The heterocycle protoporphyrin and its derivatives can be found in the active sites of many enzymes. The coordination of metal ions in the macrocyclic ring depends on the size of the open space in the ring and the ionic radius of the metal ion. Depending on the size of the ion, metal ions can be coordinated in the plane of the ring or slightly above it (in-plane or out-of-plane). The macrocycle is then doubly deprotonated.


Variations of the ring skeleton allow a certain amount of variation of the ring size in order to adapt to the requirements of the metal ions (cf. Corrin).

Schematic depiction

Metal ions and their coordination to protoporphyrins


To the 3D-depiction of uncoordinated protoporphyrin

Metal Ion Ionic radius (pm) Suitability for coordination to tetrapyrrol ligands Examples
Be2+ 45 too small
Mg2+ 72 good fit (Chlorin) Chlorophyll
Ca2+ 100 too big
Fe2+ (low-spin) 61 a bit small O2 Transport
Fe2+ (high-spin) 70 too big (out-of-plane) O2 Transport
Co2+ (low-spin) 65 good fit (Corrin) Cobalamines
Ni2+ 69 good fit Coenzyme F430
Cu2+ 73 a bit big (not biol. relevant),
coordination to His, Cys
Zn2+ 74

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