The Bohr effect

The binding of O₂ to hemoglobin is usually affected by the concentration of H+ ions and CO₂ in surrounding tissue; the Bohr effect. In the actively metabolizing tissue, like muscle, the concentrations of the two substances are high relatively.This causes a shift of the O₂ dissociation curve for the hemoglobin to the right, promoting the release of O2. This comes about as there are H⁺ binding sites, primarily His146 in the β-chain, which has higher affinity for  binding H⁺ in deoxyhemoglobin than in the oxyhemoglobin. An increase in CO2 causes an increase in H+ because of the action of the enzyme carbonic anhydrase that catalyzes the reaction:

                                                                                  CO2   +   H2O     <------->  HCO^-  +  H⁺

 
Additionally, CO₂ can react with the primary amino groups in polypeptide chain to form the negatively charged carbamate. Again, this change from positive to negative charge favors the conformation of deoxyhemoglobin. On returning in blood to the lungs, concentrations of H+ and CO2 are lower relatively and that of O₂ higher, so that the process is made reverse and O2 binds to the hemoglobin. Therefore, it can be seen that not only hemoglobin carry O₂ but it carries CO₂ back to the lungs where it is expelled.2,3-Bisphosphoglycerate is a highly anionic organic phosphate molecule.

                                          Figure- 2,3-Bisphosphoglycerate

that is  it is present  in  erythrocytes  along  with  the  hemoglobin.  This  molecule promotes  release  of  O₂ from  hemoglobin  by  reducing the  affinity of  the protein for O₂. 2,3-Bisphosphoglycerate binds in small cavity in center of four subunits.  In oxyhemoglobin this cavity is very small for it, while in deoyhemoglobin it is sufficiently large to accommodate a single molecule of 2,3- bisphosphoglycerate. On binding the  central  cavity  of deoxyhemoglobin it develops ionic bonds with the positively  charged amino acid side-chains  in β- subunits, stabilizing  quaternary  structure.  H, CO and 2,3-bisphosphoglycerate are all the allosteric effectors as they favor the conformation  of deoxyhemoglobin and thus  promote  the release of O₂. As these three molecules act at the different sites, their effects observed are additive.