Oxygen affinity of hemoglobin and myoglobin
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Oxygen affinity of hemoglobin and myoglobin
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[post_date] => 2024-12-23 18:32:57
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Adult hemoglobin (HbA) differs significantly from fetal hemoglobin (HbF), the primary hemoglobin component found in fetal blood, and from myoglobin (Mb), the oxygen carrier present in muscle cells. These differences are essential for facilitating the appropriate transport of oxygen between the mother and baby during fetal development, as well as between the blood and muscle tissues throughout the body.
Figure 1 shows the oxygen saturation curves of normal HbA, HbF, and Mb. The oxygen saturation curve, also known as the oxygen dissociation curve, measures the relationship between the partial pressure of oxygen (PO₂) in the surrounding environment and the percentage of hemoglobin molecules that are bound to oxygen in solution (% saturation, or SO₂).
Figure 1 Oxygen saturation/dissociation curves for HbA, HbF, and Mb at physiological pH
The p50 value on the oxygen dissociation curve refers to the PO₂ at which the hemoglobin molecule is 50% saturated with oxygen. It is an important parameter on the oxygen dissociation curve because it indicates the affinity of hemoglobin for oxygen.
Oxygen affinity, and therefore the p50 value, can be influenced by a variety of factors, including:
- pH levels: Acidic conditions lead to a decrease in hemoglobin’s affinity for oxygen, a phenomenon known as the Bohr effect. This effect is more pronounced in HbA compared with HbF, meaning that in acidic conditions, HbA releases oxygen more readily than HbF, enhancing oxygen unloading in tissues with higher metabolic activity. Changes in pH may also affect oxygen binding by myoglobin.
- Temperature: Higher temperatures reduce the oxygen affinity of both HbF and HbA. In warmer tissues, hemoglobin tends to release oxygen more efficiently, ensuring oxygen supply to cells with higher energy demands. Unlike hemoglobin, myoglobin does not show a significant temperature dependence on oxygen affinity.
- 2,3-Bisphosphoglycerate (BPG) concentrations: BPG is a molecule in red blood cells that binds to hemoglobin and decreases its oxygen affinity. Higher levels of BPG are generally found in situations with low oxygen availability (e.g., at high altitudes or in chronic lung diseases). HbF naturally has a higher affinity for BPG compared with HbA, which contributes to its ability to unload oxygen in fetal tissues more effectively. Myoglobin does not bind to BPG like hemoglobin does. Instead, myoglobin has a higher affinity for oxygen and acts as an oxygen reservoir in muscle tissues, allowing efficient oxygen transfer from hemoglobin in the blood to the muscles during periods of high demand.
[post_title] => Oxygen affinity of hemoglobin and myoglobin
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[question] => Why do fetuses use HbF instead of HbA?
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[answer] => 2
[description] => Reason for Correct Answer:
Iron is used in HbF as well as HbA.
Fetuses get their oxygen from their mother’s blood.
In order to take enough oxygen to provide for the fetus, HbF has a higher affinity for oxygen than HbA does. This allows it to extract oxygen from the mother’s blood through the placenta, even when the oxygen concentration in the maternal blood is relatively low. It does not allow the fetus to store extra oxygen in the blood, as its oxygen is still delivered to tissues.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Fetuses need to store oxygen in case the mother’s oxygen supply drops.
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[1] => Array
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[each_answer] => B. Fetuses need to be able to extract oxygen from the mother’s blood.
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[2] => Array
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[each_answer] => C. In order to grow, fetuses need more oxygen than adults.
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[each_answer] => D. Fetuses lack the iron stores needed for making HbA.
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Which of the following is INCORRECT regarding an HbA solution in equilibrium with a gas at PO₂ = 80 mmHg?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Oxygen saturation curves show the percent oxygen saturation of hemoglobin molecules in solution at equilibrium with different partial pressures of oxygen. The oxygen saturation curve in Figure 1 shows that most of the HbA is bound to oxygen at a PO₂ of 80 mmHg.
If a gas and solution are in equilibrium, there is no net flux of gas from one to the other.
At a PO₂ of 80 mmHg, the saturation curves for HbA and HbF superimpose, meaning that their % saturations will be equivalent in these conditions.
However, the concentration of oxygen in the HbA solution will NOT be the same as in the gas. The concentration of oxygen in the solution is related to its partial pressure (PO₂) AND the solubility of oxygen in the particular solution for a given temperature (the constant of proportionality is called Henry’s constant).
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[answers] => Array
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[0] => Array
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[each_answer] => A. The amount of oxygen leaving the solution will be equal to the amount of oxygen entering the solution.
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[1] => Array
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[each_answer] => B. The % saturation of HbA will be approximately equal to the % saturation of HbF in similar conditions.
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[2] => Array
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[each_answer] => C. The concentration of oxygen in the HbA solution will be the same as in the gas.
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[each_answer] => D. Over 95% of HbA’s oxygen binding sites will be bound to oxygen.
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => Which is a correct cause or effect of the sigmoidal shape of the HbF and HbA oxygen dissociation curves?
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[answer] => 2
[description] => Reason for Correct Answer:
Hemoglobin is composed of four subunits, each containing a heme group that can bind to one molecule of oxygen.
The sigmoidal shape of the oxygen dissociation curves is a result of cooperative binding behavior.
When one oxygen molecule binds to a heme group in a hemoglobin molecule, it induces a conformational change that increases the affinity of the remaining heme groups in the same hemoglobin molecule for oxygen. This positive cooperativity allows hemoglobin to rapidly pick up oxygen in areas of high oxygen pressure (e.g., in the lungs) and efficiently release oxygen in areas of low oxygen pressure (e.g., in metabolically active tissues).
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[each_answer] => A. Cause: Hemoglobin contains three separate subunits that each bind to an oxygen molecule.
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[each_answer] => B. Cause: Oxygen binding to one heme group increases the oxygen affinity of other groups.
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[each_answer] => C. Effect: In areas of high oxygen pressure, hemoglobin readily releases oxygen.
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[each_answer] => D. Effect: In states of low oxygen pressure, hemoglobin holds onto oxygen.
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[quiz_unique_key] => 1403770772
[question] => The following graph shows what is called a “left shift” and “right shift” of the oxygen dissociation curve.
According to the passage, which of the following would result in a right shift in the HbA binding curve?
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[answer] => 3
[description] => Reason for Correct Answer:
A right shift means that a higher partial pressure of oxygen is needed to saturate the hemoglobin to the same level. This corresponds to decreased oxygen affinity.
The passage states that higher acidity causes a decrease in oxygen affinity. This means that decreased pH (higher acidity) would cause a right shift.
The passage also states that higher temperature and 2,3-BPG cause decreased oxygen affinity and would also cause a right shift. Here is a summary of things that would cause a right shift vs. a left shift:
(Here, BPG is labeled DPG for diphosphoglycerate – either name may be used on the MCAT.)
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[answers] => Array
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[0] => Array
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[each_answer] => A. Increased temperature, increased 2,3-BPG, and increased pH
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[each_answer] => B. Decreased temperature, decreased 2,3-BPG, and decreased pH
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[each_answer] => C. Increased temperature, increased 2,3-BPG, and decreased pH
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[each_answer] => D. Decreased temperature, decreased 2,3-BPG, and increased pH
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[quiz_unique_key] => 1403770772
[question] => Which graph best represents relative values and effects of pH on the p50 values of HbA and HbF?
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[answer] => 3
[description] => Reason for Correct Answer:
To answer this question, you want to consider: (a) how the relative p50 of HbA and HbF compare, and (b) how pH affects each of them, according to the passage.
The passage defines p50 as “the partial pressure of oxygen (PO2) at which the hemoglobin molecule is 50% saturated with oxygen.” Figure 1 shows that HbA has a higher p50 than HbF, as shown here. So, you can eliminate Choice B.
As oxygen affinity decreases, the p50 will increase, as more oxygen is required to saturate half of the lower-affinity hemoglobin molecules.
The passage states that acidity decreases the affinity, which would increase the p50 level. Therefore, you should see higher p50 levels at the lower pH, so you can eliminate Choice A.
The passage also states that pH has a greater effect on the affinity of HbA for hemoglobin. Therefore, you should see a steeper slope between the HbA points compared with the HbF points, and Choice C must be correct.
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
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[3] => Array
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[each_answer] => D.
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[quiz_unique_key] => 1997864699
[question] => Assume a person is on Mount Everest (PO2 = 50 mmHg). His goal is to maintain as much oxygen as possible in his tissues. Would it be better to have Mb or HbA in his blood, and why?
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[answer] => 4
[description] => Reason for Correct Answer:
The passage discusses how BPG works to decrease the oxygen affinity of hemoglobin under high altitude conditions.
It also states that BPG does not affect the affinity of myoglobin.
This difference helps myoglobin in the muscle continue to pull oxygen from the blood, into the tissues, even in low oxygen conditions.
Likewise, you would want to keep HbA in the blood, as its relatively lower oxygen affinity would help it to feed as much oxygen as possible to myoglobin in the tissues.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Mb, because it would carry the most oxygen from the lungs.
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[each_answer] => B. Mb, because it would deposit the most oxygen in the tissues.
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[each_answer] => C. HbA, because it would carry the most oxygen from the lungs.
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[each_answer] => D. HbA, because it would deposit the most oxygen in the tissues.
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[554842|4] => C
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Figure 1
