 The α-axial ligand 5,6-dimethylbenzimidazole (DMB) is involved in organizing the active site to enable histidine-610 to bond with Co, instead of DMB (the reason for the ‘DMB-off’/’His-on’ notation).Binding of histidine-610 residue increases the rate of homolytic β-axial ligand – Co bond breakage by a factor of 1012.Other important residues of methylmalonyl-CoA mutase include Histidine-244, which acts as a general acid near the substrate and shields the radical species from side reactions involving oxygen, Glutamate-370, whose hydrogen bond with the 2’-OH group of the ribose of the β-axial ligand forces interaction between the β-axial ligand radical species and the substrate, and tyrosine-89 which stabilizes reactive radical intermediates and accounts for the stereo-selectivity of the enzyme.