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This Sandbox is Reserved from February 27 through August 31, 2023 for use in the course CH462 Biochemistry II taught by R. Jeremy Johnson at the Butler University, Indianapolis, USA. This reservation includes Sandbox Reserved 1765 through Sandbox Reserved 1795.

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Sodium Bile Salt Co-Transporting Protein

spinqualitylabelsall models Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Structure Binding Sites Sodium NTCP, among others in the SLC10 family, have two sodium binding. Many polar and negatively charged residues are characteristic of these active sites. The high level of conservation among sodium binding placement and interacting residues suggests sodium binding is coupled to bile salt transport. Additional mutations in the X-motif near sodium binding sites have shown that bile salt transport function is lost also suggesting that sodium allows bile salt binding. It is understood that these sodium binding sites facilitate changes from open-pore to closed pore states of NTCP that allow for the binding or release of bile salts. Closed-pore state is favored in the absence of sodium ions, while open-pore state is favored in the presence of sodium ions. This also allows for sodium concentrations to regulate uptake of taurocholates. When intracellular sodium levels are higher, open-pore state is favored allowing for the diffusion of taurocholates. However, when extracellular sodium levels are high, closed-state is favored preventing diffusion of taurocholates. Bile Salt The amphipathic pore is also characteristic of NTCP. The pore surface remains Hydrophobic, while lining of the open pore state is largely Polar. This pattern is believed to follow similar amphipathic patterns within taurocholate and other NTCP substrates, such as steroids or hormones Thus the channel provides specificity while preventing leakage of other substrates. When observing the relevant bile salt binding residues it is shown that some residues form Van der Waals interactions while others will form dipole-dipole or ionic interactions with bile salt substrates. The core domain appears to contribute most of the polar domains, while the panel domain contributes more hydrophobic residues. Conformational Change   Cartoon representation of NTCP conformational change.   Cartoon representation of NTCP conformational change. Bile Salt Transport A proposed pathway for NTCP bile salt transport suggests that both sodium ions are translocated with the transport of one bile salt. In this mechanism both sodium ions are released along with the inner bile salt into the cytoplasm (Fig. 5). The outermost bile salt remains bound however in the pore, likely helping to prevent leakage <Ref name = "Liu"> The movement of sodium ions then facilitates the conformational change to the inward-facing, pore inaccessible conformation that is believed to displace the outer bile salt into the inner bile salt placement (Fig. 5). </Ref name = "Liu"> Sodium ion then bind to NTCP, favoring the open-pore state and also allowing for the binding of another outer bile salt (Fig 5). The process can then start again releasing the next inner bile salt with the translocation of the sodium ions into the cytoplasm. Medical Relevancy This is a sample scene created with SAT to color by Group, and another to make a transparent representation of the protein. You can make your own scenes on SAT starting from scratch or loading and editing one of these sample scenes.

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Jaime Prilusky