During the uptake of low-density lipoprotein (LDL) by a liver cell, LDL-receptor
ID: 3167117 • Letter: D
Question
During the uptake of low-density lipoprotein (LDL) by a liver cell, LDL-receptor protein complexes are internalized by endocytosis. The endosomes then fuse with lysosomes, where protein components of LDL are hydrolyzed to free amino acids, while cholesterol esters are hydrolyzed by a lysosomal acid lipase. The LDL receptor itself is not affected by lysosomal enzymes.
a. Briefly describe what would happen to cholesterol metabolism in a cell deficient in lysosomal acid lipase.
Since the cholesterol esters are not hydrolyzed to form free cholesterol, the cells would not have enough free cholesterol to meet their needs. This would cause the liver to continually synthesize cholesterol for transport to peripheral tissues, leading to a buildup of LDL.
b. Why is it important that LDL receptors are not degraded by lysosomal enzymes?
They must be recycled back to the cell surface so they can be reused to internalize more LDL.
The correct answer to the quesion is provided in italics, but I am having trouble understanding the backgorund info. Can you please break down the questio stem and reasoning for answers in more detail? Thank you
Explanation / Answer
a. If the cell is deficient in lysosomal acid lipase, then the breakdown of fats and cholesterol will be affected. This leads to deposition of fats in the liver in the beginning. This causes enlargement of the liver and the spleen. When cholesterol esters are not hydrolyzed, free cholesterol is not formed. So, need of cholesterol is not fulfilled. Cell need cholesterol for various function, most importantly membrane fluidity. This causes liver to form cholesterol for tarnsport to peripheral tissues. As a result LDL builds up.
b. LDL receptor is a transmembrane protein. It is involved in receptor mediated endocytosis. And helps in maintaining the plasma level of LDL. As it is recyclable receptor and it can be reused again. When LDL enters the lysosomes along with its ligand, the ligand dissociate at low pH and receptor go back to the surface.
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