page_title: Carboxysome
hierarchical_section_title: Carboxysome
caption_attribution_description: English: (A) A thin-section electron micrograph of H. neapolitanus cells with carboxysomes inside. In one of the cells shown, arrows highlight the visible carboxysomes. (B) A negatively stained image of intact carboxysomes isolated from H. neapolitanus. The features visualized arise from the distribution of stain around proteins forming the shell as well as around the RuBisCO molecules that fill the carboxysome interior. Scale bars indicate 100 nm.
context_page_description: Carboxysomes are bacterial compartments consisting of polyhedral protein shells filled with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase -the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin Cycle-and a carbonic anhydrase. 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. 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 production 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. The carboxysome is an essential part of the carbon dioxide-concentrating mechanism.
context_section_description: Carboxysomes are bacterial compartments consisting of polyhedral protein shells filled with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) -the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin Cycle-and a carbonic anhydrase. 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. 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 production 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. The carboxysome is an essential part of the carbon dioxide-concentrating mechanism (CCM).
  Carboxysomes are the best studied example of a bacterial microcompartment (BMC), the term for functionally diverse organelles that are alike in having a protein shell.
caption: 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]
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