Aquaporin, transmembrane protein

Dendritic Dipeptides as Aquaporin Transmembrane Protein Mimics. 188... 

Propose the key elements of a transmembrane protein that transports water through a lipid bilayer (consider the aquaporins). Draw a picture. 

A long with a small diameter of 8.1 A. In another study, Kalra et al. [6] reported a through carbon nanotube comparable to the flow rate measured for the transmembrane protein aquaporin-1, and are practically independent of the length of the nanotube, in contrast to predictions of macroscopic hydrodynamics. A study by N. Alum s group [7] examined ionic flow in... 

FIGURE 11-46 Structure of an aquaporin, AQP-1. The protein is a tetramer of identical monomeric units, each of which forms a transmembrane pore (derived from PBD ID 1J4N). (a) Surface model viewed perpendicular to the plane of the membrane. The protein contains four pores, one in each subunit. (The opening at the junction of the subunits is not a pore.) (b) An AQP-1 tetramer, viewed in the plane of the membrane. The helices of each subunit cluster around a central transmembrane pore. In each monomer, two short helical loops, one between helices 2 and 3 and the other between 5 and 6, contain the Asn-Pro-Ala (NPA) sequences found in all aquaporins, and form part of the water channel, (c) Surface representation of a single subunit, viewed in the plane of the membrane. The near side of the AQP-1... 

Specifically mediate water fluxes within the brain. Among several members of the aquaporin family, aquaporin 4 and 9 are expressed in astrocytes. The activity of aquaporins is regulated by transmembrane G-protein coupled receptor (GPCR) family of hormonal receptors. 

As discussed in Chapter 7, the plasma membranes of animal cells contain a water channel called aquaporin. Like most other integral proteins, aquaporin contains multiple transmembrane a helices. Thus, despite its name, aquaporin differs structurally from the porins as well as functionally in that it mediates transport of a single molecule—namely, water. 

Water as the unique natural medium for native cells contribntes a major portion of their weight. Water (with dissolved inorganic and organic compounds) can penetrate into or from cells throngh membrane channels or by transport protein molecules (e.g., aquaporins) (Finkelstein 1987, Murata et al. 2000, Vorisek 2000, Fujiyoshi et al. 2002). The length of transmembrane channels... 

Another important class of proteins that contain water channels are the aquaporins, which regulate the flow of water in and out of cells. They will let water through but not salts or other dissolved substances, and as such, they act as molecular water filters. Water transport occurs via a chain of nine hydrogen-bonded molecules (Fig. 6.13). But if this chain were to permit rapid transmembrane proton motion, that would disturb the delicate charge balance across the membrane. So aquaporin must somehow disrupt the potential proton wire that threads through it. The mechanism has been much debated, but it now seems that the inhibition of proton transport is dominated by electrostatic repulsion by positively charged groups in a narrow constriction in the middle of the pore [72]. 

This selective transport across cellular membranes is carried out by two broad classes of specialized proteins, which are assodated with or embedded in those lipid bilayers channels and transmembrane transporters. They work by different mechanisms Whereas channels catalyze the passage of ions (or water and gas in the case of the aquaporin channel) (Agre, 2006) across the membrane through a watery pore spanning the membrane-embedded protein, transporters are working via a cycle of conformational changes that expose substrate-binding sites alternately to the two sides of the membrane (Theobald Miller, 2010). 

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