Section 6
Hemoglobin: Protein Structure and Function
Pre-Activity
Assignment
1. Produce a reading log for the chapter covering myoglobin and hemoglobin.
2. Produce a description of hemoglobin function in terms of the molecular level changes in hemoglobin
structure. Begin with the binding of O2 to the low affinity conformer of hemoglobin in the lung and end
back at the lung with a hemoglobin molecule ready to again bind O2. As you move from lungs to tissue,
incorporate a change in pH from 7.4 to 7.2 and an increase in PCO from 20 to 40 mm Hg.
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Why
Protein function is intimately linked to protein structure. Understanding this structure/function relationship is
important in understanding the limits of protein function as well as recognizing that variation in structure will
ultimately affect function. Many genetically inherited diseases are the result of the production of structurally
aberrant proteins. You will better understand how proteins work by examining the protein hemoglobin, a wellstudied model of structure/function relationship.
Outcomes
1. Depict myoglobin and hemoglobin oxygen binding function using a graph and correlate the graph with each
protein’s function.
2. Explain the structural features that are important for both myoglobin and hemoglobin to carry out their
functions.
3. Describe how the change in conformation of hemoglobin caused by allosteric binding is important in
hemoglobin function. Depict the effect of allosteric binding using a graph.
4. Predict how changes in hemoglobin structure might lead to pathological abnormalities in hemoglobin
function.
Plan
1. Form Structure/Function teams as instructed.
2. Answer the Critical Thinking Questions.
3. Prepare your spokesperson to share two discoveries your team made while discussing the material.
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Critical Thinking Questions
1. Consider the biological roles of myoglobin and hemoglobin and sketch oxygen binding curves for myoglobin
and hemoglobin. Be sure to label axes. Try talking through a strategy for the sketch before looking at the
book. In what ways do the binding curves of myoglobin and hemoglobin correlate with their respective
functions?
Information: Some textbooks use the symbol θ whereas others use YO in the formula for fractional saturation
2
of the protein with oxygen.
Stes occuped
θ or YO =
2
Total avalable stes
2. Explain why YO ranges from 0 to 1. What does YO = 0.5 mean at the molecular level with a populatio of
2
2
hemoglobin molecules?
3. When YO = 0.5 the value of pO is equal to p50. How does the value of p50 relate to binding affinity?
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2
Foundations of Biochemistry (Selected Activities)
4. When O2 binds to Fe the ring flattens out. What residues are most affected by the flattening?
5. What is the order of molecular level changes that occur upon binding of O2 to the low affinity conformer of
hemoglobin that result in the high affinity conformer? Be brief.
6. At the molecular level, how do H+, CO2, and bisphosphoglycerate (BPG) act as allosteric effectors of
hemoglobin? (Use the equilibria, H-Hbdeoxy + O2 E Hb-O2 + H+)
7. Draw binding curves for hemoglobin in the presence of each effector individually and label the p50.
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Section 6 — Hemoglobin: Protein Structure and Function
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8. Choose a hemoglobin mutation from the table in your text (or provided by your instructor) and describe
specifically how the structural change affects hemoglobin function. Decide whether p50 would be higher or
lower than normal.
9. Identify two discoveries your team made.
1. In the mutant hemoglobin known as HB Providence, an asparagine residue in the β-chain replaces Lys82. In normal hemoglobin, Lys-82 projects into the central cavity of the hemoglobin molecule. Predict the
effect of the Lys → Asn mutation on the affinity of allosteric modifiers (relative to normal Hb e.g. HbA) and
describe the effect the mutation would have on the function of HB Providence.
2. Under appropriate conditions, hemoglobin dissociates into its four subunits. Isolated α subunits bind
oxygen, but the O2-saturation curve is hyperbolic instead of sigmoidal. In addition, the saturation curve is not
affected by the presence of H+, CO2, or BPG. What do these observations indicate about the cooperativity
and allosterism observed in hemoglobin?
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Foundations of Biochemistry (Selected Activities)