Chap. 5. Problem 1. The lower the K d of a protein for a ligand, the higher its affinity. Thus, Protein B has a higher affinity for ligand X than Protein A. Because K a = 1/K d , the K a of Protein A for ligand X is 10 6 M -1 , and the K a of Protein B for ligand X is 10 9 M -1.
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The lower the Kd of a protein for a ligand, the higher its affinity. Thus, Protein B has a higher affinity for ligand X than Protein A. Because Ka = 1/Kd, the Ka of Protein A for ligand X is 106 M-1, and the Ka of Protein B for ligand X is 109 M-1.
Hb’s affinity for O2 is regulated by the allosteric modulators, H+, CO2, and BPG. In each case, the binding of these modulators to Hb decreases its affinity for O2 (i.e., shifts the O2-binding curve to the right). Thus, (a) a drop in the pH of blood plasma from 7.4 to 7.2 decreases the affinity of Hb for O2; (b) a decrease in the partial pressure of CO2 in the lungs from 6 kPa to 2 kPa increases the affinity of Hb for O2; (c) an increase in the BPG level from 5 mM to 8 mM decreases the affinity of Hb for O2; and (4) an increase in CO in the atmosphere decreases the affinity of Hb for O2.
A sigmoidal-shaped O2 binding curve is indicative of O2 binding cooperativity in Hb. Because O2 binding cooperativity is not observed for the isolated subunit of Hb, it suggests that cooperativity in the Hb tetramer arises from interactions between subunits in the tetramer.
Because the O2 binding curve for HbF is shifted to the left compared to HbA, HbF has a higher affinity for O2. At identical O2 concentrations, HbF binds O2 more avidly than HbA.
The differences in the O2 binding affinities for the two Hbs ensures that O2 will be transferred from maternal HbA to HbF in the placental circulation.
BPG decreases the affinity of both Hbs for O2, however, the affinity of HbA for O2 is more strongly affected. This indicates that HbA binds BPG with higher affinity than HbF. This could account for the differences observed in the O2 affinities of the two Hbs.
Because tetrameric Hb binds O2 less tightly due to interactions between subunits, Hb ß dimers are more likely to bind O2 with higher affinity than native Hb. In addition, Hb dimers would probably not bind BPG, and therefore this should increase their affinity for O2 compared to native Hb. Normally, BPG stabilizes the low affinity T state of tetrameric Hb.