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.
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.