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, either in a linear or cyclic fashion.
Linear tetrapyrroles, using three one-carbon bridges, include:
Cyclic tetrapyrroles, using four one-carbon bridges, include:
Corrins are cyclic tetrapyrroles that have replaced one of the conventional methine bridges with a direct interpyrrole bond.