any of a group of pyrrole derivatives, found in cytoplasm, that combine with iron and magnesium to form heme and chlorophyll, respectively
See porphyrin in American Heritage Dictionary 4
(pôrˈfə-rĭn)
noun
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.