porphyrin

Any of various organic compounds containing four pyrrole rings, occurring universally in protoplasm, and functioning as a metal-binding cofactor in hemoglobin, chlorophyll, and certain enzymes.

Any of a class of heterocyclic compounds containing four pyrrole rings arranged in a square; they are important in biochemistry in a form with a metal atom in the central cavity (hemoglobin with iron, chlorophyll with magnesium, etc.).

any of various pigments distributed widely in living tissues