Now, at the center of that protoporphinen is an inorganic atom, a metal atom, the iron atom. And this is what makes up the inorganic component of that Hen group. And it's this fe atom that is actually responsible to not only binding to the protein, but also to binding to that oxygen, as we'll see in just a moment. Now notice, as shown in this diagram, this fe atom is bound to four nitrogen atoms. We have 1234. Now, fe can have an oxidation state of positive six.