Structure of Water ChannelsSince th

Structure of Water Channels

Since the initial discovery that a protein acts as a water channel in mammalian red blood cell membranes, additional water channels have been discovered in other tissues, with currently 10 (or 11) known mammalian water channels. CHIP28 [also known as aquaporin-1 (AQP1)] is now identified as a representative of this new class of transport proteins known as aquaporins. AQP1 has 269 amino acids forming two tandem repeats of three membrane-spanning -helices plus two short helical loops (B and E loops) within the lipid bilayer (31). Figure 2 shows the “hourglass” model of AQP1. The carboxy and amino termini are both cytoplasmic. The B loop connects helices 2 and 3 and the E loop connects helices 5 and 6. The connecting loops each contain an Asn-Pro-Ala (NPA) motif that appears to be the site of the channel for water. Sui et al. (26) conducted definitive analysis of AQP1 by X-ray crystallography down to a resolution of 2.2 Å and in so doing clarified the selectivity of the pore region for water molecules. The short helical B and E loops are two membrane-inserted non-membrane- spanning helices capped by Asn residues. The centrally located channel adjacent to these loops has a constriction of
2.8 Å (water molecules have a radius of 2.8 Å). Thus the pore itself consists of an extracellular and a cytoplasmic vestibule connected by an extended narrow pore with a long hydrophobic core and a minimal number of solute-binding sites. Residues in the region of the constriction (particularly histidine-182, which is conserved in the aquaporin family) are critical for the selectivity of the channel for water molecules. The hydrophilic face of the pore provides the chemical groups for displacing waters of hydration to establish a pathway for coordinating the
transport of water molecules. It appears that the formation of hydrogen bonds between water molecules and the pore residues causes the specificity of the channel for water. Water molecules permeate the channel single file and break hydrogen bonds with each other to form one and then the other hydrogen bonds with residues of the B and E loops within the channel

Hasil (Bahasa Indonesia) 1:

Struktur saluran airSince the initial discovery that a protein acts as a water channel in mammalian red blood cell membranes, additional water channels have been discovered in other tissues, with currently 10 (or 11) known mammalian water channels. CHIP28 [also known as aquaporin-1 (AQP1)] is now identified as a representative of this new class of transport proteins known as aquaporins. AQP1 has 269 amino acids forming two tandem repeats of three membrane-spanning -helices plus two short helical loops (B and E loops) within the lipid bilayer (31). Figure 2 shows the “hourglass” model of AQP1. The carboxy and amino termini are both cytoplasmic. The B loop connects helices 2 and 3 and the E loop connects helices 5 and 6. The connecting loops each contain an Asn-Pro-Ala (NPA) motif that appears to be the site of the channel for water. Sui et al. (26) conducted definitive analysis of AQP1 by X-ray crystallography down to a resolution of 2.2 Å and in so doing clarified the selectivity of the pore region for water molecules. The short helical B and E loops are two membrane-inserted non-membrane- spanning helices capped by Asn residues. The centrally located channel adjacent to these loops has a constriction of
2.8 Å (water molecules have a radius of 2.8 Å). Thus the pore itself consists of an extracellular and a cytoplasmic vestibule connected by an extended narrow pore with a long hydrophobic core and a minimal number of solute-binding sites. Residues in the region of the constriction (particularly histidine-182, which is conserved in the aquaporin family) are critical for the selectivity of the channel for water molecules. The hydrophilic face of the pore provides the chemical groups for displacing waters of hydration to establish a pathway for coordinating thetransport of water molecules. It appears that the formation of hydrogen bonds between water molecules and the pore residues causes the specificity of the channel for water. Water molecules permeate the channel single file and break hydrogen bonds with each other to form one and then the other hydrogen bonds with residues of the B and E loops within the channel