tetrapyrroles

Tetrapyrroles are a class of organic compounds characterized by a macrocyclic ring formed by four pyrrole units linked by methine bridges. The macrocycle often coordinates a central metal ion, enabling a range of essential biological roles. Common metals include iron in heme and magnesium in chlorophyll, which determine the molecule’s function.

Major members of the family include porphyrins, such as heme in blood and myoglobin and chlorophyll in plants and algae; siroheme, a cofactor in sulfite and nitrite reductases; and cobalamins (vitamin B12), which contain a related corrin ring. Linear tetrapyrroles, called bilins, also occur, including phycoerythrobilin and phycocyanobilin that act as pigments in photosynthetic organisms.

Biosynthesis converges on the porphyrin macrocycle via condensation of four pyrrole units to form the porphyrin core. In animals and many microbes, the pathway proceeds from δ-aminolevulinic acid to porphobilinogen, uroporphyrinogen, coproporphyrinogen, and protoporphyrin IX, before insertion of iron to yield heme. In plants and algae, magnesium is inserted and the ring is elaborated with side chains to produce chlorophyll, accompanied by additional modifications required for light harvesting.

Functions are diverse: heme and its metalloproteins catalyze oxygen transport, storage, and electron transfer; chlorophylls harvest light energy for photosynthesis; siroheme participates in electron transfer in assimilatory pathways; cobalamins act as cofactors in methyl transfer and rearrangement reactions. Due to their central biological roles, tetrapyrroles are prominent in biochemistry, medicine, and agriculture.