Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 341: What is the end product of even-chain fatty acid oxidation?

A. Acetyl Co-A (Correct Answer)
B. Propionyl Co-A
C. Malonyl Co-A
D. None of the above

Explanation: **Explanation:** The oxidation of fatty acids primarily occurs via the **$\beta$-oxidation pathway** within the mitochondria. **1. Why Acetyl-CoA is correct:** In the $\beta$-oxidation of **even-chain fatty acids** (e.g., Palmitic acid), the fatty acid chain undergoes a repetitive four-step cycle (oxidation, hydration, oxidation, and thiolysis). In each cycle, two carbon atoms are cleaved from the carboxyl end of the fatty acyl-CoA molecule. The final cleavage of a four-carbon intermediate (Acetoacetyl-CoA) yields **two molecules of Acetyl-CoA**. Therefore, the entire even-chain fatty acid is completely degraded into Acetyl-CoA units, which then enter the TCA cycle for ATP production. **2. Why other options are incorrect:** * **Propionyl-CoA:** This is the end product of **odd-chain fatty acid** oxidation. In the final round of $\beta$-oxidation for odd-chain fats, a five-carbon fragment is cleaved into one Acetyl-CoA and one three-carbon Propionyl-CoA. * **Malonyl-CoA:** This is an intermediate of **fatty acid synthesis** (lipogenesis), not oxidation. It acts as a potent inhibitor of Carnitine Palmitoyltransferase-I (CPT-I), preventing a futile cycle by stopping fatty acid breakdown while synthesis is active. **Clinical Pearls for NEET-PG:** * **Site:** $\beta$-oxidation occurs in the mitochondrial matrix; Fatty acid synthesis occurs in the cytosol. * **Rate-limiting step:** The transport of fatty acids into the mitochondria via the **Carnitine Shuttle** (inhibited by Malonyl-CoA). * **Energy Yield:** Oxidation of one molecule of Palmitate (16C) yields a net of **106 ATP**. * **Odd-chain metabolism:** Propionyl-CoA is converted to Succinyl-CoA (a TCA cycle intermediate) via a Vitamin B12-dependent pathway. This makes odd-chain fatty acids **glucogenic**.

Question 342: What is the cofactor for lipoprotein lipase?

A. apo A-I
B. apo A-II
C. apo C-II (Correct Answer)
D. apo C-III

Explanation: **Explanation:** Lipoprotein Lipase (LPL) is a key enzyme located on the luminal surface of capillary endothelial cells (primarily in adipose tissue, cardiac, and skeletal muscle). Its primary function is to hydrolyze triglycerides within Chylomicrons and Very Low-Density Lipoproteins (VLDL) into free fatty acids and glycerol. **Why apo C-II is correct:** **Apo C-II** acts as a mandatory **obligatory cofactor** for LPL. It is acquired by chylomicrons and VLDL from HDL in the circulation. Without apo C-II, LPL remains inactive, preventing the peripheral tissues from utilizing the triglycerides carried by these lipoproteins. **Analysis of Incorrect Options:** * **Apo A-I:** This is the major structural protein of HDL and serves as the activator for **LCAT** (Lecithin-Cholesterol Acyltransferase), which esterifies cholesterol. * **Apo A-II:** Primarily found in HDL; its exact physiological role is less clear, but it may inhibit LPL or modulate hepatic lipase, rather than activate it. * **Apo C-III:** This protein actually **inhibits** LPL. High levels of apo C-III are associated with hypertriglyceridemia because it prevents the breakdown of triglyceride-rich lipoproteins. **NEET-PG High-Yield Pearls:** 1. **Insulin** stimulates the synthesis and secretion of LPL in adipose tissue (promoting fat storage). 2. **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome):** Caused by a genetic deficiency of either **LPL** or **apo C-II**. Patients present with eruptive xanthomas, pancreatitis, and milky plasma. 3. **Heparin** releases LPL from the endothelial surface into the blood; this is used clinically to measure "post-heparin lipolytic activity."

Question 343: Which of the following is not a ketone body?

A. Acetone
B. Acetic acid (Correct Answer)
C. Acetoacetate
D. Beta-hydroxybutyric acid

Explanation: **Explanation:** Ketone bodies are water-soluble molecules produced by the liver from fatty acids during periods of low food intake (fasting), carbohydrate restrictive diets, or untreated type 1 diabetes. The process, known as **ketogenesis**, occurs in the mitochondrial matrix of hepatocytes. **Why Acetic Acid is the Correct Answer:** Acetic acid (Option B) is a two-carbon carboxylic acid. While it is related to the metabolism of Acetyl-CoA, it is **not** classified as a ketone body. In the context of metabolism, acetic acid is usually found in its activated form, Acetyl-CoA, which serves as the precursor for ketogenesis but is not a ketone body itself. **Analysis of Incorrect Options:** * **Acetoacetate (Option C):** This is the "primary" ketone body synthesized from Acetyl-CoA. It is a true keto-acid. * **Beta-hydroxybutyric acid (Option D):** Formed by the reduction of acetoacetate. Although technically a hydroxy-acid (lacking a keto group), it is clinically and biochemically classified as a ketone body. It is the most abundant ketone body in the blood during ketosis. * **Acetone (Option A):** Produced by the spontaneous non-enzymatic decarboxylation of acetoacetate. It is highly volatile and excreted via the lungs. **NEET-PG High-Yield Pearls:** 1. **Site of Synthesis:** Liver (mitochondria); however, the liver **cannot** utilize ketone bodies because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). 2. **Rate-limiting Enzyme:** HMG-CoA Synthase (Mitochondrial). 3. **Detection:** The **Rothera’s Test** detects Acetoacetate and Acetone, but it does **not** detect Beta-hydroxybutyrate. 4. **Clinical Sign:** The "fruity odor" of the breath in Diabetic Ketoacidosis (DKA) is due to the excretion of Acetone.

Question 344: What is the basic structure of a triglyceride?

A. Two molecules of fatty acids esterified to glycerol
B. Three molecules of fatty acids esterified to glycerol (Correct Answer)
C. Two molecules of fatty acids and 2,3-diphosphoglycerate
D. Three molecules of fatty acids and 2,3-diphosphoglycerate

Explanation: ### Explanation **Correct Answer: B. Three molecules of fatty acids esterified to glycerol** **1. Underlying Concept:** Triglycerides (also known as triacylglycerols or TAGs) are the primary storage form of lipids in the human body, predominantly found in adipose tissue. Chemically, they are **esters**. The structure consists of a single molecule of **glycerol** (a 3-carbon sugar alcohol) acting as a backbone. Each of the three hydroxyl (-OH) groups of the glycerol molecule undergoes an esterification reaction with the carboxyl group (-COOH) of a **fatty acid**. This results in a neutral fat molecule with three fatty acid chains. **2. Analysis of Incorrect Options:** * **Option A:** Describes a **diglyceride** (diacylglycerol). While these exist as intermediates in lipid metabolism and act as second messengers (e.g., in the PIP2 pathway), they are not the "basic structure" of the storage lipid. * **Options C & D:** These are incorrect because **2,3-diphosphoglycerate (2,3-DPG)** is a glycolytic intermediate found in RBCs that regulates hemoglobin’s affinity for oxygen. It is not a structural component of lipids. **3. NEET-PG High-Yield Clinical Pearls:** * **Storage:** TAGs are stored in a dehydrated form in adipocytes, making them a highly efficient energy reserve (9 kcal/g). * **Transport:** Being non-polar and hydrophobic, TAGs cannot float freely in the blood; they are transported within **lipoproteins** (primarily Chylomicrons and VLDL). * **Clinical Correlation:** Elevated serum triglycerides (>150 mg/dL) are a risk factor for cardiovascular disease. Extreme elevations (>1000 mg/dL) significantly increase the risk of **acute pancreatitis**. * **Enzyme Link:** **Lipoprotein Lipase (LPL)** is the key enzyme that hydrolyzes TAGs in chylomicrons and VLDL into free fatty acids for tissue uptake.

Question 345: Squalene is the intermediate product during the synthesis of which of the following?

A. VLDL
B. Cholesterol (Correct Answer)
C. Tachysterol
D. Lanosterol

Explanation: **Explanation:** The synthesis of **Cholesterol** occurs in the cytosol and endoplasmic reticulum of cells (primarily in the liver). Squalene is a critical intermediate in this multi-step de novo biosynthetic pathway. **Why Cholesterol is correct:** Cholesterol synthesis follows a specific sequence: 1. **Acetate** (2C) → **Mevalonate** (6C) via the rate-limiting enzyme HMG-CoA Reductase. 2. Mevalonate → **Isopentenyl Pyrophosphate (IPP)** (5C). 3. Condensation of six IPP units forms **Squalene**, a 30-carbon linear hydrocarbon. 4. Squalene undergoes cyclization to form **Lanosterol**, which is eventually converted into **Cholesterol** (27C). **Why other options are incorrect:** * **VLDL:** This is a lipoprotein responsible for transporting endogenous triglycerides from the liver to peripheral tissues. While it carries cholesterol, it is a transport vehicle, not a biosynthetic product of squalene. * **Tachysterol:** This is a byproduct of Vitamin D synthesis formed during the irradiation of 7-dehydrocholesterol. It is not an intermediate in the primary cholesterol pathway. * **Lanosterol:** While Lanosterol is indeed synthesized from Squalene, the question asks for the final product of the pathway. Lanosterol is the first *sterol* formed, but it is an intermediate that must be converted into Cholesterol. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** HMG-CoA Reductase (Target of **Statins**). * **Squalene Epoxidase:** The enzyme that converts squalene to squalene-2,3-epoxide; it is inhibited by the antifungal **Terbinafine**. * **Carbon Count:** Acetate (2C) → Mevalonate (6C) → IPP (5C) → Geranyl PP (10C) → Farnesyl PP (15C) → Squalene (30C) → Cholesterol (27C). * **Farnesyl Pyrophosphate (FPP):** An intermediate used for the synthesis of Coenzyme Q (Ubiquinone) and Dolichol.

Question 346: During fatty acid synthesis, humans cannot introduce double bonds beyond which position?

A. 4
B. 7
C. 9 (Correct Answer)
D. 11

Explanation: ### Explanation **1. Why Option C is Correct:** Human cells possess fatty acid desaturase enzymes (specifically $\Delta^4$, $\Delta^5$, $\Delta^6$, and $\Delta^9$ desaturases) that can introduce double bonds at specific locations in a fatty acid chain. However, humans **lack the enzymatic machinery** to introduce double bonds beyond the **$\Delta^9$ position** (counting from the carboxyl end). Specifically, we lack $\Delta^{12}$ and $\Delta^{15}$ desaturases, which are found in plants. Consequently, any fatty acid with a double bond beyond carbon 9 cannot be synthesized *de novo* and must be obtained through the diet. **2. Why Other Options are Incorrect:** * **Option A (4):** Humans do possess $\Delta^4$ desaturase, which is involved in the synthesis of highly unsaturated fatty acids like Docosahexaenoic acid (DHA). * **Option B (7):** While not a primary regulatory desaturase, the body can easily desaturate at positions before carbon 9. * **Option D (11):** This is beyond the human enzymatic capability. Double bonds at $\Delta^{12}$ or $\Delta^{15}$ are characteristic of essential fatty acids. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Essential Fatty Acids (EFAs):** Because we cannot desaturate beyond $\Delta^9$, **Linoleic acid (18:2; $\Delta^{9,12}$)** and **Linolenic acid (18:3; $\Delta^{9,12,15}$)** are nutritionally essential. * **Arachidonic Acid:** It is a $\omega$-6 fatty acid. It is considered "semi-essential" because it can be synthesized from Linoleic acid. * **Site of Synthesis:** Fatty acid desaturation occurs in the **Smooth Endoplasmic Reticulum (SER)**. * **Key Components:** The desaturase system requires **NADH, Cytochrome b5, and Mg²⁺**. * **Palmitoleic (16:1; $\Delta^9$) and Oleic acid (18:1; $\Delta^9$):** These are the most common monounsaturated fatty acids synthesized by the human $\Delta^9$ desaturase.

Question 347: A 45-year-old man on a fat-free, carbohydrate-rich diet for a certain time continues to gain weight and become obese. Which of the following lipoproteins is likely to be elevated in his blood?

A. Chylomicrons
B. VLDL (Correct Answer)
C. LDL
D. HDL

Explanation: ### Explanation **Correct Option: B (VLDL)** The core concept here is **Endogenous Lipogenesis**. When a person consumes a diet high in carbohydrates but low in fat, the liver processes the excess glucose through glycolysis and the Citric Acid Cycle. The resulting excess Acetyl-CoA is diverted toward **De Novo Lipogenesis** (fatty acid synthesis). These newly synthesized triglycerides are packaged into **Very Low-Density Lipoproteins (VLDL)** in the liver and secreted into the bloodstream to be transported to adipose tissue for storage. Therefore, a high-carbohydrate diet directly stimulates hepatic VLDL production, leading to weight gain and elevated plasma VLDL levels. **Why Incorrect Options are Wrong:** * **A. Chylomicrons:** These transport **exogenous (dietary) lipids** from the intestine. Since the patient is on a fat-free diet, chylomicron production will be minimal. * **C. LDL:** While VLDL is eventually converted to LDL, the primary lipoprotein elevated due to immediate hepatic synthesis from carbohydrate excess is VLDL. LDL elevation is usually associated with cholesterol metabolism defects rather than acute carbohydrate loading. * **D. HDL:** Known as "good cholesterol," HDL is involved in reverse cholesterol transport. High carbohydrate intake often correlates with *decreased* HDL levels, not elevation. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme for Fatty Acid Synthesis:** Acetyl-CoA Carboxylase (stimulated by Insulin). * **Apolipoprotein of VLDL:** Apo B-100 (Endogenous pathway). * **Apolipoprotein of Chylomicrons:** Apo B-48 (Exogenous pathway). * **Clinical Correlation:** High-carbohydrate diets are a common cause of **Hypertriglyceridemia** because VLDL is rich in triglycerides.

Question 348: High-density lipoprotein (HDL) levels are increased by which of the following?

A. Exercise (Correct Answer)
B. 400 mg Vitamin E daily
C. Aspirin
D. DHEA

Explanation: **Explanation:** **High-density lipoprotein (HDL)**, often referred to as "good cholesterol," plays a critical role in **Reverse Cholesterol Transport**, where it removes excess cholesterol from peripheral tissues and transports it back to the liver for excretion. **Why Exercise is Correct:** Physical activity, particularly aerobic exercise, is one of the most effective non-pharmacological ways to raise HDL levels. Exercise increases the activity of **Lipoprotein Lipase (LPL)** and decreases the activity of **Cholesteryl Ester Transfer Protein (CETP)**. This shift promotes the maturation of HDL particles and reduces their clearance, leading to a sustained increase in plasma HDL-C levels. **Analysis of Incorrect Options:** * **Vitamin E:** While Vitamin E is a potent lipid-soluble antioxidant that prevents the oxidation of LDL, clinical trials have not demonstrated a significant or consistent effect on increasing HDL levels. * **Aspirin:** Aspirin is an antiplatelet agent used for secondary prevention of cardiovascular events. It does not have a direct biochemical effect on lipoprotein synthesis or HDL concentration. * **DHEA (Dehydroepiandrosterone):** DHEA is an endogenous steroid. Clinical studies generally show that DHEA supplementation either has no effect or may actually **decrease** HDL levels, particularly in women. **Clinical Pearls for NEET-PG:** * **ApoA-I** is the primary apolipoprotein associated with HDL. * **LCAT (Lecithin-Cholesterol Acyltransferase)** is the enzyme responsible for converting nascent discoid HDL into mature spherical HDL. * **Factors that decrease HDL:** Smoking, obesity, sedentary lifestyle, and uncontrolled diabetes. * **Drug of choice to raise HDL:** Niacin (Vitamin B3) is the most potent pharmacological agent for increasing HDL, though its clinical use is limited by side effects like flushing.

Question 349: Which of the following is NOT a component of the multifunctional enzyme Fatty Acid Synthase Complex?

A. Acetyl transacylase
B. Enoyl reductase
C. Malonyl transacylase
D. Monoamine oxidase (Correct Answer)

Explanation: **Explanation:** The **Fatty Acid Synthase (FAS) complex** is a multi-enzyme system responsible for the de novo synthesis of palmitate from acetyl-CoA and malonyl-CoA. In humans, it exists as a **homodimer**, where each monomer contains seven distinct enzyme activities and an Acyl Carrier Protein (ACP) domain. **Why Monoamine Oxidase (MAO) is the correct answer:** Monoamine oxidase is an enzyme located on the **outer mitochondrial membrane** involved in the oxidative deamination of neurotransmitters (like epinephrine and serotonin). It has no role in lipid biosynthesis. Therefore, it is not a component of the FAS complex. **Analysis of incorrect options (Components of FAS):** * **Acetyl transacylase:** Transfers the acetyl group from Acetyl-CoA to the Cys-SH group of the enzyme to initiate synthesis. * **Malonyl transacylase:** Transfers the malonyl group from Malonyl-CoA to the -SH group of the ACP. * **Enoyl reductase:** Responsible for the second reduction step (using NADPH) to saturate the double bond, forming a saturated acyl-ACP. * *Other components include:* Ketoacyl synthase (condensing enzyme), Ketoacyl reductase, and Hydratase. **High-Yield NEET-PG Pearls:** 1. **Location:** Fatty acid synthesis occurs in the **cytosol**, whereas beta-oxidation occurs in the mitochondria. 2. **The "Primer":** Acetyl-CoA is the starting primer, but **Malonyl-CoA** is the immediate donor of 2-carbon units. 3. **Reducing Power:** **NADPH** is the essential coenzyme for FAS, primarily supplied by the Hexose Monophosphate (HMP) shunt. 4. **End Product:** The primary end product of the FAS complex is **Palmitate** (a 16-carbon saturated fatty acid). 5. **Structural Organization:** In eukaryotes, FAS is a **Type I system** (linked polypeptide chain), whereas in bacteria, it is a **Type II system** (individual separate enzymes).

Question 350: Which lipase is regulated by glucagon?

A. Lipoprotein lipase
B. Hormone-sensitive lipase (Correct Answer)
C. Gastric lipase
D. Pancreatic lipase

Explanation: **Explanation:** The correct answer is **Hormone-sensitive lipase (HSL)**. **1. Why Hormone-sensitive lipase is correct:** HSL is the key regulatory enzyme for **lipolysis** (the breakdown of stored triacylglycerols into free fatty acids and glycerol) within adipose tissue. It is regulated via covalent modification (phosphorylation). During fasting or stress, **Glucagon** and **Epinephrine** bind to G-protein coupled receptors, increasing cAMP levels. This activates Protein Kinase A (PKA), which phosphorylates and activates HSL. Conversely, insulin dephosphorylates and inactivates HSL, promoting fat storage. **2. Why the other options are incorrect:** * **Lipoprotein lipase (LPL):** Found on the capillary endothelium, LPL functions in the **fed state** to clear triglycerides from chylomicrons and VLDL. It is stimulated by **Insulin**, not glucagon. * **Gastric lipase:** Secreted by the chief cells of the stomach, it initiates lipid digestion (primarily in neonates). Its regulation is primarily mechanical/hormonal via gastrin, not glucagon. * **Pancreatic lipase:** The primary enzyme for dietary fat digestion in the small intestine. It is secreted as an active enzyme in response to **Cholecystokinin (CCK)**, independent of the glucagon-insulin ratio. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** HSL is the rate-limiting enzyme for the mobilization of stored energy from adipose tissue. * **Perilipin:** When HSL is phosphorylated, **Perilipin** (a protein coating the lipid droplet) is also phosphorylated, allowing HSL access to the triglyceride core. * **Inhibitor:** Niacin (Vitamin B3) inhibits HSL, which is why it is used to reduce plasma free fatty acid levels and VLDL synthesis.

Practice by Chapter

Lipid Classification and Chemistry

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Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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