Copper serves catalytic and structural roles in many essential enzymes in biology. In the context of catalytic activity, copper proteins function as electron or oxygen carriers, oxidases, mono- and dioxygenases and nitrite reductases. In particular, copper-containing enzymes include hemocyanins, one flavor of superoxide dismutase (SOD), metallothionein, cytochrome c oxidase, multicopper oxidase and particulate methane monooxygenase (pMMO).