Solutions to Selected End of Chapter 5 Problems
1. Affinity of binding and dissociation. Protein B (Kd = 10-9M) has the higher affinity because it will
bind ligand X well at a much lower concentration of X. It takes a thousand times higher amount of X
to get similar binding Protein A (Kd = 10-6 M)
Ka = 1/Kd so for Protein A with a Kd of 10-6 M, it has a Ka of 106 M-1.
3. Hb binding oxygen. The affinity of Hb binding O2 is affected by pH, CO2 and 2,3
bisphosphoglycerate (BPG): increasing any of these decreases Hb affinity for O2.
a. Drop in pH from 7.4 to 7.2: decreased affinity (shifts the O2 binding curve to the right).
b. Decrease in CO2 increases affinity.
c. Increase in BPG decreases affinity.
d. Increase in CO decreases affinity.
5. Membrane proteins binding a hormone: Three different proteins binding the same hormone: below
the theta ( θ ) is shown at different concentrations of the hormone.
Hormone Conc. nM Protein 1 Protein 2 Protein 3
0.2
0.048
0.29
0.17
0.5
0.11
0.50
0.33
1
0.2
0.67
0.5
4
0.5
0.89
0.8
10
0.71
0.95
0.91
20
0.83
0.97
0.95
50
0.93
0.99
0.98
a. What is the Kd for Protein 2: By simple inspection you can see that it is 0.5 nM
(concentration of ligand that results in half of the binding sites being bound by ligand
(hormone). The Kd of the others can be gotten just as easy.
b. Which of these proteins binds hormone most tightly (highest affinity). Protein 2 is the
winner! Protein1 Kd = 4 nM, and Protein 3 Kd is 1 nM.
6. Hb cooperativity is a property of the interaction of the α and β subunits. It is totally lost when the
subunits are dissociated. We shall see this in Question 7 on the next page.
7. Maternal (α2β2) and Fetal (α2γ2) Hb binding curves:
a. You can see that HbF binds oxygen with a higher affinity than HbA. The fetal curve is the
left, it takes less O2 to bind HbF than it does for HbA.
b. What is important is the flow of O2 in the placenta from HbA to HbF, because HbF has a
higher affinity for O2, it lowers [O2] which causes HbA, then to release its O2.
c. Removing BPG shifts both Hb’s to the left, making them almost the same, but HbA came
further. This suggests that HbA has a tighter affinity for BPG than HbF.