 Proteins based on cobalamin use corrin to hold the cobalt . Coenzyme B12 features a reactive C @-@ Co bond that participates in the reactions . In humans , B12 has two types of alkyl ligand : methyl and adenosyl . <unk> promotes methyl ( <unk> ) group transfers . The adenosyl version of B12 catalyzes rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent , X , which may be a carbon atom with substituents , an oxygen atom of an alcohol , or an amine . <unk> coenzyme A <unk> ( <unk> ) converts <unk> @-@ CoA to Su @-@ CoA , an important step in the extraction of energy from proteins and fats . 
