Figure 1.

Comparison of the molecular structure of (a) hydroxocobalamin to (b) cobinamide (lower panel). Cobinamide lacks the dimethyl-benzimidazole ribonucleotide tail coordinated to the cobalt atom in the lower axial position. Whereas cobalamin has only an upper ligand binding site, cobinamide has both an upper and a lower ligand binding site. The dimethylbenzimidazole group on cobalamin has a negative trans-effect on the upper binding site, thereby reducing cobalamin’s affinity for ligands.¹¹ The combined effect is that each cobinamide molecule can bind two cyanide molecules and that cobinamide has a much greater affinity for cyanide than cobalamin.