Hemoglobin Structure

Question 1

Describe the overall structure of hemoglobin, indicating the site of oxygen binding. Explain the concepts of allostery and positive cooperativity as they relate to hemoglobin function and explain what is meant by taut (T) and relaxed (R) configurations.

Answer 1

• Tetramer, 2a 2B
• Heme group linked covalently to each chain (Fe where O2 binds)
• Fe must be reduced (ferrous, 2+) form to bind O2. Ferric +3 cannot bind (methemoglobin)
• Whole structure changes as O2 binds to molecule. No O2 = T. As O2 binds —> R
• Allostery: substrate binding at one site on a protein leads to altered conf at other binding sites on the protein
Allosteric changes lead to cooperatively
Binding of O2 at one site increases binding affinity at other sites

Question 2

Draw a typical oxygen dissociation curve. Explain why it is sigmoidal in shape. Define the p50. Explain the effects of pH, [CO2], temperature, and [2,3-BPG].

Answer 2

Sigmoidal shape due to positive cooperativity.

p50 is pO2 at which hemoglobin is 50% bound (around 27mmHg), myoglobin is 2.7mmHg

Curve shifts to the RIGHT when you workout:
high temp, dec pH, high CO2, more 2,3-BPG

Question 3

Compare oxygen dissociation curves for myoglobin and hemoglobin and explain the reason for the differences.

Answer 3

Myoglobin lives in the cells. It binds one O2, so it undergoes no allosteric changes - it’s either bound or not bound
It quickly & tightly binds one O2 (ready to grab it of Hgb), so graph is purely hyperbolic

Question 4

List and describe the typical hemoglobin variants seen during fetal development and in adulthood and explain how amounts of these different hemoglobins change during development.
zeta chain early in development

Answer 4

• Adult = a2B2, 97% (HbA). a2d2 (HbA2)
• A2: functions like A but is more heat stable and slightly higher affinity for O2. Elevated in B-thalassemia, sickle cell, hyperthyroidism, megaloblastic anemia
• Fetus = a2g2 (HbF). Dominant after 8-week of gestation. Higher affinity (left shift) b/c fetus must pull O2 from maternal circulation. It poorly binds 2,3-BPG
• Embryonic: don’t worry about. Higher affinity to get blood to the embryo
• alpha production remains the same, B/g switch at birth (HbF —> HbA)

Question 5

Describe how structural differences in hemoglobin affect oxygen affinity and explain the physiologic effects of altered oxygen affinity.

Answer 5

• Hb precipitates in the cell (stain and see Heinz bodies) —> anemia
• Hb Koln: most common, mutation in B chain —> increased O2 affinity, left shift
• Hb Poole: mutation in gamma, hemolytic anemia at birth but resolves in a few months (stop making g)
• Hb Chesapeake: a-globin chain has inc affinity
• Low affinity variants shift the curve to the right (body thinks it has enough O2 so RBC production is lower —> anemia)

Question 6

Describe what methemoglobinemia is, what causes it, how to diagnose it, and how to treat it.

Answer 6

• Fe becomes oxidized to 3+ state (ferric) and cannot bind O2
• Cause: oxidation of the heme by reaction with free radicals, H2O2, NO, OH-. Acquired: drug exposure (benzocaine, well water cont w/ nitrates), newborns with lower activity of cytochrome b5 reductase. Congenital: b5, HbM
Dx: Person looks cyanotic but PAO2 is normal. Blood looks chocolate brown and doesn’t change with oxygen exposure