Defining Substrate Binding Site in Sodium-Dependent Bile Acid Transporters

Abstract

Bile acids are stored in the gallbladder and excreted into the digestive tract where they assist absorption of lipids and vitamins. Bile acids are synthesized from cholesterol in the liver before they are stored and entered into the small intestine. In the small intestine, more than 95% of the bile acids are absorbed and sent back to the liver through a process known as enterohepatic circulation. In humans, there are two Na+-dependent transporters that play an essential role in enterohepatic circulation—Na+-taurocholate co-transporting polypeptide (NTCP) and apical sodium-dependent bile acid transporter (ASBT). NTCP and ASBT share 35% sequence identity and both belong to the solute carrier 10 (Slc10A) family of secondary transporters. NTCP, in addition to its role in bile acid transport, is required for infection of hepatitis B virus because it serves as a co-receptor for human hepatitis B and D virus. ASBT is crucial to enterohepatic circulation as it has been correlated to multiple digestive illnesses, and has become a potential drug target for hypercholesterolaemia, type 2 diabetes, and constipation. Crystals structures of two bacterial homologs of human ASBT, ASBTNm and ASBTYf, have been reported, however, the two reports differ in their definition of the bile acid binding site. We will better define the bile acid binding site and its selectivity to various bile acids by making mutations to solvent exposed residues on the ASBTYf transporter and measure binding and transport of bile acids. These studies will give a better understanding of the substrate binding site of ASBT.