The prosthetic group
(Hb), consisting of a porphyrin
ring (protoporphyrin IX) with an iron group in the center. This is what binds the oxygen
by forming an iron-oxygen bond. The binding is mediated by the protein microenvironment
and the sixth ligand, which in Hb is a histidine
The oxygen binding changes the electronic state of the Fe(II)-haem, which therefore changes colour from cherry red (oxygenated) to dark red (deoxygenated). Unfortunately small molecules such as CO, NO and H2S bind to haem better, which accounts for their toxic properties.
PostScript: I reluctantly add a hardlink to the interesting alternative spelling of 'heme' used by the evil overlords of the world.