Chap. 5. Problem 1.
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.
Chap. 5. Problem 3.
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.
Chap. 5. Problem 6.
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.
Chap. 5. Problem 7.
(a) 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.
(b) 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.
(c) 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.
Chap. 5. Problem 9.
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.