Porphyrin

**Semantic type:** Biologically Active Substance|Organic Chemical

**Definition:** A group of organic heterocyclic compounds that are comprised of four modified pyrrole subunits connected via methine bridges and form an aromatic macrocyclic structure, which has one or more side chains attached. Many porphyrins are naturally occurring pigments. Porphyrins with metal ions bound to the porphin ring can be cofactors for biological processes, such as oxygen transport by hemes and photosynthesis by chlorophyll.