how does iron bind to heme?
 The iron atom has a very strong covalent bond with the globular protein on the proximal histidine of the protein, on histidine's imidazole ring. Even with this strong covalent bond, the iron still has the ability to bind with oxygen via a dipolar or coordinate covalent bond. That is the electrostatic bond that hemoglobin is involved in, not the one that binds heme to the globin proteins in the first place. So the iron atom is not only involved in keeping the heme group attached to the globin proteins, but it is the functional oxygen-carrying component of the hemoglobin molecule. 