Carboxysomen

Carboxysomes are protein-based bacterial microcompartments that encapsulate the carbon-fixing enzyme RuBisCO in many cyanobacteria and some chemoautotrophic bacteria. They function as a carbon-concentrating mechanism by concentrating CO2 around RuBisCO and limiting oxygenase activity, thereby improving carbon fixation under low CO2 conditions.

The carboxysome is a polyhedral shell assembled from multiple shell proteins of the BMC (bacterial microcompartment) family. The shell forms a selectively permeable boundary that allows diffusion of inorganic carbon and metabolic intermediates while restricting CO2 leakage. Inside, RuBisCO is positioned with matrix-associated proteins, and, in many lineages, a carbonic anhydrase enzyme converts bicarbonate to CO2 to sustain the RuBisCO substrate supply.

There are two major classes of carboxysomes: alpha-carboxysomes and beta-carboxysomes. They differ in shell protein composition, RuBisCO isoforms, and gene organization. Alpha-carboxysomes are found in certain proteobacteria and some cyanobacteria, while beta-carboxysomes are common in many marine cyanobacteria. The genetic loci encoding carboxysomes are typically organized in operons that include shell genes along with those for RuBisCO and carbonic anhydrase.

Functionally, carboxysomes enable efficient CO2 fixation by spatially separating RuBisCO from the cytosol and concentrating CO2, which is especially advantageous in environments with limited inorganic carbon. They are central to the bacterial carbon-concentrating mechanism and have attracted interest in synthetic biology for enabling engineered microreactors and enhanced CO2 capture.