Aquqporin (AQP) is a integral membrane proteins that control the amount of water that travels in and out of the cell[1]. Before the discovery of aquaporin by Peter Agre (1992); it was thought that water molecules leaked through the phospholipid bilayer one by one. However this could not be the case for cells that need to released and uptake water quickly like in the kidneys.
Aquaporins allow water to flow rapidly to the inside of cell then it would by crossing the bilayer.[1] The presence of water channels increases membrane permeability to water[1]. This protein is highly selective to water molecules and prevents the passage of ions and other solutes.There are multiple types of aquaporins in the human boby, some can allow the transport of other molecules such as glycerol,CO2, ammonia and urea. For example aquaporin 3 has a pore width of 8-10 Ångströms and allows the passage of hydrophilic molecules ranging between 150-200 Da[1].
Structure
AQP1 monomer contains 269 amino acid residues, which form two tandem repeats of three membrane-spanning α-helices.Aquaporin is a protien with six transmembrane alpha helices with the amino and craboxyl terminal tails located in the cytoplasm.[2]. Water molecules traverse through the pore of the channel in single file[1].Each monomer is able to channel water.There is a conserved Asn-Pro-Ala sequence that overlap in the middle of the lipid bilayer membrane which creates trail for the water molecules to be carried along in the channel.
The hydrogen-bonding interactions of water molecules with the polar side chains of Asn-76 and Asn-192 on the strictly conserved Asn-Pro-Ala sequence motifs were found to be essential for maintaining the connectivity of water flow in the narrow constriction region[3].
The hourglass shape allows the water flows, these water pores are completely impermeable to charged species, such as protons. Which is very important to the conservation of membrane's electrochemical potential[1].
Water Enters
Water molecules enter aquaporin and move through the narrow channel by orienting themselves by the electrical field created channel wall.
-The water molecule is oriented with their oxygen atom facing down the channel.
-If water is flowing outside of the cell then the water molecule is oriented with the oxygen atom facing down.[4]
Aquaporin's channel contains asparagines that interact with the hydrogen bonds on the water molecule which allows the orientation of water.
The narrow pore acts to weaken the hydrogen bonds between the water molecules allowing the water to interact with the positively charged arginine, which also acts as a proton filter for the pore.[2]
Function
There are thirteen known types of aquaporins in mammals, and six of these are located in the kidney. The most studied aquaporins are AQP1, AQP2, AQP3, and AQP4. The location of Aquaporin is usually found in the kidney, eye in the body. [4]
In the Kidney, they are found in the basolateral and apical plasma membranes of the proximal tubules and the limb of the loop of Henle in the kidney.Aquaporin are concentrated in the kidney is where there is a need to transporting large amounts of water in and out of the cell.[4]
Additionally, it is found in red blood cells, vascular endothelium, the gastrointestinal tract, sweat glands, and lungs.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Takata K, Matsuzaki T, Tajika Y. Aquaporins: water channel proteins of the cell membrane. Prog Histochem Cytochem. 2004;39(1):1-83. PMID:15242101
- ↑ 2.0 2.1 Murata K, Mitsuoka K, Hirai T, Walz T, Agre P, Heymann JB, Engel A, Fujiyoshi Y. Structural determinants of water permeation through aquaporin-1. Nature. 2000 Oct 5;407(6804):599-605. PMID:11034202 doi:10.1038/35036519
- ↑ PMID:PMC64684
- ↑ 4.0 4.1 4.2 Rutkovskiy A, Valen G, Vaage J. Cardiac aquaporins. Basic Res Cardiol. 2013 Nov;108(6):393. doi: 10.1007/s00395-013-0393-6. Epub 2013, Oct 25. PMID:24158693 doi:http://dx.doi.org/10.1007/s00395-013-0393-6
