Deoxyhemoglobin, proton binding

The H+ concentration also affects the abihty of deoxyhemoglobin to bind oxygen. When deoxyhemoglobin binds oxygen, it releases protons. These are quickly removed in a reaction catalyzed by the enz)une carbonic anhydrase. 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

Deoxyhemoglobin has a higher affinity for protons than does oxyhemoglobin, so that the binding of protons competes with the binding of oxygen (though at different sites) ... 

Oxygen and H are not bound at the same sites in hemoglobin. Oxygen binds to the iron atoms of the hemes, whereas H binds to any of several amino acid residues in the protein. A major contribution to the Bohr effect is made by His (His HC3) of the 3 subunits. When protonated, this residue forms one of the ion pairs—to Asp (Asp FGl)—that helps stabilize deoxyhemoglobin in the T state (Fig. 5-9). The ion pair stabilizes the protonated form of His HC3, giving this residue an abnormally high piCa in the T state. The falls to its normal value of 6.0 in the R state because the ion pair cannot form, and this residue is largely unpro-... 

---