7.1 Myoglobin and Hemoglobin Bind Oxygen at Iron Atoms in Heme

Myoglobin is a largely α-helical protein that binds the prosthetic group heme. Heme consists of protoporphyrin, an organic component with four linked pyrrole rings, and a central iron ion in the Fe²⁺ state. The iron ion is coordinated to the side chain of a histidine residue in myoglobin, referred to as the proximal histidine. One of the oxygen atoms in O₂ binds to an open coordination site on the iron. Because of partial electron transfer from the iron to the oxygen, the iron ion moves into the plane of the porphyrin on oxygen binding. Hemoglobin consists of four polypeptide chains, two α chains and two β chains. Each of these chains is similar in amino acid sequence to myoglobin and folds into a very similar three- dimensional structure. The hemoglobin tetramer is best described as a pair of αβ dimers.

7.2 Hemoglobin Binds Oxygen Cooperatively

The oxygen-binding curve for myoglobin reveals a simple equilibrium binding process. Myoglobin is half-saturated with oxygen at an oxygen concentration of approximately 2 torr. The oxygen-binding curve for hemoglobin has an “S”-like (sigmoid) shape, indicating that the oxygen binding is cooperative. The binding of oxygen at one site within the hemoglobin tetramer affects the affinities of the other sites for oxygen. Cooperative oxygen binding and release significantly increase the efficiency of oxygen transport. The amount of the potential oxygen- carrying capacity utilized in transporting oxygen from the lungs (with a partial pressure of oxygen of 100 torr) to tissues (with a partial pressure of oxygen of 20 torr) is 66% compared with 7% if myoglobin had been used as the oxygen carrier.

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The quaternary structure of hemoglobin changes on oxygen binding. The structure of deoxyhemoglobin is referred to as the T state. The structure of oxyhemoglobin is referred to as the R state. The two αβ dimers rotate by approximately 15 degrees with respect to one another in the transition from the T to the R state. Cooperative binding can be potentially explained by concerted and sequential models. In the concerted model, each hemoglobin adopts either the T state or the R state; the equilibrium between these two states is determined by the number of occupied oxygen-binding sites. Sequential models allow intermediate structures. Structural changes at the iron sites in response to oxygen binding are transmitted to the interface between αβ dimers, influencing the T-to-R equilibrium.

Red blood cells contain 2,3-bisphosphoglycerate in concentrations approximately equal to that for hemoglobin. 2,3-BPG binds tightly to the T state but not to the R state, stabilizing the T state and lowering the oxygen affinity of hemoglobin. Fetal hemoglobin binds oxygen more tightly than does adult hemoglobin owing to weaker 2,3-BPG binding. This difference allows oxygen transfer from maternal to fetal blood.

7.3 Hydrogen Ions and Carbon Dioxide Promote the Release of Oxygen: The Bohr Effect

The oxygen-binding properties of hemoglobin are markedly affected by pH and by the presence of carbon dioxide, a phenomenon known as the Bohr effect. Increasing the concentration of hydrogen ions—that is, decreasing pH—decreases the oxygen affinity of hemoglobin, owing to the protonation of the amino termini and certain histidine residues. The protonated residues help stabilize the T state. Increasing concentrations of carbon dioxide decrease the oxygen affinity of hemoglobin by two mechanisms. First, carbon dioxide is converted into carbonic acid, which lowers the oxygen affinity of hemoglobin by decreasing the pH inside the red blood cell. Second, carbon dioxide adds to the amino termini of hemoglobin to form carbamates. These negatively charged groups stabilize deoxyhemoglobin through ionic interactions. Because hydrogen ions and carbon dioxide are produced in rapidly metabolizing tissues, the Bohr effect helps deliver oxygen to sites where it is most needed.

7.4 Mutations in Genes Encoding Hemoglobin Subunits Can Result in Disease

Sickle-cell disease is caused by a mutation in the β chain of hemoglobin that substitutes a valine residue for a glutamate residue. As a result, a hydrophobic patch forms on the surface of deoxy (T-state) hemoglobin that leads to the formation of fibrous polymers. These fibers distort red blood cells into sickle shapes. Sickle-cell disease was the first disease to be associated with a change in the amino acid sequence of a protein. Thalassemias are diseases caused by the reduced production of either the α or the β chain, yielding hemoglobin tetramers that contain only one type of hemoglobin chain. Such hemoglobin molecules are characterized by poor oxygen release and low solubility, leading to the destruction of red blood cells in the course of their development. Red-blood-cell precursors normally produce a slight excess of hemoglobin α chains compared with β chains. To prevent the aggregation of the excess α chains, they produce α-hemoglobin stabilizing protein, which binds specifically to newly synthesized α-chain monomers to form a soluble complex.

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