Aquaporins
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Aquaporins
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[ID] => 554021
[post_author] => 12815
[post_date] => 2024-12-23 17:49:40
[post_date_gmt] => 2024-12-23 22:49:40
[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.
Movement of water across cell membranes occurs by simple diffusion. However, a large amount of highly selective and osmotic gradient driven water transport through the cell membranes of renal tubules, secretory glands, and other tissues is aided by channel proteins called the aquaporins (AQPs). These proteins are categorized as (i) water permeable AQPs (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6) or (ii) water and or glycerol transporting aqua-glyceroporins (AQP3, AQP7, AQP9, AQP10).
AQP1 is a transmembrane tetrameric protein with four identical subunits (Figure 1). Each subunit contains six membrane spanning alpha helices (A to F) connected by five loops. Helices B and E from each subunit form a 2.8 angstrom central pore which is just wide enough to allow a single-file of water molecules. A conserved motif called the asparagine-proline-alanine (NPA) motif) and nearby aromatic/arginine (ar/R) selectivity filter play a crucial role in excluding protons and other small molecules from the channel. The NPA motifs are located in loops B and E of each subunit, contributing to the formation of the pore's narrowest region. The ar/R selectivity filter is typically composed of residues located near the extracellular side of the channel, including a conserved arginine residue and aromatic residues.
Figure 1. AQP1 tetrameric structure showing the water transport; adapted from Nakhjavani M et al
Liposomal lipid bilayers harboring AQPs can serve as a model system to study the transport of water through these channels in vitro. For example, as shown in Figure 2, liposomes expressing water permeable (NtPIP2;1) or non-water permeable (NtAQP1) AQPs are generated. When these AQP integrated liposomes are shifted to hypo-osmotic conditions, their volume is expected to increase due to AQP mediated influx of water. A change in scattered light signal intensity recorded via a stopped-flow spectrophotometer is indicative of change in liposomal volume. Phosphatidylcholine (PC) is used as a control.
Figure 2. Fitted light-scattering curves recorded via a stopped-flow spectrophotometer. (Figure adapted Kai et al). (Nt: Nicotiana tabacum; PC: phosphatidylcholine)
References: Nakhjavani M et al. Potential Molecular Targets and Therapeutic Indication in Metastatic Breast Cancer, 2019; Kai L et al. A refined model of water and CO₂ membrane diffusion: effects and contribution of sterols and proteins, 2014; Khan, N., Kushwah. The Aquaporins: Regulator for Brain Pathophysiology, 2015
[post_title] => Aquaporins
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[question] => Aquaporins are integral membrane proteins that primarily facilitate:
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Primary active transport is a type of transport where energy (usually from ATP) is directly used to move molecules across a membrane against their concentration gradient, from an area of lower concentration to an area of higher concentration.
In secondary active transport, energy is used indirectly. Instead of directly using ATP, it relies on the energy stored in the form of an ion gradient, created by primary active transport. This gradient is then used to drive the transport of another substance against its concentration gradient.
Antiport is a type of transport mechanism where two or more different molecules or ions are transported in opposite directions across a membrane. One molecule moves in while another moves out, typically using the energy from the movement of one molecule down its concentration gradient to drive the movement of the other molecule against its gradient.
Diffusion is the movement of molecules across a cell membrane without the use of energy (ATP). It occurs when molecules move from an area of higher concentration to an area of lower concentration, down their concentration gradient.
Aquaporins specifically facilitate the passive diffusion of water molecules, allowing water to move freely across cell membranes according to its concentration gradient, without the need for cellular energy.
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[answers] => Array
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[each_answer] => A. primary active transport of water molecules.
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[each_answer] => B. secondary active transport of water molecules.
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[each_answer] => C. facilitated diffusion of water molecules.
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[each_answer] => D. antitransport of water and solutes.
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Which molecular mechanism helps to selectively filter water molecules through AQP1?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
The passage states that the NPA motif, which stands for asparagine-proline-alanine, and the aromatic/arginine selectivity filter help to ensure selective passage of water molecules and exclude protons.
Electrostatic repulsion would help to keep protons out of the channel.
This repulsion would be between protons and positively charged residues.
In the selectivity filter, the conserved arginine residue as well as aromatic histidine residues are both positively charged amino acids that would help to exclude protons through electrostatic repulsion.
https://commons.wikimedia.org/wiki/File:Homemade_Chart_of_the_20_Amino_Acids.jpg
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[answers] => Array
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[0] => Array
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[each_answer] => A. Electrostatic repulsion of protons by arginine and histidine in the selectively filter
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[each_answer] => B. Binding of protons by arginine and histidine residues in the selectively filter
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[2] => Array
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[each_answer] => C. Electrostatic repulsion of protons by the A residue of NPA
)
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[each_answer] => D. Binding of protons by the N residue of NPA
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => The following images are of X. laevis oocytes. Image A shows a control oocyte (-ve AQP1 expression) on the left and test oocyte (+ve AQP1 expression) on the right. Image B shows the same oocytes 3 min after transfer to hypo-osmotic culture medium.
Which of the following statements does NOT describe the results?
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
The control oocyte is missing aquaporin 1 expression, whereas the test oocyte has intact function.
The control oocyte is shown on the left, and the test oocyte is on the right, in both pictures.
The images show that the test oocyte ruptures in the hypo-osmotic solution, whereas the control oocyte did not swell.
This is because the test oocyte had functional AQP1, which allowed water to enter the cell; too much water entered because the solution was hypoosmotic.
Therefore, oocyte rupture occurred with functional AQP, NOT because of AQP dysfunction.
)
[answers] => Array
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[each_answer] => A. Functional demonstration of AQP mediated transport in X. laevis oocytes.
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[each_answer] => B. The test oocyte swelled and ruptured due to osmosis.
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[2] => Array
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[each_answer] => C. The control oocyte failed to swell.
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[each_answer] => D. Oocyte rupture was caused by dysfunction of AQP.
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[quiz_unique_key] => 1403770772
[question] => Which statement describes the light scattering data in Figure 2?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
In Figure 2, a change in light scattering reflects a change in water inside the cell.
The dotted line in Figure 2 represents cells with functional AQP. Aquaporins transport water via facilitated diffusion.
Accordingly, these cells showed the fastest increase in light scattering.
The liposomes with a greater increase in light scattering were able to take in water faster; this was due to facilitated diffusion through AQP channels and not simple diffusion.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Liposomes containing the functional AQP scattered the highest percentage of light within 1/10 of a second, which is indicative of sudden increase in volume.
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[each_answer] => B. In the presence of hypo-osmotic solution the liposomes with non-water permeable AQP swelled rapidly and bursted, hence a decrease in scattered light.
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[2] => Array
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[each_answer] => C. The percentage of scattered light in the case of liposomes with functional AQP was comparable to that of the PC control.
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[each_answer] => D. Increase in scattered light intensity in the case of liposomes with functional AQP was due to simple diffusion of water.
)
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[quiz_unique_key] => 1403770772
[question] => This figure shows a liposome harboring AQP, and the graph below represents light scattering curves. Both curves are data points from water permeability assay under the same osmotic conditions. The top curve indicates increase in cell volume of the liposome due to AQP mediated influx of water, and bottom curve is not a control. What could correspond to I, II, and the bottom curve III?
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[answer] => 1
[description] => Reason for Correct Answer:
Hypo-osmotic conditions cause cells to swell.
Hyper-osmotic conditions cause cells to shrink.
I and II therefore correspond to hyper-osmotic conditions and hypo-osmotic conditions, respectively.
Water moved into or out of the cell through aquaporins.
The Figure shows less change in volume in the bottom curve (III).
Less change in volume/water movement could be due to an AQP inhibitor or mutation.
)
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[each_answer] => A. I: hyper-osmotic conditions; II: hypo-osmotic conditions; III: the presence of an AQP inhibitor
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[each_answer] => B. I: hyper-osmotic conditions; II: hypo-osmotic conditions; III: the presence of an AQP enhancer
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[each_answer] => C. I: hypo-osmotic conditions; II: hyper-osmotic conditions; III: mutant AQP
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[each_answer] => D. I and II: hypo-osmotic conditions; III: mutant AQP
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[quiz_unique_key] => 1115843717
[question] => Why is phosphatidylcholine (PC) alone used as a control in experiments involving liposomal lipid bilayers harboring aquaporins (AQPs)?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
Controls are typically used to provide a baseline or standard for comparison, ensuring that the experimental results are due to the variable being tested and not other factors.
In this experiment, the primary variable being tested is the water transport capability of liposomes with different AQPs integrated into their membranes.
Phosphatidylcholine (PC) is a common phospholipid found in biological membranes.
Its primary role in this experiment is not to interact with AQPs or affect water transport but to provide a baseline for comparison.
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[answers] => Array
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[0] => Array
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[each_answer] => A. PC has no effect on the transport of water, thus serving as a baseline for comparison.
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[1] => Array
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[each_answer] => B. PC enhances the water permeability of liposomes, providing a positive control for water transport studies.
)
[2] => Array
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[each_answer] => C. PC interacts with AQPs, increasing their water transport activity.
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[each_answer] => D. PC destabilizes the liposomal lipid bilayers, ensuring they break down under hyper-osmotic conditions.
)
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Figure 1.
