Heme

Heme is an iron-containing molecule made of a ring called protoporphyrin IX that holds a central iron atom. It is the oxygen-binding part of hemoglobin and myoglobin, and it serves as a cofactor in many other proteins, including cytochromes and catalase.

The iron atom sits at the center of the flat ring, held by four nitrogen atoms, and can bind a gas molecule above the ring. In hemoglobin, ferrous iron (Fe²⁺) reversibly binds O₂, which the blood carries from lungs to tissues. Oxidized iron (Fe³⁺) cannot bind oxygen, which is why methemoglobin is harmful. The same ring family gives chlorophyll its structure, with magnesium instead of iron.

Heme groups in cytochromes pass electrons in the electron transport chain of mitochondria, and the enzyme cytochrome P450 uses heme to process drugs and toxins. Catalase uses it to break down hydrogen peroxide. Heme also acts as a signal that regulates gene expression.

Old heme is degraded to biliverdin and then bilirubin, the yellow pigment that causes jaundice when it accumulates. The parasite behind malaria, Plasmodium, digests hemoglobin inside red blood cells and detoxifies the free heme by crystallizing it into hemozoin, a process targeted by some antimalarial drugs. Defects in heme synthesis cause the porphyrias.

• Chlorophyll — the magnesium analog
• Malaria — where heme detoxification is a drug target
• Cytochrome P450 — a heme enzyme family