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Aquaporins are membrane proteins of a cell which helps in regulating the flow of water. It is a 28Kda protein . This is a kind of water channels which facilitate the movement of water by incresing the permiability of water towards membrane . It is found in all kingdom of life like plants, animals and micro-organisms.
The structure of Aquaporin is made up of 6 membrane helices.
Functions -
1. In cell migration .
2.Skin hydration . (AQP3 &AQP7)
3.Cell proliferation particularly AQP3.
4.AQP7 found to be involved in fat metabolism .
5.Conduct water molecules in and out of the cell and preventing the flow of ions and other molecules.
6. AQPs have also been proposed to transport other small molecules and gases, including carbon dioxide, ammonia, nitric oxide and hydrogen peroxide .
7.The AQP4 water channel is expressed in astrocytes throughout the central
nervous system.
The strucuture of aquaporins have been defined by mutagenesis, spectroscopy and freeze fracture electron microscopy method.
CSIR QUESTIONS Q.Which of the following statements describe the characteristics of an aquaporin-
A)they are membrane proteins that are soluble in water.
B)they are protein molecules with channels through which water molecules can diffuse.
C)their numbers in a membrane can change due to differing conditions or hormonal stimulation.
D)B& C
Ans. (D) They are actually hydrophobic proteins and therefore not soluble in water. Aquaporins are especially abundant in kidney and intestinal epithelial cells where water transport occurs extensively across tubular and epithelial cells. Dehydration condition can stimulate the kidney tubules to increase the number of water channels under stimulation by the posterior pituitary hormone ADH.
Q2.Aquaporins are a class of proteins that are relatively abundant in plant membranes .Following are certain statements -
a.Aquaporins form water channels in membrane.
b.some aquaporins also transport uncharge molecules such as ammonia.
c.Activity of aquaporins is not regulated by phosphorylation.
d.Activity is regulated bu calcium concentration and reactive oxygen species .
Which of the combinations are correct -
a.A,B& D C.A,C&D
b.B,C &D D.A,B& C
Ans. a (A,B &D ) the aquaglyceroporins transport uncharged molecules such as ammonia,urea,glycerol and CO2.
NOTE- Aquaporin proteins are composed of a bundle of six transmembrane α-helices. They are embedded in the cell membrane. The amino and carboxyl ends face the inside of the cell. The amino and carboxyl halves resemble each other, apparently repeating a pattern of nucleotides. Some researchers believe that this was created by the doubling of a formerly half-sized gene. Between the helices are five regions (A – E) that loop into or out of the cell membrane, two of them hydrophobic (B, E), with an asparagine–proline–alanine ("NPA motif") pattern. They create a distinctive hourglass shape, making the water channel narrow in the middle and wider at each end.
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