Hemoglobin is an oxygen-transport protein found in virtually all vertebrates. Hemoglobin A is the main type found in human adults. It is a tetramer consisting of two alpha and two beta subunits. Each of the four monomeric units contain a heme prosthetic group in which a ferric cation is bound between four nitrogen atoms of a porphyrin ring. Along with a histidine, the apo form has five ligands surrounding the iron atom. Oxygen binds to the empty sixth position to form an octahedral complex in the holo form. Oxygen binding is fully cooperative for each of the subunits because as the first oxygen binds to one of the four heme groups, the protein undergoes a drastic conformational change that sharply increases the oxygen affinity of the other three subunits.
