Carboxysomes

Carboxysomes are protein-based organelles within certain bacteria that concentrate carbon dioxide for carbon fixation by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). They are a type of bacterial microcompartment found in cyanobacteria, some proteobacteria, and other autotrophic lineages, enabling efficient RuBisCO activity under low CO2.

Structurally, carboxysomes are 80–200 nm polyhedral shells assembled from thousands of shell proteins of the BMC family. The shell forms a selective barrier that confines the core enzymes—RuBisCO and carbonic anhydrase—while allowing diffusion of inorganic carbon substrates. The internal core includes scaffolding proteins that organize the enzymes and coordinate assembly. Two major lineages exist: alpha- and beta-carboxysomes, with distinct shell protein composition and organization.

Alpha-carboxysomes generally contain Form I RuBisCO in certain chemoautotrophs, while beta-carboxysomes are common in cyanobacteria and many proteobacteria. The internal carbonic anhydrase converts bicarbonate to CO2, supporting a high local CO2 concentration around RuBisCO and reducing oxygenase activity.

Physiological role: carboxysomes support a carbon-concentrating mechanism that elevates CO2 for RuBisCO, enabling growth in low-CO2 environments. Inorganic carbon is imported as bicarbonate and converted inside the shell, increasing the CO2/RuBisCO ratio.

Genetics and biogenesis: carboxysomes are encoded by operons containing shell, RuBisCO, and carbonic anhydrase genes, with scaffolding proteins guiding assembly. The process is thought to initiate with shell formation followed by core encapsulation.

Relevance: carboxysomes illustrate bacterial compartmentalization and have attracted interest for engineering enhanced photosynthesis, CO2 capture, and metabolic organization.