Illustration showing the structure of a carboxysome. Carboxysomes are organelles found in the cells of photosynthetic bacteria and are primarily involved in the process of carbon fixation. They are composed of a polyhedral protein shell filled with the enzyme Ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase (CA). RuBisCO is the predominant enzyme in carbon fixation, the process of converting inorganic carbon from forms such as carbon dioxide, to organic compounds by living organisms. Carbonic anhydrases are enzymes that catalyse the rapid interconversion of carbon dioxide and water to bicarbonate and protons, a reversible reaction that occurs relatively slowly in the absence of a catalyst.