Tetrapyrrole
Tetrapyrroles are a class of chemical compounds whose molecules contain four pyrrole rings held together by direct covalent bonds or by one-carbon bridges (=(CH)- or -CH
2- units), in either a linear or a cyclic fashion. A pyrrole ring in a molecule is a five-atom ring where four of the ring atoms are carbon and one is nitrogen. In cyclic tetrapyrroles, lone electron pairs on nitrogen atoms facing the center of the macrocycle ring can bond or chelate with a metal ion such as iron, cobalt, or magnesium.
Some tetrapyrroles are the active cores of some compounds with crucial biochemical roles in living systems, such as hemoglobin and chlorophyll. In these two molecules, in particular, the pyrrole macrocycle ring frames a metal atom, that forms a coordination compound with the pyrroles and plays a central role in the biochemical function of those molecules.
Structure
Linear tetrapyrroles (called bilanes) include:[1]
- Heme breakdown products (e.g., bilirubin, biliverdin)
- Phycobilins (found in cyanobacteria)
- Luciferins as found in dinoflagellates and euphausiid shrimps (krill)
 |  |  |  |
| Bilirubin | Biliverdin | Phycoerythrobilin | Dinoflagellate (R = H) and krill (R = OH) luciferin |
Cyclic tetrapyrroles having four one-carbon bridges include:[2]
- Porphin, the simplest tetrapyrrole
- Porphyrins, including heme, the core of hemoglobin
- Chlorins, including those at the core of chlorophyll.
Cyclic tetrapyrroles having three one-carbon bridges and one direct bond between the pyrroles include:
- Corrins, including the cores of cobalamins, when complexed with a cobalt ion.
 |  |  |
| Heme group of hemoglobin with a porphin macrocycle. | The chlorin section of the chlorophyll a molecule. The green box shows a group that varies between chlorophyll types. | Cobalamin structure includes a corrin macrocycle. |
The tetrapyrrole portions of the molecules typically act as chromophores because of a high degree of conjugation in them. Therefore, these compounds are commonly colored.