Question

Regarding the effects of the affinity of hemoglobin when 2,3-biphosphoglycerate (BPG) is regulating, analyze the biochemical mechanisms that occur to cause the catch and release of oxygen in this manner.

Which of the following structural changes occur when deoxyhemoglobin binds to oxygen? Choose the two correct answers and briefly explain why the other two are incorrect:

1.) The proximal histidine (His F8) moves helix F towards the planar heme.

2.) The heme releases CO2 from the other subunits.

3.) A salt bridge is disrupted between the C-terminal His (HC3) on one beta-subunit and a Lys side chain on the adjacent alpha-subunit.

4.) One subunit converts to the tense conformation.

Answer 1

1) 2, 3 BPG is binds to oxyhaemoglobin and shift this curve to right side, thus release oxygen from it. 2, 3 BPG is synthsized by 1,3 BPG, intermediate of glycolysis cycle.

This 2, 3BPG preferentially binds with de oxy Hb in the cavity of globin chain ( through ionic intrtaction ) and stabilizeses its T conformation , thus it allow release more and more oxygen from it at tissue level. While in lungs , due to increse concentration of oxygen it bind with deoxy Hb . Thus it converts T form to again R form and eject 2, 4BPG from its pocket .

2) option 1 and 3 are correct

Option 1 explanation

Upon oxygenation , iron atom moves into the porphyrin plane and histidine residue is drawn closer to the heme group. Thus this movement of the histidine residue later shifts the other amino acids position also which are located near the histidine. In this way by attaching of oxygen to Hb changes the or altered the interface between the four subunits and one oxygen attach to it which later enhances its affinity to other oxygen molecules.

Option 3 explanation

Upon binding of oxygen molecule to deoxy Hb, it destabilizes some of the the salt bridges between the subunits. Thus by doing this it cause conversion of T to R form thus it enhances , affinity of Hb to coming next oxygen and responsible for cooperative behavior.

Option 2 is wrong- heme never bind directly to Co2, it always binds with H + ions, which are releases after binding with oxygen.

Option 4 is also wrong upon oxygenation T form convert in R

Not Hb convert in to T form.