Carboxysome

Carboxysomes are bacterial organelles consisting of polyhedral protein shells filled with the enzyme Ribulose-1,5-biphosphate 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-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.

Polyhedral bodies were discovered by transmission electron microscopy in the cyanobacterium Phormidium uncinatum in 1956. These were later observed in other cyanobacteria and in some chemotrophic bacteria that fixed carbon dioxide—many of them are sulfur reducers or nitrogen fixers (for example, Halothiobacillus, Acidithiobacillus, Nitrobacter and Nitrococcus). The polyhedral bodies were first purified from Thiobacillus neapolitanus (now Halothiobacillus 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.

...
Wikipedia