Tetrapyrrole

Tetrapyrroles are compounds containing four pyrrole rings. With the exception of corrin, the four pyrrole rings are interconnected through one-carbon (methine or methylene) bridges, in either a linear or a cyclic fashion. Because of their ability to form metal complexes, these compounds are particularly important in biological systems.

Linear tetrapyrroles (bilanes), using three one-carbon bridges, include:
 * Heme breakdown products (e.g., bilirubin)
 * Phycobilins (found in cyanobacteria)

Cyclic tetrapyrroles, using four one-carbon bridges, include:
 * Porphyrins (e.g., heme)
 * Chlorins (e.g., Chlorophyll a)
 * Corrins are cyclic tetrapyrroles that have replaced one of the conventional methine bridges with a direct interpyrrole bond.