What are the components of hemoglobin?

Globin: Hemoglobin consists of four polypeptide subunits called globins. These subunits are arranged in a tetrameric structure, with two alpha and two beta globin chains. Each globin chain is made up of a linear sequence of amino acids that fold into a specific three-dimensional structure.

Heme: Each globin subunit binds to a heme group. Heme is an iron-containing porphyrin molecule. The iron ion in heme is bound to four nitrogen atoms of the porphyrin ring and to a fifth nitrogen atom of a histidine residue from the globin chain. The sixth coordination position of the iron ion is available to bind oxygen molecules.

Oxygen binding: Oxygen molecules bind to the iron ions of the heme groups in hemoglobin. Each hemoglobin molecule can bind up to four oxygen molecules, one to each of its four heme groups. The binding of oxygen to hemoglobin is a cooperative process, meaning that the binding of oxygen to one heme group increases the affinity of the other heme groups for oxygen. This cooperative binding allows hemoglobin to bind and release oxygen efficiently in response to changes in the oxygen concentration in the body.

Allosteric regulation: The binding of oxygen to hemoglobin is regulated by allosteric effectors. Allosteric effectors are molecules that bind to hemoglobin and change its affinity for oxygen. The most important allosteric effector is carbon dioxide. Carbon dioxide binds to hemoglobin and decreases its affinity for oxygen. This is why the release of oxygen from hemoglobin is increased in tissues where carbon dioxide levels are high, such as in respiring tissues.

Other allosteric effectors of hemoglobin include hydrogen ions (H+), chloride ions (Cl-), and certain organic phosphates, such as 2,3-bisphosphoglycerate (BPG). These allosteric effectors can bind to hemoglobin and either increase or decrease its affinity for oxygen, depending on the specific effector.