Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. 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.
Using electron microscopy the first carboxysomes were seen in 1956, in the cyanobacterium Phormidium uncinatum in the early 1960s similar polyhedral objects were observed in other cyanobacteria. 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).
Carboxysomes were first purified from Thiobacillus neapolitanus in 1973 and shown to contain RuBisCo, held within a rigid outer covering. The authors proposed that since these appeared to be organelles involved in carbon fixation, they should be called carboxysomes.
Structurally, carboxysomes are icosahedral, or quasi-icosahedral, typically around 80 to 120 nm in diameter. 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. 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. In addition, non-icosahedral faceted shapes of some carboxysomes can naturally be explained within the elastic theory of heterogeneous thin shells. 
Studies in Halothiobacillus neapolitanus have shown that empty shells of normal shape and composition are assembled in carboxysomal RuBisCO-lacking mutants suggesting that carboxysome shell biogenesis and enzyme sequestration are two independent, but functionally linked processes. Intriguingly, carboxysomes of Halothiobacillus neapolitanus have been found to accommodate chimeric and heterologous species of RuBisCO and it is the large subunit of RuBisCO that determines whether the enzyme is sequestered into carboxysomes or not.
Carboxysomes are found in all cyanobacteria, some nitrifying bacteria, and thiobacilli.
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