 Although far less common than other <unk> ( e.g. those of zinc and iron ) , <unk> are known aside from B12 . These proteins include methionine <unk> 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 <unk> , an enzyme in bacteria that metabolize nitriles . 
