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Heme Group of Hemoglobin and Myoglobin
The cells of our body use oxygen to produce ATP molecules via aerobic cellular respiration. This oxygen is brought to the cells by two proteins - myoglobin and hemoglobin. Both of these proteins have the ability to bind to oxygen molecules by using prosthetic groups called heme groups. Myoglobin consists of a single polypeptide chain and so contains a single heme group. On the other hand, hemoglobin consists of four polypeptide chains and so contains four heme groups. Each heme group consists of an organic component called protoporphyrin and an inorganic component that consists of a single iron atom. The iron atom is located at the center of the organic component and is bound to four different nitrogen atoms. The iron atom is attached to the protein via a proximal histidine residue. The other side of the iron can bind to oxygen. In the deoxygenated state, the iron atom is too large to fit into the center of the protoporphyrin. When the iron binds to oxygen however, the oxygen pulls away some of the electron density and allows the iron to fit snuggly into the center. The diatomic oxygen, being more electronegative, pulls away an electron and forms the superoxide ion. The iron atom goes from the ferrous state (+2) to the ferric state (+3). The negative charge on the superoxide is stabilized by the distal histidine residue of the protein.
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