 Although far less common than other metalloproteins ( e.g. those of zinc and iron ) , cobaltoproteins are known aside from B12 . These proteins include methionine aminopeptidase 2 , an enzyme that occurs in humans and other mammals that does not use the corrin ring of B12 , but binds cobalt directly . Another non @-@ corrin cobalt enzyme is nitrile hydratase , an enzyme in bacteria that metabolize nitriles . 
