Binding of Oxygen to Heme
The heme prosthetic group in the myoglobin and hemoglobin is made from a protoporphyrin IX ring structure with the iron atom in ferrous (Fe2+) oxidation state as shown in the figure given below. This Fe2+ bonds with the four nitrogen atoms in center of protoporphyrin ring and forms the two additional bonds on either side of the plane of protoporphyrin ring. One of these is to a histidine residue that lies eight residues along helix F of hemoglobin, the proximal histidine (His F8). The sixth bond is to one of the oxygen atoms in a molecule of O2 as shown in figure given below. Near to where the O2 binds to the heme group is the other histidine remains, the distal histidine. This serves two very significant functions. At first, it prevents heme groups on the neighboring hemoglobin molecules coming into contact with each other and oxidizing to the Fe3+ state in which they cannot bind O2. Second, it prevents the carbon monoxide (CO) binding with most favorable con?guration to Fe2+ , thus lowering the af?nity of heme for CO. This is significant because once CO has bound irreversibly to the heme, the protein cannot bind O2. Thus, the oxygen binding site in hemoglobin and myoglobin is only small part of the entire protein; the polypeptide chain modulates function of heme prosthetic group.