 This refers to the nature of the AdoCbl cofactor in the active site of methylmalonyl CoA.AdoCbl is composed of a central cobalt-containing corrin ring, an upper axial ligand (β-axial ligand), and a lower axial ligand (α-axial ligand).In methylmalonyl-CoA mutase, the β-axial ligand 5’-deoxy-5’-adenosine reversibly dissociated to give the deoxyadenosyl radical.