Heme is a molecule that is synthesized by the sequential actions of eight enzymes and is everywhere in nature. A heme is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are known as hemoproteins. As a prosthetic group, heme mediates reversible binding of oxygen by hemoglobin.
The heme synthesis begins in the mitochondria and continues in the cytoplasm. The process begins in the mitochondria because one of the precursors is found only there. Since this reaction is regulated in part by the concentration of heme, the final step which produces the heme is mitochondrial. Most of the intermediate steps are cytoplasmic. Heme is also catabolized to yield biliverdin, one atom of iron, and one molecule of carbon monoxide and is subsequently reduced to bilirubin.
There are several biologically important kinds of heme; the most common type is heme B; other important types include heme A and heme C. Isolated hemes are commonly designated by capital letters while hemes bound to proteins are designated by lower case letters.