1. Hemoglobin F (HbF), known as fetal hemoglobin, is the predominant hemoglobin in the human fetus at 8 weeks of gestation. Because it is gradually replaced by normal HbA, at birth it constitutes approximately 60% of the total hemo- globin, decreasing to less than 1% by 6 months of age. In comparison to normal, adult HbA comprised of two a-chains and two ẞ-chains, HbF is comprised of two a-chains and two y-chains (gamma-chains). Important differences in the amino acid sequence of y-chains compared to ẞ-chains concern the residues that bind 2,3-bisphosphoglycerate (2,3-BPG), as shown in the table on the right. In addition to the sequence differences in the table, the N-terminal amine group of the y- chain is acetylated having an -NH-COCHз structure com- pared to the -NH3* group of ẞVal-1 in HbA. (a) (' N His 2 His 143 2, 3-BPG B₂ subunit Decide on the basis of the sequence comparison in the table how the interaction of 2,3-BPG with HbF differs from that with HbA. Explain what physiological advantage this difference provides to the fetus. (b) | Several diseases in humans are associated with high levels of HbF, e.g., hereditary persistence of hemoglobin F; ẞ-thalassemia in which there is deficient synthesis of ẞ- chains; a-thalassemia in which there is a deficiency of a-sub- units resulting in formation of ẞ4 molecules (known as HbH) a that have high O2-binding affinity and do not undergo a con- formational change upon O2 dissociation; sickle cell anemia, etc. Under conditions of high concentrations of HbF, O2 re- lease to tissues is controlled by the O2-binding properties of HbF. In efforts to alleviate the ischemic conditions generated by high concentrations of HbF with high O2 affinity, allosteric modulators of hemoglobin have been synthesized that lower the O2 affinity of both HbA and HbF. RSR-4 shown in the di- agram on the right is an allosteric modulator of HbA and HbF, lowering their O2 affinity when bound. The O2 equilibrium dis- sociation curves on the right illustrate the effect of the allo- steric modulator RSR-4 on the O2 affinity of HbF. The table below summarizes the p50 values of HbF at pH 7.4 as Curve B1 RSR-4 (μM) 0 B2 250 B3 500 p50 (mmHg) 19 26 44 Lys 82 Lys 82 B₁ subunit His 2 His 143 Sequence position ẞ-subunit y-subunit (HbA) (HbF) 1 Val His 2 His Phe 82 Lys Gln His Ser CI 143 0 NH−C− CH2 Ө CH3 0- COO- CH3 [2-[4-[[(3,5-dichloroanilino) carbonyl]-methyl]- phenoxy]-2-methylpropionic acid] = RSR-4 1.0 0.9 0.8 0.7 Y 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 40 80 120 160 P(02) [mm Hg] a function of the concentration of the allosteric modulator RSR-4. Assume that HbF constitutes at least 90% of the hemoglobin content of circulating erythrocytes (RBC ) in a patient with ẞ-thalassemia. Considering that 20 mmHg represents the O2 pressure in venous blood and that 100 mmHg correspondingly represents the O2 pressure in the lungs, estimate the frac- tion of O2 bound by HbF in the lungs that will be released by the binding of RSR-4 as the erythrocytes enter venous blood for each of the O2 equilibrium dissociation curves. For your estimates use the en- larged copy of the O2 equilibrium dissociation curves in the answer template. (c) ( ) What other modulating factor in the blood stream will add to the decrease in the O2-binding affinity of HbF?