page_title: Carboxysome
section_title: Two Types of Carboxysomes
hierarchical_section_title: Carboxysome / Two Types of Carboxysomes
caption_attribution_description: Electron micrograph of (A) alpha-carboxysomes in Halothiobacillus neapolitanus and (B) beta-carboxysomes in Synechococcus elongatus PCC 7942. Arrowheads highlight carboxysomes. Scale bars indicate 200 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: There are two types of carboxysomes. Although they may seem similar in appearance, they differ in their protein composition, including the form of RuBisCO they enclose. Furthermore, studies have revealed fundamental differences in their gene organization and possibly in how they assemble.
caption: Electron micrograph of (A) alpha-carboxysomes in Halothiobacillus neapolitanus and (B) beta-carboxysomes in Synechococcus elongatus PCC 7942, indicated by arrows
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