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.