Question

7. (15 pts) One of the allosteric effectors of hemoglobin (Hb) is 2,3 bisphosphoglycerate (2,3 BPG).

a) Where does 2,3 bisphosphoglycerate bind in hemoglobin?

b) What is its effect on the conformational state of hemoglobin.

c) What is the effect of increasing the [2,3 BPG] on the Kd values of hemoglobin at atmospheric pO2, and at the pO2 present in tissues?

d) Will 2,3 BPG bind myoglobin? If it does, what will be the effect on the saturation curve?

Answer 1

(a) 2,3-bisphosphoglycerate or 2,3-BPG is a three carbon sugar formed in the process of cell glycolysis.

Negatively charged phosphate groups are attched to the central carbon. It binds to deoxygenated hemoglobin with greater affinity. Deoxygenated hemoglobin is typically found in tissues than to oxygenated hemoglobin found in the lungs. Deoxyhemoglobin has a cavity between ß  subunits to which 2,3-BPG binds via electrostatic forces. 2,3-BPG interacts with the positively charged species on the amino acids on each beta chain within the deoxyhemoglobin. It forms salt bridges with lysine and histidine residues in the ß subunits of hemoglobin.

b) When 2,3-BPG binds to deoxyhemoglobin, the conformation of the deoxyhemoglobin is altered. A greater concentration of oxygen molecules is needed to bind to the free sites in the deoxyhemoglobin. Selectively binding to deoxyhemoglobin, 2,3-BPG stabilizes the T state (low oxygen affinity state) conformation, making it harder for oxygen to bind Hb and more likely to be released to adjacent tissues.

c) 2,3-BPG when binds to deoxyhemoglobin reduces the ability of deoxyhemoglobin to bind oxygen. The presence of 2,3-BPG releases more oxygen to the tissues. At a partial pressure of 40 mm Hg, there is less percent saturation of Hb (i.e. in the presence of 2,3-BPG). So, whe 2,3-BPG binds to Hb, it makes it less likely for Hb to bind oxygen and hence less Hb is saturated.

K_d = dissociaton constant = [Hb] [O₂]/[HbO₂]

From the plot it is clear that at 40 mm Hg, in the presence of  2,3-BPG the percentage saturation of Hb is less or oxygen affinity of Hb is less in the tissues, i.e. [HbO₂] is less. So, K_d value will be more.

d) 2,3-BPG has no effect on myoglobin since behaviour of myoglobin is different.