriol. |volume=97 |issue=3 |pages=1486–93 |year=1969 |month=March |pmid=5776533 |pmc=249872 |url=] These structures were named "polyhedral bodies" in 1961 and over the next few years were also discovered in some chemotrophic bacteria that fixed carbon dioxide (for example, "Halothiobacillus", "Acidithiobacillus", "Nitrobacter" and "Nitrococcus"). [cite journal |author=Shively JM |title=Inclusion bodies of prokaryotes |journal=Annu. Rev. Microbiol. |volume=28 |pages=167–87 |year=1974 |pmid=4372937 |doi=10.1146/annurev.mi.28.100174.001123]

Carboxysomes were first purified from "Thiobacillus neapolitanus" in 1973 and shown to contain RuBisCo, held within a rigid outer covering. [cite journal |author=Shively JM, Ball F, Brown DH, Saunders RE |title=Functional organelles in prokaryotes: polyhedral inclusions (carboxysomes) of Thiobacillus neapolitanus |journal=Science (journal) |volume=182 |issue=112 |pages=584–6 |year=1973 |month=November |pmid=4355679] The authors proposed that since these appeared to be organelles involved in carbon fixation, they should be called "carboxysomes". [cite journal |author=Shively JM, Ball FL, Kline BW |title=Electron microscopy of the carboxysomes (polyhedral bodies) of Thiobacillus neapolitanus |journal=J. Bacteriol. |volume=116 |issue=3 |pages=1405–11 |year=1973 |month=December |pmid=4127632 |pmc=246500 |url=]


Structurally, carboxysomes are icosahedral, or icosahedral, typically around 80 to 120 nm in diameter.cite journal |author=Tanaka S, Kerfeld CA, Sawaya MR, "et al" |title=Atomic-level models of the bacterial carboxysome shell |journal=Science (journal) |volume=319 |issue=5866 |pages=1083–6 |year=2008 |month=February |pmid=18292340 |doi=10.1126/science.1151458] The carboxysome has an outer shell composed of a few thousand protein subunits, which encapsulates the two carbon fixing enzymes, carbonic anhydrase and RuBisCO. Proteins known to form the shell have been structurally characterized by X-ray crystallography.cite journal |author=Kerfeld, CA, Sawaya, MR, Tanaka S, Nguyen CV, Phillips M, Beeby M, Yeates TO | title =Protein structures forming the shell of primitive bacterial organelles | journal=Science(journal) |volume=309 |issue=5736|pages=936-8|year=2005|pmid=16081736] The protein that constitutes the majority of the shell forms a cyclical hexamer. These hexamers constitute the basic building blocks of the shell. X-ray structural data suggest that the hexamers assemble further in a side-by-side fashion to form a tightly packed molecular layer, which represents the outer shell. Small pores are present through the hexamers and may serve as the route for diffusion of substrates (bicarbonate and ribulose-1,5-bisphosphate) and products (3-phosphoglycerate) into and out of the carboxysome. Positively charged amino acids in the pores might help promote the diffusion of the negatively charged substrates and products. Other minor structural components of the shell that have been characterized include pentameric proteins, which have been proposed to occupy the vertices of the icosahedral shell.

A number of viral capsids are also constructed from hexameric and pentameric proteins, but whether any evolutionary relationship exists between the carboxysome and viral capsids is unknown. Electron cryo-tomography studies have confirmed the approximately icosahedral geometry of the carboxysome, and have visualized enzymes molecules inside, presumed to be RuBisCO, arranged in a few concentric layers [cite journal | author=Schmid MF, Paredes AM, Khant HA, Soyer F, Aldrich HC, Chiu W, Shively JM| title=Structure of Halothiobacillus neapolitanus carboxysomes by cryo-electron tomography | journal = J. Mol. Biol. | volume = 364 | issue=3 | pages = 526-35|year=2006|pmid=17028023] [cite journal | author=Iancu CV, Ding HJ, Morris DM, Dias DP, Gonzales AD, Martino A, Jensen GJ | title=The structure of isolated Synechococcus strain WH8102 carboxysomes as revealed by electron cryotomography | journal=J. Mol. Biol. | volume=372 | issue=3 | pages=764-73|pmid=17669419] .


Carboxysomes are found in all cyanobacteria, some nitrifying bacteria, and thiobacilli.


External links

* [ Mysterious Bacterial Microcompartments Revealed By Biochemists]
* [ Not so simple after all. A renaissance of research into prokaryotic evolution and cell structure]

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