An iron-containing molecule that binds with protein as a cofactor or prosthetic group to form the haemoproteins. These are haemoglobin, myoglobin, and cytochromes

Haem comprises a porphyrin with its four nitrogen atoms holding the iron (II) atom as a chelate.

This iron can reversibly bind oxygen (as in haemoglobin and myoglobin) or (as in the cytochromes) conduct electrons by conversion between the iron (II) and iron (III) series.