Carboxysome

A carboxysome is a protein-shelled compartment found in cyanobacteria and some other bacteria that fix carbon dioxide. It packs the enzyme Rubisco together with carbonic anhydrase, which creates a CO₂-rich environment that makes carbon fixation more efficient.

The carboxysome is a polyhedral body about 100 to 150 nanometers across, whose shell is made of thousands of small proteins arranged in hexagonal and pentagonal tiles, much as in a virus capsid. Unlike an organelle of eukaryotes, it has no lipid membrane. Inside, hundreds of copies of Rubisco are held together by linker proteins. Two types are known, alpha and beta, in different bacterial lineages.

The cell pumps bicarbonate across its membranes, so it builds up in the cytoplasm. Inside the carboxysome, carbonic anhydrase converts it to CO₂, which is released next to Rubisco. The shell slows the escape of CO₂ and keeps oxygen out, so Rubisco makes fewer wasteful reactions, as in photorespiration. It is a bacterial version of the CO₂ concentrating mechanism that C4 plants have in another form.

Cyanobacteria with carboxysomes fix a large share of the world's carbon in the ocean, as part of the carbon cycle. Researchers in synthetic biology are trying to introduce carboxysomes into crop chloroplasts to improve photosynthesis in a plant's Calvin cycle.

• Rubisco — the enzyme it packages
• Cyanobacteria — the main bacteria that have it
• Calvin Cycle — the pathway it supports