Water flow through aquaporin channels

Water molecules flow through an AQP-1 channel at the rate of about 109 s 1. For comparison, the highest known turnover number for an enzyme is that for catalase, 4 X 107 s-1, and many enzymes have turnover numbers between 1 s 1 and 104 s 1 (see Table 6-7). The low activation energy for passage of water through aquaporin channels (AG < 15 kJ/mol) suggests that water moves through the channels in a continuous... 

Aquaporins comprise six transmembrane a-helices, and five interhelical loop regions (A-E) that form the extracellular and cytoplasmic vestibules. Loops B and E are hydrophobic loops which contain the highly, although not completely, conserved Asn-Pro-Ala (NPA) motif, which overlaps the middle of the lipid bilayer of the membrane, forming a 3D hourglass structure through which water flows. Two constrictions in the channel act as selectivity Alters. 

Water is considered to diffuse through membranes by unspecific movement through ion channels, pores, or around proteins embedded in the lipids. Certain cells (e.g., renal tubule cells) also contain large protein pores, called aquaporins, which permit a high rate of water flow from a region of a high water concentration (low solute concentration) to one of low water concentration (high solute concentration). 

The spectmm of protein functions in a biological organism is manifold. Proteins are involved in almost all cell processes. Ion channels and nanopores formed by membrane proteins control ion and water flow into and out of the cell [4]. Figure 1.3 shows the atomic stmcture of the membrane protein aquaporine, a pore embedded in the cell membrane that is permeable for water molecules. The efficiency is extreme. Up to 3 billion water molecules can msh through this pore per second [5,6]. Other cell proteins like actin, for example, are responsible for the mechanical stability of the cell backbone. Actin polymerizes to filaments and these filaments can form stable networks. Besides cell stability, these networks also enable transport processes along these tracks, e.g., vesicle transport mediated by myosin proteins. The interplay between actin and myosin is also important for the ability of muscle tissue to contract. Biochemical reactions are catalyzed... 

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]. 

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