Iron is also the most abundant transition metal in the human body and it is used in various processes like oxygen transport and ATP production. It plays a key role in the function of enzymes like cytochrome a, b and c as well as iron-sulfur complexes which play an important role in ATP production. It is present in every type of cell in the brain as the brain itself has a very high energy requirement and by extension a very high iron requirement. In animals, iron plays a very important role in transporting oxygen from the lungs to tissues and CO from tissues to the lungs. It does this via two important transport proteins called hemoglobin and myoglobin. Hemoglobin in the blood transports oxygen from the lungs to myoglobin in tissues. Both proteins are tetramer complexes with iron protein complexes called hemes built into each subunit of the tetramer. The oxygen binds to the iron in the heme via affinity-based binding or liganding and dissociates from the protein once it has reached its destination. Iron can also be a potential carcinogen in three ways; first being the production of hydroxyl radicals. Ferric ions can be reduced via superoxide and the product can be reoxidized via peroxide to form hydroxyl radicals. Hydroxyl radicals and other reactive oxygen species when generated near DNA can cause point mutations, cross-linkage and breaks. The second being the bolstering of the growth of neoplastic cells by suppressing host defenses. Excessive iron inhibits the activity of CD lymphocytes and suppresses the tumoricidal activity of macrophages. The third way it can act as a carcinogen is by functioning as an essential nutrient for unrestricted proliferation of tumor cells.