Globular Protein

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 three-dimensional structure of the protein. In hemo globin and myoglobin, the two most abundant hemeproteins in humans, the heme group serves to reversibly bind oxygen.


Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+). 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 myo globin 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 oxygen.

Under homeostasis, the reactivity of heme is controlled by its insertion into the “heme pockets” of hemoproteins. Under oxidative stress however, some hemoproteins, e.g. hemoglobin, can release their heme prosthetic groups. The non-protein-bound (free) heme produced in this manner becomes highly cytotoxic, most probably due to the iron atom contained within its protoporphyrin IX ring, which can act as a Fenton's reagent to catalyze in an unfettered manner the production of free radicals. This property of free heme can sensitize a variety of cell types to undergo programmed cell death in response to pro-inflammatory agonists, a deleterious effect that plays an important role in the pathogenesis of certain inflammatory diseases such as malaria and sepsis. There is an association between high intake of heme iron sourced from meat and increased risk of colon cancer,  however, there is no solid evidence that this is a causal relationship. The heme content of red meat is 10-fold higher than that of white meat such as chicken.