Name__________________________________________
CHEM 431/531 - Biochemistry I/Exam III/November 06, 2006
Part I: Hemoglobin and Cooperativity- Answer ONLY 4 out of the 8 questions.
1. Why is it advantageous for hemoglobin to have allosteric properties?
Hemoglobin binds oxygen in a positive cooperative manner. This allows it to become
saturated in the lungs, where oxygen pressure is high. When the hemoglobin moves to
tissues, the lower oxygen pressure induces it to release oxygen and thus deliver oxygen
where it is needed.
2. Explain the structural differences between fetal hemoglobin and adult hemoglobin. Explain
how fetal hemoglobin can bind oxygen released by adult hemoglobin inside the uterus. This
must include a discussion of 2,3 BPG as part of your answer.
Fetal hemoglobin contains two
and two
chains, in contrast to adult hemoglobin with two
and two
chains. The fetal hemoglobin
chain is probably a result of gene duplication
and divergence. The difference in the chains results in a lower binding affinity of 2-3 BPG to
fetal hemoglobin. Thus, the fetal hemoglobin has a higher affinity for oxygen, and the
oxygen is effectively transferred from the mother’s hemoglobin to fetal hemoglobin.
3. Describe the octahedral coordination sphere of the iron ion in hemoglobin and myoglobin.
The Fe +2
ion is coordinated to the four nitrogens in the center of the protoporphyrin of the heme. The fifth
coordination site is occupied by the “proximal histidine” of the globin chain. The oxygen is bound to the
sixth coordination site of the iron.
4. Briefly describe the cause of sickle-cell anemia.
Sickle-cell anemia is a genetic disorder that is the result of a single substitution of β6 Glu with a Val. This
changes a negatively charged side chain to a nonpolar, hydrophobic side chain. This Val binds into a
hydrophobic pocket on the β chain of an adjacent molecule whose β6 Val binds to another molecule, thus
hemoglobin aggregates. These aggregates form long fibers that strain the RBC and force into a sickled
shape. The distorted red blood cells clog capillaries and impair blood flow, resulting in the sickle-cell
crisis. The sickled cells are then destroyed, resulting in the anemia.
5. Describe how carbon dioxide affects the oxygenation of hemoglobin.
Increased levels of carbon dioxide cause hemoglobin to release oxygen. The more active the
tissue, the more fuel is burned and the more CO2 is produced. These active tissue cells have
the greatest need for oxygen to produce more energy. The CO2 combines with the N-
terminal amino groups to form negatively charge carbamate groups. The negatively charge
carbamate groups form salt bridges that stabilize the T-state. Thus, the increase of carbon
dioxide causes the conversion of the R-state to the T-state, releasing the bound oxygen to
the tissues producing the most CO2.
6. Describe the role of 2,3-bisphosphoglycerate in the function of hemoglobin..
2,3-bisphosphoglycerate, 2,3-BPG, is a relatively small, highly anionic molecule found in the RBC. 2,3-
BPG only binds to the center cavity of deoxyhemoglobin (T-state). The size of the center cavity decreases
upon the change to the R-form so that it cannot bind to the R-state. Thus, the presence of 2,3-BPG shifts the
equilibrium toward the T-state. T-state is unstable, and without BPG, the equilibrium shifts so far toward
the R-state that little oxygen would be released under physiological conditions.
