14.1
Aquaporins
Aquaporins or AQPs are a family of integral membrane proteins whose primary function is to transport water, while some called aquaglyceroporins also t…
Water molecules are polar in nature, hence, they cannot diffuse rapidly across the non-polar lipid bilayer.
Therefore, cell membranes have special channel proteins called aquaporins that selectively transport water molecules according to the osmotic gradient.
These proteins function as tetrameric clusters, with each monomer acting as an independent water channel.
A single aquaporin channel comprises six transmembrane alpha-helices connected by five loops, of which two hydrophobic loops contain a conserved amino acid motif – Asparagine-Proline-Alanine or NPA.
The NPA residues form a constricted site that confines the water molecules into a single file as they pass through the channel.
Additionally, repulsion from the hydrophobic walls of the channel accelerates the movement of the water molecules.
While passing through the constricted site, the oxygen atoms interact with the asparagine residues of the NPA motif, interrupting the hydrogen-bonded bridges between the water molecules that would have otherwise allowed protons to hop in and pass through the channel.
Disruption of the hydrogen-bonded network prevents proton movement, making the aquaporin selectively permeable to water.
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Q1: Why do water molecules need special channel proteins to cross cell membranes?
Water molecules are polar and cannot diffuse rapidly across the non-polar lipid bilayer. Aquaporins are specialized channel proteins that selectively transport water molecules according to the osmotic gradient, enabling efficient water movement across membranes where passive diffusion would be too slow.
Q2: What is the structural organization of aquaporin proteins?
Aquaporins function as tetrameric clusters, with each monomer acting as an independent water channel. Each channel comprises six transmembrane alpha-helices connected by five loops, two of which contain a conserved NPA (Asparagine-Proline-Alanine) motif that forms a constricted site confining water molecules into single-file passage.
Q3: How does the NPA motif prevent protons from passing through aquaporins?
As water molecules pass through the NPA constricted site, oxygen atoms interact with asparagine residues, interrupting hydrogen-bonded bridges between water molecules. This disruption of the hydrogen-bonded network prevents protons from hopping through the channel, making aquaporins selectively permeable to water only.
Q4: Where are water-selective aquaporins most highly expressed in the body?
Water-selective aquaporins are widely expressed in water-transporting organs and tissues including the kidneys, exocrine glands, and central nervous system. They are also found in the lungs, skeletal muscles, and gastrointestinal organs, where they facilitate cell migration, neuroexcitation, and epithelial fluid transport.
Q5: What human diseases are associated with aquaporin mutations or deficiency?
Aquaporin mutations or deficiency cause renal dysfunction, epilepsy, skin disease, cancer, neurological disorders, and cardiac ailments. For example, mutations in AQP1 or AQP2 impair urine concentration, while AQP2 loss-of-function mutations cause non-X-linked nephrotic diabetes insipidus, characterized by unusually high urine output.
Q6: What additional functions do some aquaporins perform beyond water transport?
Aquaglyceroporins transport both water and glycerol, playing roles in adipocyte metabolism, skin hydration, and cell proliferation. Some aquaporins also function as gas channels, transporting volatile substances like carbon dioxide and ammonia, and are highly expressed in cells involved in gaseous exchange such as red blood cells and pulmonary capillaries.
Q7: How do plant aquaporins respond to environmental stress?
Some plant aquaporins have evolved gated mechanisms that close in response to harsh environmental conditions such as drought, stress, or flooding. This gating prevents water exchange when it would be harmful to the organism, demonstrating adaptive regulation of water transport across membranes.