haem

The iron-containing pigment (chemically a tetrapyrrole) which forms the oxygen-binding site of haemoglobin and myoglobin. It is also part of a variety of other proteins, collectively known as haem proteins, including the cytochromes.

A ring-structured porphyrin group containing ferrous iron. It is the prosthetic (non-protein) group of haemoglobin. Haem gives the blood its red colour and its ability to transport large amounts of oxygen. See also haem iron.