MODULE 4: Chapter 6.2 Flashcards
What are the main oxygen binding proteins discussed?
Myoglobin and hemoglobin
These proteins are key examples of how protein structure mediates function.
Who were the pioneers in studying hemoglobin and myoglobin?
Max Perutz and John Kendrew
They conducted studies in the 1940s.
What is the role of heme in myoglobin and hemoglobin?
Heme allows reversible binding of O2 to Fe2⁺
Heme is a prosthetic group needed for oxygen binding.
What is the oxidation state of iron in heme when it binds oxygen?
Fe2⁺
Iron must be in the +2 oxidation state to bind O2.
What happens to the color of blood or meat when iron oxidizes from Fe2⁺ to Fe3⁺?
Changes from red to brown
This color change indicates reduced oxygen binding ability.
How many polypeptide chains does myoglobin consist of?
One polypeptide chain
Myoglobin has one heme group.
How many polypeptide chains are in hemoglobin?
Four polypeptide chains
Hemoglobin contains two α and two β subunits.
What is the term used to describe the structural similarity between myoglobin and hemoglobin despite low amino acid sequence similarity?
Globin fold
Both share the same protein fold containing eight α helices.
What are the names of the two critical histidine residues in globin proteins involved in oxygen binding?
Proximal histidine (His F8) and distal histidine (His E7)
His F8 coordinates with Fe2⁺; His E7 stabilizes O2 binding.
What happens to the heme group in the absence of O2?
The heme group is puckered
The larger ionic radius of Fe2⁺ causes this puckering.
What effect does oxygen binding have on the heme group?
Makes the heme planar
Oxygen binding reduces the radius of Fe2⁺.
What is the consequence of carbon monoxide binding to hemoglobin?
Displaces O2 and causes carbon monoxide poisoning
CO binds with 200 times higher affinity than O2.
What is a ligand?
A molecule that binds to another molecule
In this context, ligands refer to molecules like O2.
What is the equilibrium equation for protein-ligand binding?
P + L ⇌ PL
P is the protein, L is the ligand, and PL is the complex.
What does the association constant (Ka) represent?
The binding (or association) of the protein and ligand
Ka has units of M−1.
What does a larger dissociation constant (Kd) indicate?
Lower affinity between two molecules
A larger Kd means more unbound species are present.
What is fractional saturation (θ)?
The fraction of protein binding sites that are occupied
It can be calculated from the concentrations of the protein, ligand, and complex.
What is the reaction for oxygen binding to myoglobin?
Mb + O2 ⇌ MbO2
MbO2 is oxymyoglobin; Mb is deoxymyoglobin.
What type of binding curve is produced when plotting fractional saturation versus ligand concentration?
Hyperbolic curve
It shows the relationship between ligand binding and saturation.
What does it mean when the ligand concentration equals Kd?
θ = 0.5
This indicates half of the binding sites are occupied.
How can Kd values be used in biochemistry?
To assess relative affinities of proteins for ligands
Different Kd values indicate how strongly a protein binds to a ligand.
What does a smaller Kd value indicate about a protein’s affinity for a ligand?
Higher affinity
It takes less ligand to achieve half-saturation.
What is the reaction for oxygen binding to myoglobin?
Mb + O2 ⇌ MbO2
MbO2 is oxymyoglobin, and Mb is deoxymyoglobin.
What does fractional saturation (θ) represent?
The fraction of oxygen binding sites on myoglobin that are occupied.
What shape does the oxygen binding curve for myoglobin take?
Hyperbolic.
What is the P50 in the context of myoglobin’s oxygen binding?
The partial pressure of oxygen that results in a fractional saturation value of 0.5.
What are the pO2 values in resting and active muscle?
- Resting muscle: ~4 kPa
- Active muscle: ~1.3 kPa.
What physiological role does myoglobin serve?
Oxygen storage protein.
How does myoglobin behave under high oxygen concentrations?
Myoglobin O2 binding sites are almost saturated.
What type of curve does the oxygen binding of hemoglobin produce?
Sigmoidal.
What is the primary function of hemoglobin?
To deliver O2 obtained from the lungs to the tissues.
What is cooperative binding in the context of hemoglobin?
The binding of the first O2 facilitates the binding of additional O2 molecules.
What are the three key features of ligand–protein interactions?
- Ligand binding is reversible.
- Ligand binding induces structural conformations.
- Equilibrium can be altered by effector molecules.
What are the T state and R state in hemoglobin?
- T state: tense conformation (deoxyhemoglobin).
- R state: relaxed conformation (oxyhemoglobin).
What happens to hemoglobin crystals upon exposure to O2?
They shatter.
What is the effect of O2 binding on the F helix of hemoglobin?
It stabilizes a protein conformation that fixes the position of His F8.
How do structural changes in the F helix affect hemoglobin’s quaternary structure?
They lead to large structural changes in the entire hemoglobin molecule.
What is the significance of the 15° rotational movement in hemoglobin?
It alters noncovalent interactions at the interfaces of the dimers.
What are the two models proposed to explain the cooperative binding behavior of proteins?
- Concerted model
- Sequential model.
What does the concerted model of allostery suggest?
The protein complex exists in either the T state or the R state.
How does the sequential model of allostery differ from the concerted model?
It allows for subunits to be in both the T state and R state simultaneously.
What is the role of allosteric effectors in hemoglobin function?
They induce conformational changes that shift the equilibrium toward the R state or T state.
What are the four allosteric effectors that modulate O2 affinity of hemoglobin?
- O2
- CO2
- H⁺
- 2,3-bisphosphoglycerate.
What is the Bohr Effect?
The effect of CO2 and pH on O2 binding affinity of hemoglobin.
What enzyme hydrates carbon dioxide in the blood?
Carbonic anhydrase
This enzyme converts CO2 into bicarbonate (HCO3−) and protons (H⁺), affecting blood pH.
What is the Bohr effect?
The observed pH dependence of oxygen binding to hemoglobin
At lower pH (7.2 in tissues), hemoglobin’s oxygen saturation decreases, promoting O2 unloading.
What happens to blood pH from the lungs to the tissues?
It decreases from 7.6 in the lungs to 7.2 in the tissues
This change is due to the production of H⁺ by carbonic anhydrase.
Which states of hemoglobin are affected by protonation?
T state and R state
Protonation at key residues stabilizes either the T or R state of hemoglobin.
How does CO2 affect hemoglobin?
CO2 binds to hemoglobin, forming a carbamate group
This enhances the formation of ion pairs that stabilize the T state.
What is the role of 2,3-bisphosphoglycerate (2,3-BPG) in hemoglobin function?
It traps hemoglobin in the T state
2,3-BPG inhibits O2 binding by stabilizing the T state of hemoglobin.
Where does 2,3-BPG bind in hemoglobin?
Between the β1 and β2 subunits
This binding site is crucial for its regulatory function.
What is the effect of 2,3-BPG on O2 binding?
It inhibits O2 binding to all four subunits
Only one molecule of 2,3-BPG binds to hemoglobin in the T state.
How does the concentration of O2 affect hemoglobin states?
High O2 favors the R state; low O2 favors the T state
The pO2 difference between tissues and lungs shifts the equilibrium between these states.
What is the significance of the His143Ser143 difference in fetal hemoglobin?
It reduces the affinity for 2,3-BPG
This allows fetal hemoglobin to be more often in the R-state, facilitating O2 transfer from mother to fetus.
What physiological adaptation occurs at high altitudes regarding 2,3-BPG?
Increased synthesis of 2,3-BPG
This adaptation leads to enhanced O2 unloading in tissues despite lower oxygen saturation.
What evolutionary process explains the diversity of globin genes?
Gene duplication and sequence divergence
This process leads to orthologous and paralogous genes with similar functions.
What are orthologous genes?
Genes in different organisms that have the same function
An example is the α hemoglobin gene in humans and chickens.
What are paralogous genes?
Related genes within the same organism that perform similar functions
Human α, β, and myoglobin genes are examples of paralogs.
What is a common mutation in the β-globin gene associated with sickle cell anemia?
Valine substitution for glutamate at position 6 (β-E6V)
This mutation leads to hemoglobin aggregation and clinical symptoms.
What clinical condition is characterized by reduced O2 transport efficiency?
Anemia
Anemia can result from altered hemoglobin function or reduced red blood cell counts.
How does 2,3-BPG affect oxygen saturation at high altitudes?
It results in decreased oxygen saturation
However, it improves the efficiency of O2 delivery to tissues.
What is the amino acid substitution in sickle cell anemia?
Valine for glutamate at position 6 of the β-globin polypeptide (β-E6V)
This substitution results in the βS polypeptide, leading to the formation of hemoglobin S (HbS) and causing sickle cell anemia.
What is the tetramer of hemoglobin containing α2βS2 subunits called?
Hemoglobin S, or HbS
HbS is pathologic under low O2 concentrations.
Under what conditions does HbS polymerization occur?
Under low O2 conditions
This occurs when hemoglobin is in the T state.
What causes erythrocytes to become sickle-shaped in sickle cell anemia?
Long chains of complex protein polymers formed by deoxyhemoglobin HbS
These rigid distorted cells clog microcapillaries and cause tissue damage.
What is the genetic inheritance pattern of sickle cell anemia?
Autosomal recessive
Affected individuals must inherit two copies of the defective gene from heterozygous parents.
What is the role of hydroxyurea in treating sickle cell disease?
Elevates expression of the fetal gene for the γ subunit
This helps disrupt HbS polymer formation.
What pathogen causes malaria?
Plasmodium falciparum
This protozoan is transmitted by female Anopheles mosquitoes.
How does the βS mutation provide resistance to malaria?
Heterozygous individuals with the βS mutation are more resistant to malaria
This is due to sickling of infected cells being preferentially removed by the spleen.
What are the allosteric effectors that regulate hemoglobin’s oxygen transport?
- O2
- CO2
- H⁺
- 2,3-bisphosphoglycerate (2,3-BPG)
O2 acts as a positive homotropic effector, while CO2, H⁺, and 2,3-BPG are negative heterotropic effectors.
What are the two conformations of hemoglobin?
- T state (tense)
- R state (relaxed)
T state has low-affinity O2 binding, while R state has high-affinity O2 binding.
What is the Bohr effect?
The dependence of oxygen binding in hemoglobin upon pH and carbon dioxide concentration
This effect helps facilitate oxygen release in tissues.
What is the definition of cooperativity in the context of hemoglobin?
The phenomenon where binding of a molecule lowers the energy of binding of subsequent molecules
This is also referred to as cooperative binding.
What is the difference between the association constant (Ka) and the dissociation constant (Kd)?
Ka is the equilibrium constant for binding, while Kd is the inverse of Ka
Kd indicates the strength of the interaction; a lower Kd indicates a stronger binding affinity.
What is myoglobin?
A globular transport protein concentrated in muscle tissue that functions in oxygen storage
It has a single heme group and is distinct from hemoglobin.
What is the globin fold?
A protein fold containing eight α helices
This structure is found in both hemoglobin and myoglobin.
What is the significance of the proximal and distal histidine in globin proteins?
- Proximal histidine coordinates with the Fe2⁺ of the porphyrin ring
- Distal histidine forms a hydrogen bond with oxygen
These residues stabilize the interaction of oxygen with the heme group.
What is the shape of the graph representing oxygen binding to hemoglobin?
Sigmoidal curve
This shape indicates cooperative binding behavior.
What is sickle cell anemia characterized by?
Deformed red blood cells with a sickle appearance due to intracellular deoxyHbS fibers
This deformation leads to various clinical symptoms.
