Heme is an organic compound that is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is an iron-containing molecule that combines with globin proteins to form hemoglobin, which carries oxygen in red blood cells from the lungs to the rest of the body. Heme is a coordination complex consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to one or two axial ligands. Heme is made from four pyrroles, which are small organic molecules. Heme is named for the Greek derivative for “blood”, which is where it was first identified.

Heme is biosynthesized in both the bone marrow and the liver. Within cells, heme levels are maintained by a balance of heme production, degradation, and distribution. The mechanisms of heme production and degradation have been intensively studied over several decades. However, the critical steps and location within the red blood cell where heme production takes place are largely unknown.

Heme is most commonly recognized as a component of hemoglobin, the red pigment in blood, but is also found in a number of other biologically important hemoproteins such as myoglobin, cytochromes, catalases, heme peroxidase, and endothelial nitric oxide synthase.