Tetrapyrroles are compounds containing four pyrrole

Pyrrole

Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

 rings. With the exception of corrin

Corrin

Corrin is an heterocyclic compound. It is the parent macrocycle related to substituted derivative that is found in vitamin B12. Its name reflects that it is the "core" of vitamin B12 .-Coordination chemistry:...

, the four pyrrole rings are interconnected through one-carbon (methine

Methine

In chemistry, methine is a trivalent functional group CH, derived formally from methane. The methine group consists of a carbon atom bound by two single bonds and one double bond, where one of the single bonds is to a hydrogen...

 or methylene

Methylene

Methylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....

) 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
  Bilirubin
  Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...
  
  )
• Phycobilin
  Phycobilin
  Phycobilins are chromophores found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads...
  
  s (found in cyanobacteria)

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

• Porphyrin
  Porphyrin
  Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...
  
  s (e.g., heme
  Heme
  A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
  
  )
• Chlorin
  Chlorin
  In organic chemistry, a chlorin is a large heterocyclic aromatic ring consisting, at the core, of three pyrroles and one pyrroline coupled through four methine linkages...
  
  s (e.g., Chlorophyll a
  Chlorophyll a
  Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...
  
  )
• Corrin
  Corrin
  Corrin is an heterocyclic compound. It is the parent macrocycle related to substituted derivative that is found in vitamin B12. Its name reflects that it is the "core" of vitamin B12 .-Coordination chemistry:...
  
  s are cyclic tetrapyrroles that have replaced one of the conventional methine bridges with a direct interpyrrole bond.