Question

In the body, hemoglobin (Hb) is the substance responsible for transporting oxygen (O2) to tissues. Hemoglobin binds oxygen to produce oxyhemoglobin (HbO2):

Hb + O2 ? HbO2

Carbon monoxide (CO) can also bind to hemoglobin to produce carboxyhemoglobin (HbCO):

Hb + CO ? HbCO

The equilibrium constant for Reaction 2 (KCO) is about 210 times larger than the equilibrium constant for Reaction 1 (KO2). Thus, carbon monoxide binds to hemoglobin much more efficiently than oxygen.

KCO/KO2 = 210

a. Write a balanced equation for the displacement of oxygen from HbO2 by CO to produce HbCO. Write the equilibrium constant expression for your overall reaction.

b. Write an expression that relates the equilibrium constant for the overall reaction (i.e., from Question 1) to the equilibrium constants KCO and KO2.

c. Use your balanced equation from part (a) and your knowledge of equilibrium constants to explain why patients who are experiencing CO poisoning are generally given pure oxygen as a remedy.

Answer 1

Answer A:

Carbon monoxide poisoning

Carbon monoxide (CO) reacts with haemoglobin in the same way as oxygen:

Hb(aq) + CO(aq) HbCO(aq)

OR

Hb₄(aq) + 4CO(aq) Hb₄(CO)₄(aq)

The equilibrium constant for this reaction is very large, about 200 times greater than that for the oxygen reaction, so that there is very little haemoglobin left to react with oxygen. The tissue cells will be starved of oxygen because carbon monoxide rather than oxygen is being transported.

Answer: C

Hemoglobin consists of four heme groups that bind each other to form a tetramer.^[2] Oxygen binding to any one of these four sites causes a conformational change in the protein structure, such that binding to each of the other sites is facilitated. The bond between carbon monoxide and hemoglobin is incredibly strong. Carbon monoxide binds to hemoglobin approximately 200 times more tightly than does oxygen.^[3] Normally, oxygen would bind to hemoglobin in the lungs and be released in areas with low oxygen partial pressure (e.g. active muscles).^[4] When carbon monoxide binds to hemoglobin, it cannot be released as easily as oxygen. The slow release rate of carbon monoxide causes an accumulation of CO-bound hemoglobin molecules as exposure to carbon monoxide continues. Because of this, fewer hemoglobin particles are available to bind and deliver oxygen, thus causing the gradual suffocation associated with carbon monoxide poisoning.