How does oxygen bind to hemoglobin?
What is hemoglobin?
Hemoglobin is a high spin Fe (II) complex coordinated by the four nitrogen atoms of protoporphyrin and fifth position is bound to the nitrogen atom of an imidazole group in histidine residue. When human beings or animals inhales oxygen, hemoglobin bind to oxygen in lungs to form an oxyhemoglobin complex. It dissociates into Hb and oxygen in the tissues of human beings or animals. Therefore, it acts as an oxygen carrier in the blood.
In absence of oxygen, the iron (II) in hemoglobin is about 36 to 40 pm out of the plane of the nitrogen atoms in the direction of the histidine group. Upon binding of dioxygen, the iron atom moves within 13 pm of the plane. Upon binding of oxygen, hemoglobin pulls the histidine group within it. The dioxygen bound in a bent fashion to iron by making a hydrogen bond with the N-H bond of histidine residue given below the picture,
The iron in oxyhemoglobin may be taken as low spin Fe (III). The covalent radius of the high spin Fe (II) is presumably too large to fit the space in the plane of the four nitrogen atoms. The formation of Fe (III) oxygen adduct reduces the effective size of the iron atom. Therefore, it can now move closer into the plane of the porphyrin ring.
