Background

Figure 1. NTCP structure with both Na ions and bile salts bound. PDB file 7ZYI.

• Sodium taurocholate co-transporting polypeptide (NTCP) is a sodium-dependent transporter in the body that is responsible for the transportation of bile salts from the blood into epithelial liver cells. This carrier protein is responsible for a conformational change that allows bile salts to cross the cell membrane and enter the inside of liver cells. ^[1]. Both sodium ions and bile salts bind to NTCP in the same binding pocket on the molecule (Fig. 1). NTCP also acts as a receptor for Hepatitis B virus and Hepatitis D virus.

• The bile salts transported by NTCP are located within the gastrointestinal tract of the body and play a very key role in many biological functions. These functions include digesting and absorbing nutrients by helping break down fats and transporting lipid soluble nutrients into the liver. ^[2].

• The NTCP carrier protein itself can be found within hepatocytes, or the epithelial cells of the liver, but more specifically, within the basolateral membrane of these cells. ^[3]. The uptake of bile salts into the liver also allow for drugs to be both absorbed and excreted, as well as essential nutrients such as Vitamin A,D,E, and K to be absorbed in the small intestine.

Structural Overview

Function

Binding Pocket

• The NTCP binding pocket represents a "tunnel" lined with hydrophilic residues within the NTCP structure to allow hydrophilic bile salts and sodium ions to bind. When there are no bile salts bound to NTCP's binding tunnel, it forms a hollow hole in the middle of the structure, from . As bile salts bind to this binding pocket, the bile salts very nicely fill the hole within NTCP and fit almost perfectly within the tunnel, to the point where the can no longer be seen, as it is completely encompassing the bile salts.

Mechanism

Significance

HBV/ HDV

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