bile acid:sodium symporter activity

Definition

Target type: molecularfunction

Enables the transfer of a solute or solutes from one side of a membrane to the other according to the reaction: bile acid(out) + Na+(out) = bile acid(in) + Na+(in). [TC:2.A.28.-.-]

Bile acid:sodium symporter activity is a crucial process in the absorption of bile acids from the intestinal lumen into the bloodstream. This process involves the coordinated movement of bile acids and sodium ions across the apical membrane of enterocytes, the cells lining the small intestine. The transport is driven by the electrochemical gradient of sodium ions, which are actively pumped out of the enterocyte by the sodium-potassium pump. This creates a favorable gradient for sodium to enter the cell, and as it does, it brings along a bile acid molecule. The symporter protein responsible for this transport is known as the apical sodium-dependent bile acid transporter (ASBT), also called ileal bile acid transporter (IBAT).

The activity of ASBT is essential for the efficient recycling of bile acids, which are critical for the digestion and absorption of lipids. Bile acids are synthesized in the liver, conjugated with glycine or taurine, and secreted into the bile. They are then stored in the gallbladder and released into the small intestine during digestion. After their role in lipid digestion, bile acids are reabsorbed in the terminal ileum by ASBT and transported back to the liver. This process is known as enterohepatic circulation.

The molecular function of ASBT involves several steps:

1. **Binding:** ASBT binds to both bile acids and sodium ions at specific binding sites on the protein.
2. **Conformation change:** The binding of bile acids and sodium ions induces a conformational change in ASBT, causing it to open and allow both molecules to move through the membrane.
3. **Translocation:** The bile acid and sodium ion move across the membrane and are released into the cytoplasm of the enterocyte.
4. **Recycling:** After release, ASBT returns to its initial conformation, ready to bind more bile acids and sodium ions.

In summary, the molecular function of bile acid:sodium symporter activity is a tightly regulated process that ensures efficient absorption of bile acids from the gut, critical for lipid digestion and recycling. The transport is mediated by the ASBT protein, which utilizes the electrochemical gradient of sodium to facilitate the movement of bile acids across the enterocyte membrane.'
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Proteins (4)

Compounds (32)