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Chemistry · Glossary

What is Porphyrins and haem?

Also known as: porphyrin · haem

Definition 24.5 University Chemistry — Year 3 · Chapter 24 — Bioinorganic Chemistry

A porphyrin is a large, flat, aromatic macrocycle of four pyrrole rings joined by methine bridges, whose four nitrogens bind a metal ion at its centre. Haem is the iron complex of protoporphyrin IX, the cofactor of haemoglobin, myoglobin and the cytochromes.

The haem iron seen edge-on, its displacement from the ring drawn to scale (1 Å as 1.35 cm; the other distances are schematic). In deoxyhaemoglobin the high-spin iron(II) lies 0.34\, Å below the mean plane of the four pyrrole nitrogens, on the side of the proximal histidine; in oxyhaemoglobin it is within 0.05\, Å of the plane, with O2 bound end-on and bent (distances computed from crystal structures).
The haem iron seen edge-on, its displacement from the ring drawn to scale (1 Å as 1.35 cm; the other distances are schematic). In deoxyhaemoglobin the high-spin iron(II) lies 0.34 A˚0.34\,\text{Å} below the mean plane of the four pyrrole nitrogens, on the side of the proximal histidine; in oxyhaemoglobin it is within 0.05 A˚0.05\,\text{Å} of the plane, with OX2\ce{O2} bound end-on and bent (distances computed from crystal structures).
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