English: Legend: (left) Purified carboxysomes (material courtesy of S. Heinhorst and G. Cannon) as visualized by cryo-electron microscopy (courtesy of M.J. Yeager and K.A. Dryden, University of Virginia). (right) Models for the structure of the carboxysome. Current data suggest that the shell is composed of several hundred hexameric protein building blocks and 12 pentameric building blocks. The three-dimensional atomic structures of the shell proteins have been determined by X-ray crystallography. RuBisCO, the main interior enzyme is shown packed inside in a regular arrangement for simplicity, though the actual organization of the enzymes is not understood yet. The other key enzyme, carbonic anhydrase, which is present in lesser amounts, is not illustrated (image by T.O. Yeates).