Hemoglobin (abbreviated Hb) is a protein that is responsible for the transport o

Question

Hemoglobin (abbreviated Hb) is a protein that is responsible for the transport of oxygen in the blood of mammals. Each hemoglobin molecules contains four iron atoms that are the binding sites for O2 molecules. The oxygen binding is pH-dependent. The relevant equilibrium reaction is

HbH4+ (aq) + 4O2 (g) Hb(O2)4 (aq) + 4H+ (aq)

Use Le Chatlier’s principle to answer the following:

a. What form of hemoglobin, HbH4+ or Hb(O2)4, is favored in the lungs? What form is favored in the cells?

b. When a person hyperventilates, the concentration of CO2 in the blood is decreased. How does this affect the oxygen binding equilibrium? How does breathing into a paper bag help to counteract this effect?

c. When a person has suffered a cardiac arrest, injection of sodium bicarbonate solution is given. Why is this necessary? (Hint: CO2 blood levels increase during cardiac arrest.)

Explanation / Answer

Ans. Le Chatelier’s principle states “if a dynamic equilibrium is disturbed by changing the conditions (Concentration, Volume, Pressure, temperature, etc.), the position of equilibrium shifts to counteract the change to reestablish an equilibrium”.

#A. In lungs, [O2] is highest compared to rest of the tissues and organs. So, the equilibrium shifts to the right [ more Hb-(O2)4 formed] so that the equilibrium constant is least affected.

#B.1. In case of hyperventilation (excess O2), the O2 -binding equilibrium shifts to the right similar as it does in lungs.

B.2. Hyperventilation is the state of inhalation of excess O2 (more than required). As a result, there is lesser amount of CO2 in blood.

            CO2 + H2O -<----1----> H2CO3 <-----2----> H+ + HCO3-

When [CO2] is reduced, the equilibrium for reaction 2 shifts towards the left. As a result, less number of protons is available in blood- it causes increase in pH above the normal 7.4 (called respiratory alkalosis, say pH 7.8, due to hyperventilation) due to loss of protons in blood.

Breathing in paper bag (a closed paper bag in which air is inhaled-exhaled few times at certain intervals). During the process, exhaled air is trapped in bag simultaneously with less fresh air incoming, causing relatively high [CO2] and low [O2] in the bag. When breather in again, more CO2 but less O2 enters the lungs. Therefore, breathing in bag increases CO2 in blood. Increase in [CO2] further shifts the equilibrium to the right of reaction 2, thus restoring the pH back from alkaline to 7.4.

#C. When [CO2] is high, the equilibrium shifts to the right of reaction 2- it causes acidosis (drop in pH below 7.4, say to 7.0) due to presence of more protons.

When sodium bicarbonate (NaHCO3) is injected, it forms bicarbonate ion (HCO3).

NaHCO3(aq) <-----3------> Na+(aq) + HCO3-(aq)

The resultant bicarbonate ion associates with H+ to form carbonic acid- removal of H+ thus increases pH from acidic to 7.4. The resultant carbonic acid further dissociates into CO2 and H2O (equilibrium shifts to right for reaction 2, 1). CO2 formed is exhaled out though respiration.

            HCO3-(aq, from injection) + H+ (excess, due to higher CO2) -- 4--> H2CO3 (aq)

            H2CO3 (aq) ---5----> CO2 (exhaled out) + H2O

Note that reaction 4 is reverses of reaction 2, and 5 is reverse of reaction 1.

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