Lipid Metabolism Practice Questions

Q: Reverse cholesterol transport is mediated by which lipoprotein?

HDL. **Explanation:** **Reverse Cholesterol Transport (RCT)** is the physiological process by which excess cholesterol is removed from peripheral tissues (like macrophages in the arterial wall) and transported back to the liver for excretion in bile. **HDL (High-Density Lipoprotein)** is the primary mediator of this process, which is why it is clinically referred to as "Good Cholesterol." The process involves: 1. **Efflux:** Free cholesterol is moved from cells to nascent HDL via **ABCA1 transporters**. 2. **Esterification:** The enzyme **LCAT** (Lecithin-Cholesterol Acyltransferase) converts free cholesterol into cholesterol esters, trapping them in the HDL core. 3. **Hepatic Uptake:** HDL delivers these esters to the liver via **SR-BI receptors** or transfers them to other lipoproteins via **CETP**. **Why other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** Synthesized in the liver to transport endogenous triglycerides to peripheral tissues. * **LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," it transports cholesterol **from** the liver **to** peripheral tissues. High levels are associated with atherosclerosis. * **IDL (Intermediate-Density Lipoprotein):** A transient product formed during the conversion of VLDL to LDL; it is not involved in RCT. **High-Yield NEET-PG Pearls:** * **Apo A-I:** The major apoprotein associated with HDL and a potent activator of LCAT. * **Tangier Disease:** A genetic deficiency of ABCA1 transporters resulting in extremely low HDL levels and orange-colored tonsils. * **Anti-atherogenic property:** HDL prevents foam cell formation, reducing the risk of Coronary Artery Disease (CAD).

Q: Which lipoprotein has the highest concentration of endogenous triglycerides?

VLDL (Very-low-density lipoprotein). **Explanation:** The core of this question lies in distinguishing between **exogenous** and **endogenous** lipid transport. **Why VLDL is correct:** VLDL (Very-low-density lipoprotein) is synthesized in the **liver**. Its primary physiological role is to transport **endogenous triglycerides** (lipids synthesized by the body) from the liver to peripheral tissues. It contains approximately 50-60% triglycerides by weight, making it the lipoprotein with the highest concentration of triglycerides of internal origin. **Why the other options are incorrect:** * **Chylomicrons:** While chylomicrons have the highest *overall* triglyceride content (85-90%), these are **exogenous** (dietary) triglycerides absorbed from the intestines. The question specifically asks for endogenous sources. * **LDL:** Known as the primary carrier of **cholesterol** to peripheral tissues. It is a metabolic byproduct of VLDL (via IDL) and has a low triglyceride concentration. * **HDL:** Known as "good cholesterol," it is involved in **reverse cholesterol transport** (carrying cholesterol from tissues back to the liver). It has the highest protein content and the lowest lipid content. **High-Yield Clinical Pearls for NEET-PG:** * **Apolipoprotein Marker:** B-100 is the characteristic apolipoprotein for VLDL and LDL, while B-48 is unique to Chylomicrons. * **Rate-limiting enzyme:** HMG-CoA reductase is the rate-limiting enzyme for endogenous cholesterol synthesis (target of Statins). * **Lipoprotein Lipase (LPL):** This enzyme, located on capillary endothelium, is responsible for clearing triglycerides from both Chylomicrons and VLDL. * **Friedewald Equation:** LDL = Total Cholesterol – HDL – (Triglycerides/5). Note: This is invalid if TG >400 mg/dL.

Q: Which lipoprotein has the highest quantity of lipid and the lowest concentration of protein?

Chylomicrons. ### Explanation Lipoproteins are classified based on their **density**, which is determined by the ratio of lipids to proteins. Since lipids have a lower density than water and proteins have a higher density, the lipoprotein with the most lipid and least protein will be the least dense. **1. Why Chylomicrons are correct:** Chylomicrons are the largest and least dense of all lipoproteins. They consist of approximately **98-99% lipid** (predominantly exogenous triglycerides) and only **1-2% protein**. Because they have the highest lipid-to-protein ratio, they float to the top when plasma is left standing, forming a creamy supernate. **2. Analysis of Incorrect Options:** * **VLDL (Very Low Density Lipoproteins):** These are the second least dense. They contain about 90% lipid (mainly endogenous triglycerides) and 10% protein. * **LDL (Low Density Lipoproteins):** Known as "bad cholesterol," they contain about 75-80% lipid (primarily cholesterol) and 20-25% protein. * **HDL (High Density Lipoproteins):** Known as "good cholesterol," these are the smallest and densest. They have the **highest concentration of protein** (approx. 40-55%) and the lowest quantity of lipid. **3. High-Yield NEET-PG Pearls:** * **Electrophoretic Mobility:** On electrophoresis (pH 8.6), the order of mobility from origin to anode is: **Chylomicrons (Origin) < LDL (Beta) < VLDL (Pre-beta) < HDL (Alpha).** Note that VLDL moves faster than LDL despite being less dense. * **Apolipoprotein Markers:** * Chylomicrons: **Apo B-48** (unique marker). * VLDL/IDL/LDL: **Apo B-100**. * HDL: **Apo A-I** (activates LCAT). * **Function:** Chylomicrons transport **exogenous** (dietary) triglycerides, whereas VLDL transports **endogenous** triglycerides from the liver.

Q: Which of the following amino acids is involved in the synthesis of ceramide?

Serine. **Explanation:** **1. Why Serine is Correct:** Ceramide is the fundamental structural unit of all sphingolipids. Its synthesis begins in the endoplasmic reticulum with the **rate-limiting step**: the condensation of **Palmitoyl-CoA** and the amino acid **L-Serine**. This reaction is catalyzed by the enzyme **Serine Palmitoyltransferase (SPT)**, which requires Pyridoxal Phosphate (Vitamin B6) as a cofactor. This condensation produces 3-ketosphinganine, which is subsequently reduced and acylated to form Ceramide. **2. Why Other Options are Incorrect:** * **Tyrosine:** Primarily involved in the synthesis of catecholamines (Dopamine, Epinephrine), Thyroid hormones, and Melanin. * **Arginine:** A precursor for Nitric Oxide (NO), Creatine, and Urea. It is not involved in sphingolipid backbone formation. * **Threonine:** An essential amino acid involved in mucin synthesis and O-linked glycosylation, but it does not contribute to the ceramide structure. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sphingosine Backbone:** Ceramide consists of a sphingosine base attached to a fatty acid via an **amide bond**. * **Sphingomyelin:** Ceramide + Phosphocholine = Sphingomyelin (the only phospholipid that does not contain glycerol). * **Glycosphingolipids:** Ceramide + Sugars = Cerebrosides/Gangliosides. * **Clinical Correlation:** Deficiencies in enzymes that break down sphingolipids lead to **Sphingolipidoses** (e.g., Gaucher’s, Niemann-Pick, and Tay-Sachs disease). * **Key Enzyme:** Serine Palmitoyltransferase is the primary target for regulating sphingolipid levels in the cell.

Q: What is true about the conversion of 20-carbon fatty acids to prostaglandins by prostaglandin synthase?

The primary prostaglandin produced is PGH2.. **Explanation:** The synthesis of prostaglandins begins with the release of **Arachidonic acid** (a 20-carbon polyunsaturated fatty acid) from membrane phospholipids by Phospholipase A2. This arachidonic acid is then acted upon by the bifunctional enzyme **Prostaglandin Synthase** (also known as **Cyclooxygenase or COX**). **Why Option D is Correct:** Prostaglandin Synthase possesses two distinct catalytic activities: **Cyclooxygenase** (which forms PGG2) and **Peroxidase** (which reduces PGG2 to PGH2). **PGH2** is the "parent" or primary prostaglandin from which all other series-2 prostanoids (like PGE2, PGF2α, PGI2, and Thromboxanes) are derived via tissue-specific isomerases. **Analysis of Incorrect Options:** * **Option A:** PGI2 (Prostacyclin) is a downstream product synthesized from PGH2 specifically in the vascular endothelium; it is not the first one formed. * **Option B:** The rate-limiting step of the overall eicosanoid pathway is the release of arachidonic acid from the cell membrane by **Phospholipase A2**, not the COX enzyme itself. * **Option C:** Prostaglandin synthase converts Arachidonic acid to PGH2. The conversion of PGH2 to PGE2 is performed by a specific downstream enzyme, **PGE synthase**. **High-Yield Clinical Pearls for NEET-PG:** * **COX-1 vs. COX-2:** COX-1 is constitutive (gastric protection), while COX-2 is inducible (inflammation). * **Drug Targets:** Aspirin irreversibly inhibits COX-1 and COX-2 by acetylating a serine residue in the active site. * **Glucocorticoids:** These inhibit Phospholipase A2 (via Lipocortin/Annexin A1), blocking the production of all eicosanoids. * **PGI2 vs. TXA2:** PGI2 (Prostacyclin) inhibits platelet aggregation and causes vasodilation, whereas TXA2 (Thromboxane) promotes aggregation and vasoconstriction.

Question 1

Reverse cholesterol transport is mediated by which lipoprotein?

Accepted Answer

HDL

Answer

VLDL

Answer

LDL

Answer

IDL

Question 2

Which lipoprotein has the highest concentration of endogenous triglycerides?

Accepted Answer

VLDL (Very-low-density lipoprotein)

Answer

Chylomicrons

Answer

LDL (Low-density lipoprotein)

Answer

HDL (High-density lipoprotein)

Question 3

Which lipoprotein has the highest quantity of lipid and the lowest concentration of protein?

Accepted Answer

Chylomicrons

Answer

Very low density lipoproteins

Answer

Low density lipoproteins

Answer

High density lipoproteins

Question 4

Which of the following amino acids is involved in the synthesis of ceramide?

Accepted Answer

Serine

Answer

Tyrosine

Answer

Arginine

Answer

Threonine

Question 5

What is true about the conversion of 20-carbon fatty acids to prostaglandins by prostaglandin synthase?

Accepted Answer

The primary prostaglandin produced is PGH2.

Answer

The first prostaglandin synthesized is PGI2.

Answer

Prostaglandin synthase is the rate-limiting enzyme.

Answer

It converts PGH2 to PGE2.

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?