page_title: Iron-binding proteins
section_title: Hemoglobin
hierarchical_section_title: Iron-binding proteins / Heme Proteins / Hemoglobin
caption_attribution_description: English: Animation of hemoglobin t-r state transformation
context_page_description: Iron-binding proteins are carrier proteins and metalloproteins that are important in iron metabolism and the immune response. Iron is required for life.
  Iron-dependent enzymes catalyze a variety of different biochemical reactions and can be divided into three broad classes depending on the structure of their active site: mononuclear iron clusters, Rieske centers, or heme centers. A well-known family of iron-dependent enzymes include oxygenases that facilitate hydroxyl group addition of one or both atoms from diatomic oxygen. Notable enzymes include tryptophan dioxygenase, ferredoxin, and 2-oxoglutarate dioxygenase.
context_section_description: 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.
  Hemoglobin has various affinities, depending on pH, structure, and CO₂ partial pressure. Fetal hemoglobin is a variant containing two gamma subunits instead of two beta subunits. Fetal hemoglobin is the predominant form up until the infant is several months old, and it has a greater oxygen affinity to compensate for the low oxygen tension of supplied maternal blood during pregnancy. Hemoglobin has a lower oxygen affinity at low pH. This allows for rapid dissociation as oxygenated hemoglobin is transported to cells throughout the body. Because of the CO₂ production and aqueous formation of carbonic acid in respiring cells, oxygenated hemoglobin dissociates in order to deliver the necessary oxygen to the cells. Hemoglobin has a binding affinity for carbon monoxide that is 250 times greater than for oxygen. This is the basis of carbon monoxide poisoning, as hemoglobin can no longer transport oxygen to cells.
caption: A visual depiction of the conformational change undergone by hemoglobin upon oxygen binding.