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Wednesday, January 09, 2008

The ligand-receptor complex dissociates in the endosome because Of the acidic pH of the vesicle

037. The ligand-receptor complex dissociates in the endosome because:

1. Of its large size

2. The vesicle looses its clathrin coat

3. Of the acidic pH of the vesicle

4. Of the basic pH of the vesicle

Answer

3. Of the acidic pH of the vesicle

Reference:

Lippincott 3rd Edition Page 230

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Lippincott

Discussion and Explanation

The pH of the endosome falls due to the protein pumping activity of endosomal ATPase and this allows separation of the ligand from the receptor. Late endosomes are formed as the pH continues to drop due to the activity of endosomal ATPase

Comments and Tips

The diagram below show the Fate of an LDL particle and its receptor after endocytosis. The same pathway is followed by other ligands, such as insulin and other protein hormones, that are internalized by receptor-mediated endocytosis and degraded in the lysosome. After an LDL particle binds to an LDL receptor on the plasma membrane, the receptor-ligand complex is internalized in a clathrin-coated pit that pinches off to become a coated vesicle. The clathrin coat then depolymerizes to triskelions, resulting in an early endosome. This endosome fuses with a sorting vesicle, known as a late endosome, where the low pH (approximately 5) causes the LDL particles to dissociate from the LDL receptors. A receptor-rich region buds off to form a separate vesicle that recycles the LDL receptors back to the plasma membrane. A vesicle containing an LDL particle may fuse with another late endosome but ultimately fuses with a lysosome to form a larger lysosome. There, the apo-B protein of the LDL particle is degraded to amino acids and the cholesterol esters are hydrolyzed to fatty acids and cholesterol. Abundant imported cholesterol inhibits synthesis by the cell of both cholesterol and LDL receptor protein.


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