Hemeproteins

Hemeproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. The role of the heme group is dictated by the environment created by the threedimensional structure of the protein. For example, the heme group of a cytochrome functions as an electron carrier that is alternately oxidized and reduced . In contrast, the heme group of the enzyme catalase is part of the active site of the enzyme that catalyzes the breakdown of hydrogen peroxide . In hemoglobin and myoglobin, the two most abundant hemeproteins in humans, the heme group serves to reversibly bind oxygen (O₂).
 Heme structure
Heme is a complex of protoporphyrin IX and ferrous iron (Fe²⁺), as shown in Figure 1. The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring. The heme Fe²⁺ can form two additional bonds, one on each side of the planar porphyrin ring. In myoglobin and hemoglobin, one of these positions is coordinated to the side chain of a histidine residue of the globin molecule, whereas the other position is available to bind O₂ (Fig.2). 

Figure1 : A. Hemeprotein (cytochrome c). B. Structure of heme.

Figure 2 : A. Model of myoglobin showing α-helices A to H. B. Schematic diagram of the oxygen-binding site of myoglobin.