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. Carboxysomes are an essential part of the broader metabolic network called the Carbon dioxide-Concentrating Mechanism (CCM), which functions in two parts. 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. Structurally, carboxysomes are icosahedral, or quasi-icosahedral. Electron cryo-tomography studies have confirmed the approximately icosahedral geometry of the carboxysome, and have imaged Rubisco proteins inside arranged in a few concentric layers or fibril-like structures. Here you can see a recent cryoEM structure of the alpha-carboxysome shell vertex from Prochlorococcus MED4 (PDB code: 8WXB)

#molecularart ... #carboxisome ... #prochlorococcus ... #cryoem .. #icosahedral ... #microcomparment

Structure rendered with @proteinimaging and depicted with @corelphotopaint