Any of a class of biologically important heterocyclic compound s of a characteristic chemical structure that includes four pyrrole groups (five-membered organic rings each containing a nitrogen atom) linked by additional carbon atoms to form a large flat ring.
As biological pigment s, they and closely related molecules are responsible for many of the vivid colours in living organisms, where they often occur combined with metal ion s and various substituents as coordination complexes (see compound ). These include the magnesium-containing chlorophyll s and the iron-containing heme group, a constituent (along with protein) of, e.g., hemoglobin , the cytochrome s, and the enzyme catalase. In medicine, porphyrins are used in conjunction with light, often a laser beam, to induce reactions in the body against cancer and other diseases.