Upon deprotonation, the corrinoid ring is capable of binding cobalt. In vitamin B12, the resulting complex also features a benzimidazole-derived ligand, and the sixth site on the octahedron serves as the catalytic center.
The corrin ring resembles the porphyrin ring, which occurs in hemoglobin. Both feature four pyrrole-like subunits organized into a ring. In contrast to porphyrins, corrins lack one of the carbon groups that link the pyrrole-like units. Thus the large ring has 19 carbons, whereas porphyrins have 20. Also, the pyrrole-like rings in corrin are fully saturated "edge-carbon" centers. Because of the high number of sp3 carbon centers, corrins are more flexible than porphyrins and are not as flat. They do not have a full conjugated character around the entire ring. Instead, the ring has a kind of "3/4" conjugation.