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Figure 1
Chemical constitution of B12 cofactors. X = methyl: methylcobalamin (MeCbl); X = 5′-desoxyadenosyl: 5′-desoxyadenosylcobalamin (AdoCbl); X = cyanide: cyanocobalamin (CNCbl); X = water: aquocobalamin ion (AqCbl); X = e: cob(II)alamin (B12r). In several B12-dependent enzymes the intramolecular coordination of the dimethylbenzimidazole base to the Co atom is replaced by the coordination of a protein-derived imidazole to the metal centre (`Base-off His-on' constitution). At neutral pH, AqCbl occurs as a mixture of hydroxocobalamin and aquocobalamin ion.

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