Carboxysome

Carboxysomes are bacterial microcompartments (BMCs) consisting of polyhedral protein shells filled with the enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)—the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin cycle—and 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 concentration 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. Carboxysomes are an essential part of the broader metabolic network called the Carbon dioxide-Concentrating Mechanism (CCM), which functions in two parts: (1) Membrane transporters concentrate inorganic carbon (Ci) in the cell cytosol which is devoid of carbonic anhydrases. Carbon is primarily stored in the form of HCO3- which cannot re-cross the lipid membrane, as opposed to neutral CO2 which can easily escape the cell. This stockpiles carbon in the cell, creating a disequilibrium between the intracellular and extracellular environments of about 30x the Ci concentration in water. (2) Cytosolic HCO3- diffuses into the carboxysome, where carboxysomal carbonic anhydrases dehydrate it back to CO2 in the vicinity of Rubisco, allowing Rubisco to operate at its maximal rate. Carboxysomes are the best studied example of bacterial microcompartments, 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.