Porphyrins are water-soluble, nitrogen-containing organic compounds of great biological importance which are found in a variety of pigments, including hemoglobin, chlorophyll a and b, the cytochromes (concerned with respiration in cells), and the enzymes catalase and peroxidase. The bile pigments bilirubin and biliverdin are also related.

Porphyrins are variations on the basic structure of porphin. A variety of side groups can be attached at 8 different locations around the molecule and a complex, called a chelate, can be formed with a centrally located, heavy metal atom. In hemoglobin this metal is iron, while in chlorophyll it is magnesium.