A meso position in a cyclic or linear tetrapyrrole is the bridge – a substituted or unsubstituted methine or methylene group (or a heteroatom) – that links two neighboring pyrrole rings. With the unrelated stereochemical classification meso compound, it only shares the meaning "between/middle of/amidst" of the original Greek word "mésos" (fig. 1).
Unsaturated bonds including meso positions play an important role in the natural degradation of the important biomolecules heme and chlorophyll (fig. 2): in heme, both the C4-C5 and C5-C6 bonds are cleaved by heme oxygenase to the linear tetrapyrrole biliverdin, with loss of the C5 meso atom as carbon monoxide; during chlorophyll degradation, pheophorbide a oxygenase (PAO) cleaves the C4-C5 bond of pheophorbide a, conserving the C5 meso atom as a formyl group in type-I phyllobilins (marked red in fig. 2). In further degradation processes, this is either kept, or deformylated by the enzyme CYP89A9 to type-II phyllobilins.
See also meso-Octamethylporphyrinogen Porphyrin Chlorin Corrin Corrinoid Seco (tetrapyrrole)
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