Carboxysomes are polyhedral protein shells of the bacterial microcompartment group of supramolecular complexes involved in carbon fixationcitation needed. Carboxysomes encapsulate enzymes involved in carbon fixation and are thought to concentrate carbon in the form of carbon dioxide to overcome the inefficiency of RuBisCo, the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin cycle. Carboxysomes are thought to be part of a wider group of protein micro-compartments that have dissimilar functions, but similar structures, based on homology of the two shell protein families[1]
Architecture
Structurally, carboxysomes have been shown to be icosahedral, or quasi-icosahedral, typically around 120 nm in diameter. Proteins known to form the shell have been structurally characterized by X-ray crystallography, revealing that one family forms cyclical hexamerscitation needed while another forms pentamerscitation needed. In direct analogy to many known viral structurescitation needed this suggested that the carboxysome is icosahedral. Electron cryo-tomography studiescitation needed have subsequently confirmed this, leading to a model based on a synthesis of molecular structures and gross morphology[2]
Occurrence
Carboxysomes are found in all cyanobacteria, some nitrifying bacteria, and thiobacilli.citation needed
