Question

What is the effect of the following changes on the O2 affinity of hemoglobin? Please explain step by step

(a) A decrease in the partial pressure of CO2 in the lungs from 6 kPa (holding one’s breath) to 2 kPa (normal).
(b) An increase in the BPG level from 5 mM (normal altitudes) to 8 mM (high altitudes).

A. (a)decreases, (b)increases

B. (a)increases, (b)increases

C. (a)increases, (b)decreases

Answer 1

Ans. #A. A CO₂ molecule acts as allosteric modulator as it binds to Hb at a site other than O₂-binding site. The resultant complex, HbCO₂ (carbaminohemoglobin) triggers the transition of R-state into T-state (deoxyHb) by stabilizing the later.

In turn, a lower pCO₂ in turn favors the stabilization of R-state and causes left shift, i.e. increases O₂ affinity of Hb and also % Hb saturation.

Therefore, lowering pCO₂increases the affinity of Hb for O_2.

#B. 2,3-BPG is an allosteric modulator. It binds to N-terminal valine residue of β-subunit of HbA (α₂β₂). This binding causes transition of R-state (HbO₂) into T-state (deoxyHb) by stabilizing the T-state. Stabilization of T-state by 2,3-BPG decreases the affinity of Hb for O₂, which in turn triggers the dissociation of O₂ from HbO₂.