Protein involved in the synthesis of cobalamin. Cobalamin, which is synthesized by microorganisms, has equatorial sites occupied by a modified porphyrin ring system, with two of the four pyrrol rings fused directly (without an intervening methine bridge). The modified porphyrin system binds a cobalt(III) ion in the center, and this is called a corrin ring system. One axial site is occupied usually by an intramolecularly-bound dimethylbenzimidazole nucleotide and the other axial site is occupied by a number of different ligands such as water (aquacobalamin), cyanide (cyanocobalamine=vitamin B12), glutathione (glutathionylcobalamine), 5'deoxyadenosine (adenosylcobalamine=coenzyme B12) or a methyl group (methylcobalamin). Vitamin B12, for instance, is a prosthetic group of certain mammalian enzymes, where it is essential for the normal maturation and development of erythrocytes. A deficiency in the diet or more frequently the failure to absorb the vitamin B12 give rise to pernicious anemia.