What is the function of heme in hemoglobin?

The heme group is an essential component of the protein hemoglobin, which is responsible for transporting oxygen in the bloodstream. The structure of the heme group is a porphyrin ring with an iron ion at its center. The iron ion is bound to the porphyrin ring by four nitrogen atoms and is further coordinated to a fifth ligand, typically a water molecule. The sixth coordination site on the iron ion is available for binding to an oxygen molecule.

When an oxygen molecule binds to the iron ion in hemoglobin, the iron ion changes its oxidation state from Fe(II) to Fe(III). This change in oxidation state causes the heme group to become more electron-rich, leading to stronger interactions between the heme group and the protein structure of hemoglobin. The binding of oxygen to hemoglobin also induces conformational changes in the protein, which facilitate the cooperative binding of additional oxygen molecules to hemoglobin.

The cooperative binding of oxygen molecules to hemoglobin is essential for the efficient transport of oxygen in the bloodstream. At the low partial pressures of oxygen found in the lungs, hemoglobin binds to oxygen molecules and becomes saturated with oxygen. When hemoglobin reaches tissues where the partial pressure of oxygen is lower, the oxygen molecules dissociate from hemoglobin and are released into the tissue. The cooperative binding of oxygen molecules to hemoglobin ensures that oxygen is efficiently loaded onto hemoglobin in the lungs and unloaded in the tissues, where it is needed for cellular respiration.