Metabolism of LDL
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Questions and Answers

What happens to the lipoprotein remnants in the vesicle during LDL metabolism?

  • They are converted into triglycerides.
  • They are transferred to lysosomes for degradation. (correct)
  • They are released back into the bloodstream.
  • They are recycled into receptors.

    • Endocytosed cholesterol inhibits the activity of HMG CoA reductase.

    True

    What is released from lysosomes during the degradation of lipoprotein remnants?

    Free cholesterol, amino acids, fatty acids, and phospholipids

    The degradation of lipoprotein remnants is conducted by lysosomal ______ enzymes.

    <p>hydrolytic</p> Signup and view all the answers

    Match the compounds released during LDL degradation with their purposes:

    <p>Free cholesterol = Regulates cellular cholesterol levels Amino acids = Building blocks for proteins Fatty acids = Energy source Phospholipids = Component of cell membranes</p> Signup and view all the answers

    Which enzyme's activity decreases due to high cholesterol levels?

    <p>HMG CoA reductase</p> Signup and view all the answers

    The recycling of LDL receptors is not influenced by the degradation of LDL-derived cholesterol.

    <p>False</p> Signup and view all the answers

    What effect does endocytosed cholesterol have on cellular cholesterol synthesis?

    <p>It decreases cholesterol synthesis.</p> Signup and view all the answers

    Which lipoprotein has the highest percentage of lipid and the lowest percentage of protein?

    <p>Chylomicrons</p> Signup and view all the answers

    HDL is the most dense type of lipoprotein.

    <p>True</p> Signup and view all the answers

    What primarily limits the entry of LDL cholesterol into cells?

    <p>Reduced expression of the LDL receptor gene</p> Signup and view all the answers

    What type of lipoprotein primarily consists of mostly free cholesterol?

    <p>LDL</p> Signup and view all the answers

    VLDL stands for very low density __________.

    <p>Lipoprotein</p> Signup and view all the answers

    The enzyme ACAT transfers a fatty acid to cholesterol, turning it into cholesteryl ester for storage.

    <p>True</p> Signup and view all the answers

    What happens to excess cholesterol if it is not immediately needed by the cell?

    <p>It is esterified by acyl CoA:cholesterol acyltransferase (ACAT).</p> Signup and view all the answers

    Which lipoprotein is primarily responsible for the transport of cholesterol?

    <p>LDL</p> Signup and view all the answers

    Match the following lipoproteins with their features:

    <p>Chylomicrons = Largest in size, highest lipid content LDL = Carries cholesterol primarily VLDL = Transports triglycerides HDL = Highest protein content and density</p> Signup and view all the answers

    The enzyme that converts cholesterol to cholesteryl ester is called __________.

    <p>ACAT</p> Signup and view all the answers

    Match the following processes with their descriptions:

    <p>Cholesterol esterification = Conversion of cholesterol to a storage form Decreased LDL receptor gene expression = Limits LDL cholesterol entry into cells ACAT activity = Catalyzes transfer of fatty acid to cholesterol Cholesterol synthesis = Production of new cholesterol molecules</p> Signup and view all the answers

    Both VLDLs and LDLs are successively denser than chylomicrons.

    <p>True</p> Signup and view all the answers

    Which of the following actions is a result of increased cellular cholesterol levels?

    <p>Suppression of LDL receptor gene expression</p> Signup and view all the answers

    What is the main compositional difference between HDL and LDL?

    <p>HDL has a higher percentage of protein and phospholipids compared to LDL.</p> Signup and view all the answers

    Cholesterol ester can be immediately used by the cell for structural functions.

    <p>False</p> Signup and view all the answers

    What is the functional result of endocytosed cholesterol on LDL receptors?

    <p>It reduces the synthesis of new LDL receptors.</p> Signup and view all the answers

    Which apolipoprotein is unique to chylomicrons?

    <p>Apolipoprotein B-48</p> Signup and view all the answers

    Apolipoprotein B-48 is synthesized in the smooth ER.

    <p>False</p> Signup and view all the answers

    What stimulates the synthesis of lipoprotein lipase?

    <p>insulin</p> Signup and view all the answers

    Chylomicrons are assembled with the help of the microsomal _____ transfer protein.

    <p>triacylglycerol</p> Signup and view all the answers

    Match the following components involved in chylomicron metabolism with their roles:

    <p>Apo B-48 = Unique apolipoprotein of chylomicrons Smooth ER = Location of triacylglycerol synthesis Lipoprotein lipase = Degrades triacylglycerol in chylomicrons Secretory vesicles = Fuse with plasma membrane to release lipoproteins</p> Signup and view all the answers

    What happens to the size and density of chylomicrons as they circulate?

    <p>They decrease in size and increase in density.</p> Signup and view all the answers

    The apoproteins returned to HDL after chylomicron metabolism include apo E.

    <p>False</p> Signup and view all the answers

    Where are chylomicrons released into after being packaged in secretory vesicles?

    <p>lymphatic system</p> Signup and view all the answers

    Which apoproteins must be obtained from circulating HDL for the release of VLDL?

    <p>apo C-II and apo E</p> Signup and view all the answers

    Apo C-II is required for the activation of lipoprotein lipase.

    <p>True</p> Signup and view all the answers

    What modification occurs to VLDL as it passes through circulation?

    <p>Triacylglycerol is degraded by lipoprotein lipase.</p> Signup and view all the answers

    LDL is produced from VLDL after undergoing various __________.

    <p>modifications</p> Signup and view all the answers

    Match the lipoproteins with their characteristics:

    <p>VLDL = High triacylglycerol content LDL = High concentration of cholesterol IDL = Intermediate density lipoprotein HDL = High density and removal of cholesterol from arteries</p> Signup and view all the answers

    What happens to surface components, including apo C and E, during the modification of VLDL?

    <p>They are returned to HDL.</p> Signup and view all the answers

    Triacylglycerols can be transferred from HDL to VLDL and vice versa.

    <p>True</p> Signup and view all the answers

    What role does cholesteryl ester transfer protein play in lipoprotein metabolism?

    <p>It facilitates the exchange of cholesteryl esters and triacylglycerols between HDL and VLDL.</p> Signup and view all the answers

    Study Notes

    LDL Metabolism Overview

    • LDL receptors on cells can be recycled, while lipoprotein remnants are transferred to lysosomes for degradation.
    • Lysosomal hydrolytic enzymes release free cholesterol, amino acids, fatty acids, and phospholipids for cellular reutilization.

    Cholesterol Homeostasis Effects

    • Endocytosed cholesterol influences cellular cholesterol content by:
      • Inhibiting HMG CoA reductase, reducing de novo cholesterol synthesis.
      • Decreasing synthesis of new LDL receptor proteins, limiting LDL cholesterol entry into cells.
      • Promoting esterification by ACAT when cholesterol is not needed immediately.

    Synthesis of Cholesteryl Ester

    • ACAT transfers a fatty acid from CoA derivatives to cholesterol, forming cholesteryl esters that can be stored within cells.

    Types and Composition of Lipoproteins

    • Chylomicrons: High triglyceride (TG) content, lowest density.
    • VLDL: Moderate TG content, higher density than chylomicrons.
    • LDL: Rich in cholesterol, less TG than VLDL.
    • HDL: High in protein and phospholipids, known as "good" cholesterol.

    Lipoprotein Size and Density

    • Chylomicrons are the largest and least dense lipoproteins, dominated by lipids.
    • VLDL and LDL are progressively denser with higher protein-to-lipid ratios.
    • HDL particles are the densest lipoproteins.

    Chylomicron Metabolism

    • Apolipoprotein B-48 is unique to chylomicrons, synthesized in the rough ER and glycosylated through the Golgi.
    • Enzymes for lipoprotein synthesis are located in the smooth ER where proteins and lipids assemble into chylomicrons.

    Regulation of Lipoprotein Lipase

    • Lipoprotein lipase synthesis is stimulated by insulin, allowing degradation of triacylglycerol in chylomicrons.
    • As chylomicrons lose triacylglycerol, they decrease in size and increase in density.

    VLDL Modification

    • VLDL is modified in circulation by lipoprotein lipase, which reduces size and increases density.
    • Surface components, including C and E apoproteins, are returned to HDL while retaining apo B-100.

    LDL Production from VLDL

    • Modified VLDL converts into LDL, passing through intermediate-density lipoproteins (IDL) during the process.
    • IDL can be endocytosed by cells using apo E as a ligand.

    Cholesteryl Ester Transfer

    • Triacylglycerols and cholesteryl esters can be exchanged between VLDL and HDL through cholesteryl ester transfer protein, facilitating lipid metabolism.

    LDL Characteristics

    • LDL particles have low triglyceride content compared to their VLDL precursors, with a high concentration of cholesterol and cholesteryl esters.

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    Description

    This quiz focuses on the metabolism of Low-Density Lipoproteins (LDL), including receptor recycling and the role of lysosomal enzymes in degrading lipoprotein remnants. Explore how these processes contribute to lipid metabolism and overall cellular health.

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