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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:

• Bilanes (e.g. bilirubin)
• Phycobilins (found in cyanobacteria)

Cyclic tetrapyrroles, using four one-carbon bridges, include:

• Porphyrins (e.g. heme)
• Chlorophylls

Corrins are cyclic tetrapyrroles that have replaced one of the conventional methine bridges with a direct interpyrrole bond.

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