Indicate the metal, the atom of which is part of the hemoglobin molecule.

In the human body, iron mainly exists in complex forms associated with protein (hemoprotein) in the form of heme compounds (hemoglobin or myoglobin), heme enzymes, or non-chemical compounds (flavin-iron and ferritin enzymes). The body requires iron to synthesize its oxygen transport proteins, in particular hemoglobin and myoglobin, and to form heme enzymes and other iron-containing enzymes involved in electron transport and oxidative contractions. Nearly two-thirds of iron is found in hemoglobin present in circulating red blood cells, 25% is found in easily mobilized iron storage, and the remaining 15% is associated with myoglobin in muscle tissue and in a variety of enzymes involved in oxidative metabolism and many other cellular functions.