An iron-containing molecule that binds with proteins as a cofactor or prosthetic group to form the haemoproteins. These are haemoglobin, myoglobin, and the cytochromes. Essentially, 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 iron(II) and iron(III) species.