Hemoglobin
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Hemoglobin is an oxygen-transport protein. Hemoglobin is an allosteric protein. It is a tetramer composed of two types of subunits designated α and β, whose stoichiometry is α2β2. The four subunits of hemoglobin sit roughly at the corners of a tetrahedron, facing each other across a cavity at the center of the molecule. Each of the subunits contains a heme prosthetic group. The heme molecules give hemoglobin its red color.
Each individual heme molecule contains one Fe2+ atom. In the lungs, where oxygen is abundant, an elemental oxygen molecule binds to the ferrous iron atom of the heme molecule and is later released in tissues needing oxygen. The heme group binds oxygen while still attached to the hemoglobin monomer. The spacefill view of the hemoglobin polypeptide subunit with an oxygenated heme group shows how the oxygenated heme group is held within the polypeptide. Anchoring of the heme is facilitated by a histidine nitrogen that binds to the iron. A second histidine is near the bound oxygen. the "arms" (propanoate groups) of heme face are hydrophilic and face the surface of the protein while the hydrophobic portions of the heme are buried among the hydrophobic amino acids of the protein. We can also view the anchored heme with the hemoglobin polypeptide subunit shown in a spacefill representation.