Heme

Heme is an iron-containing porphyrin complex that serves as a tightly bound component, or prosthetic group, of many proteins. Its central iron atom is coordinated within a ring-shaped porphyrin structure. A major form in humans is heme B, also called protoheme IX, which consists of iron coordinated within protoporphyrin IX and is the heme found in hemoglobin and myoglobin .^1,2

The properties of heme depend partly on the protein surrounding it and on the state of its iron atom. In hemoglobin and myoglobin, ferrous iron in heme can reversibly bind oxygen, enabling oxygen transport and storage. In other heme proteins, the iron can participate in electron-transfer or oxidation-reduction reactions. Heme is also an essential component of proteins such as cytochromes, cytochrome P450 enzymes, catalase, and nitric oxide synthase .²

Human cells synthesize heme through a multistep pathway that takes place across the mitochondria and cytosol. The final step is catalyzed by ferrochelatase, which inserts ferrous iron into protoporphyrin IX to produce heme B. Other forms, including heme A and heme C, differ in their chemical structure or in how they are associated with their proteins .^1,2

Heme is not the same as hemoglobin. Heme is the relatively small iron-containing molecular group, whereas hemoglobin is a much larger protein composed of globin chains, each associated with a heme group. Heme also occurs in many proteins unrelated to oxygen transport .^1,2