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Questions and Answers
What effect does cooperative binding have on hemoglobin's affinity for oxygen?
What effect does cooperative binding have on hemoglobin's affinity for oxygen?
In which state does deoxyhemoglobin exist, and what is its affinity for oxygen?
In which state does deoxyhemoglobin exist, and what is its affinity for oxygen?
What happens when two subunits of deoxyhemoglobin bind oxygen?
What happens when two subunits of deoxyhemoglobin bind oxygen?
How does the oxygen binding curve of hemoglobin differ from that of myoglobin?
How does the oxygen binding curve of hemoglobin differ from that of myoglobin?
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What factors can influence hemoglobin's affinity for oxygen?
What factors can influence hemoglobin's affinity for oxygen?
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What are allosteric effectors in the context of hemoglobin?
What are allosteric effectors in the context of hemoglobin?
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What primarily stabilizes the T (tense) form of deoxyhemoglobin?
What primarily stabilizes the T (tense) form of deoxyhemoglobin?
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What is the significance of hemoglobin remaining highly saturated even with a decrease in pO2?
What is the significance of hemoglobin remaining highly saturated even with a decrease in pO2?
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What does arterial oxygen saturation (SaO2) indicate?
What does arterial oxygen saturation (SaO2) indicate?
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Which method is considered more accurate for measuring arterial oxygen saturation?
Which method is considered more accurate for measuring arterial oxygen saturation?
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What describes the tertiary structure of a protein?
What describes the tertiary structure of a protein?
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What is the primary function of myoglobin?
What is the primary function of myoglobin?
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What is true regarding the oxygen dissociation curve (ODC) of myoglobin?
What is true regarding the oxygen dissociation curve (ODC) of myoglobin?
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What is the typical oxygen carrying capacity of blood with normal hemoglobin concentration?
What is the typical oxygen carrying capacity of blood with normal hemoglobin concentration?
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Which of the following statements about protein surfaces is correct?
Which of the following statements about protein surfaces is correct?
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What feature characterizes the quaternary structure of proteins?
What feature characterizes the quaternary structure of proteins?
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What does a low dissociation constant (KD) indicate about a protein's affinity for its ligand?
What does a low dissociation constant (KD) indicate about a protein's affinity for its ligand?
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How many oxygen molecules can hemoglobin (Hb) bind?
How many oxygen molecules can hemoglobin (Hb) bind?
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What conformational change occurs in hemoglobin when oxygen binds?
What conformational change occurs in hemoglobin when oxygen binds?
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Why do myoglobin (Mb) and hemoglobin (Hb) have different oxygen binding affinities?
Why do myoglobin (Mb) and hemoglobin (Hb) have different oxygen binding affinities?
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What happens to the color of hemoglobin when it binds oxygen?
What happens to the color of hemoglobin when it binds oxygen?
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What role does the ionic character of Fe2+ play in the heme complex?
What role does the ionic character of Fe2+ play in the heme complex?
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What defines the binding site of a protein?
What defines the binding site of a protein?
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What effect does the binding of H+ to hemoglobin have?
What effect does the binding of H+ to hemoglobin have?
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Study Notes
Measuring Oxygen Content in Blood
- Arterial oxygen saturation (SaO2) is expressed as a percentage, representing the proportion of hemoglobin binding sites occupied by oxygen.
- In healthy individuals at sea level, SaO2 is typically between 96% and 98%.
- The maximum amount of oxygen blood can carry when fully saturated is known as oxygen carrying capacity, which is around 20 ml oxygen per 100 ml blood with normal hemoglobin concentration.
- Oxygen content in blood can be measured using two methods:
- Pulse oximeter: Less accurate, measures pO2 in arterial blood and indicates oxygen levels. It cannot differentiate dyshaemoglobin (COHb/MetHb).
- CO oximeter: More accurate, measures SaO2 in arterial blood and can differentiate between oxyHb and dyshaemoglobins (COHb/MetHb). Ideal for critical care settings.
Protein Structure
- Proteins are composed of amino acid chains linked by peptide bonds (primary structure).
- Short amino acid sequences fold into stable structures like alpha-helices, beta-sheets, and beta-turns (secondary structure).
- A single polypeptide chain of a globular protein, like myoglobin, folds into a 3D shape (tertiary structure).
- Multi-subunit proteins (e.g., hemoglobin) have subunits associating through non-covalent forces, forming a stable complex (quaternary structure).
- A protein's surface composition reflects its environment:
- Aqueous environment (e.g., blood) has a protein surface composed of polar amino acids.
- Lipid (non-polar) environments (e.g., membrane proteins) have protein surfaces made of non-polar amino acids.
Myoglobin - Oxygen Storage
- Primarily found in muscle tissue.
- Single polypeptide chain.
- Globular protein with 8 alpha-helical segments connected by bends.
- Contains one heme prosthetic group, binding one oxygen molecule.
- Myoglobin's high affinity for oxygen makes it an efficient storage molecule.
- Myoglobin has a hyperbolic shaped oxygen dissociation curve.
Oxygen Dissociation Curve (ODC)
- A graph showing the relationship between oxygen saturation and partial pressure of oxygen (pO2) for myoglobin or hemoglobin.
- Myoglobin's ODC demonstrates near-complete saturation at high pO2, with oxygen remaining bound until pO2 becomes very low.
- This characteristic makes myoglobin suitable for oxygen storage.
- Myoglobin exhibits a hyperbolic ODC.
Myoglobin's High O2 Affinity and Low KD
- The reversible binding of oxygen to myoglobin is described by a simple equilibrium:
- Protein + ligand <=> protein-ligand complex
- KD (dissociation constant) measures the protein's affinity for its ligand.
- A small KD indicates high affinity.
- Myoglobin has a high affinity for oxygen, therefore a low KD.
- KD represents the pO2 at which 50% of the binding sites are filled with oxygen.
Hemoglobin (Hb) - Oxygen Transport
- Hb has 4 subunits (tetramers), each similar in 3D structure to the beta-subunit of myoglobin.
- Hb has 2 identical alpha-subunits and 2 identical beta-subunits (a2b2).
- Each subunit contains 1 heme group and 1 Fe2+ ion.
- Hb binds up to 4 oxygen molecules.
- Hemoglobin is more efficient at oxygen transport than myoglobin and is designed to be sensitive to small changes in pO2.
Structure of Heme
- Heme consists of a porphyrin ring covalently bound to a deep pocket in each Hb subunit, protecting the Fe2+ ion.
- Fe2+ ion is located at the center of the heme molecule.
- Fe2+ forms 6 covalent bonds: 4 to the planar heme ring, one to a histidine amino acid of Hb, and one to oxygen.
- Heme has a concave shape in the deoxy (T) state and a flat, planar shape in the oxy (R) state.
Color of Oxygenated and Deoxygenated Hemoglobin
- The electronic configuration of the heme complex changes when oxygen binds, resulting in a color shift from darker red (deoxy Hb) to scarlet red (oxy Hb).
- This underlies the working principle of pulse oximetry.
- This color change also reflects molecular changes in Hb that affect oxygen binding.
Key Principles of Protein Structure and Function
- Reversible, non-covalent binding of small molecules (ligands) to proteins occurs at specific sites called binding sites.
- The binding site is complementary to the ligand in size, shape, charge, and polarity, ensuring specificity.
- Proteins and ligands are mobile, allowing them to wiggle and find the best fit.
- Ligand binding triggers a conformational change in the protein (and/or ligand), impacting its function.
- Proteins with high affinity for a ligand bind it more tightly and release it less readily.
- Myoglobin has high affinity for oxygen.
- DeoxyHb has low affinity for oxygen, while oxyHb has high affinity.
- The question arises: how can the same protein exhibit different affinities for the same ligand?
Cooperative Binding in Hb: Changing Affinity for O2
- Hemoglobin responds to environmental changes, such as fluctuations in partial pressure of oxygen.
- In the lungs, where pO2 is high, binding of oxygen to one subunit of deoxyHb increases the affinity of neighboring subunits, making it easier for oxygen to bind.
- This enhances the transition to oxyHb. This phenomenon is known as cooperative binding.
- Hb can alter its affinity for oxygen through cooperative binding, transitioning from high affinity in the lungs to low affinity in the tissues. This allows it to efficiently pick up oxygen in the lungs and release it in the tissues.
- Myoglobin, as a single subunit protein, has a single, high affinity for oxygen.
Transition from DeoxyHb to OxyHb
- DeoxyHb is in the T (Tense) form with low affinity for oxygen.
- Subunits in the T state are stabilized by ion pairs.
- When two subunits in the T state bind oxygen, a T-R transition occurs, transitioning to the R (relaxed) state.
- This transition facilitates easier oxygen binding to the remaining subunits.
- The primary difference between the T and R states lies in the alignment of the alpha and beta subunits, which slide past each other, breaking ionic bonds.
- This process exemplifies cooperative binding.
Interpreting Hb's Oxygen Dissociation Curve
- Unlike myoglobin, Hb can change its affinity for oxygen from high to low, characterized by a sigmoidal shaped curve.
- This dynamic response is influenced by environmental changes like pH, CO2, temperature, and 2,3-BPG.
- Hb can remain highly saturated even with significant decreases in pO2.
- Oxygen binds rapidly to Hb as pO2 increases and dissociates rapidly as pO2 decreases.
- The high affinity in the lungs facilitates oxygen loading (oxyHb/R state), while the low affinity in tissues promotes oxygen unloading (deoxyHb/T state).
- The S-shaped curve is only possible for proteins with more than one subunit.
Binding of Small Molecules to Hb: Modifying O2 Affinity
- In addition to pO2, other factors influence Hb's affinity for oxygen.
- Hemoglobin can bind small molecules at sites distinct from the oxygen binding site.
- These molecules are called allosteric effectors.
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Description
This quiz covers the measurement of oxygen content in blood, discussing arterial oxygen saturation and the methods used for measurement. Additionally, it explores the primary structure of proteins and their significance in biochemistry. Perfect for students studying physiology and biochemistry.
