Studies of oxygen transport in pregnant mammals show that the O2-saturation curves of fetal and maternal blood are markedly different when measured under the same conditions. Fetal erythrocytes contain a structural variant of hemoglobin, HbF, consisting of two α and two γ subunits (α2γ2), whereas maternal erythrocytes contain HbA (α2β2).
(a) Which hemoglobin has a higher affinity for oxygen under physiological condition, HbA or HbF? Explain.
(b) What is the physiological significance of the different O2 affinities?
(c) When all the BPG is carefully removed from samples of HbA and HbF, the measured O2-saturation curves (and consequently the O2 affinities) are displaced to the left. However, HbA now has a greater affinity for oxygen than does HbF. When BPG is reintroduced, the O2-saturation curves return to normal, as shown in the graph below. What is the effect of BPG on the O2 affinity of hemoglobin? How can the above information be used to explain the different O2 affinities of fetal and maternal hemoglobin?