Lipid Metabolism Practice Questions

Q: Lingual lipase is secreted by which of the following glands?

Ebner's glands. **Explanation:** The correct answer is **Ebner's glands** (Option B). **1. Why Ebner's glands are correct:** Lingual lipase is an enzyme responsible for the initial digestion of dietary fats (triacylglycerols). It is secreted by the **Ebner’s glands** (also known as von Ebner's glands), which are minor serous salivary glands located on the dorsum of the tongue, specifically surrounding the circumvallate and foliate papillae. **2. Why other options are incorrect:** * **Parotid glands:** These are the largest salivary glands and primarily secrete serous fluid rich in **salivary amylase** (ptyalin) for carbohydrate digestion, not lingual lipase. * **Sublingual glands:** These are mixed glands (predominantly mucous) located under the tongue. While they contribute to saliva production, they are not the source of lingual lipase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Acid Stability:** Lingual lipase is highly acid-stable. Unlike pancreatic lipase, it functions optimally at a low pH (3.0–6.0), allowing it to remain active in the acidic environment of the stomach. * **Role in Neonates:** Lingual lipase (along with gastric lipase) is critically important in neonates because their pancreatic function is not yet fully developed. It helps in the digestion of milk fats. * **Short/Medium-Chain Fatty Acids:** It specifically targets the primary ester linkages of triglycerides, releasing short and medium-chain fatty acids, which are then absorbed directly into the portal circulation. * **Acid Lipases:** Together, lingual and gastric lipases are referred to as "Acid Lipases." They do not require bile salts for their activation.

Q: Lipogenesis occurs in which cellular compartment?

Cytosol. **Explanation:** **1. Why Cytosol is Correct:** Lipogenesis (De novo synthesis of fatty acids) primarily occurs in the **cytosol**. This is because the key enzymes required for the process, specifically **Acetyl-CoA Carboxylase (ACC)**—the rate-limiting enzyme—and the **Fatty Acid Synthase (FAS) complex**, are located within the cytosolic compartment. While Acetyl-CoA is generated in the mitochondria, it must be transported to the cytosol via the **Citrate-Malate Shuttle** to initiate lipogenesis. **2. Why Other Options are Incorrect:** * **Mitochondria:** This is the primary site for **Beta-oxidation** (fatty acid breakdown) and the TCA cycle. While some fatty acid elongation occurs here, it is not the site of de novo synthesis. * **Endoplasmic Reticulum (ER):** The smooth ER is responsible for **fatty acid elongation** (beyond 16 carbons) and **desaturation** (adding double bonds), as well as the synthesis of complex lipids like phospholipids and cholesterol. * **Golgi Body:** This organelle is involved in the modification, sorting, and packaging of proteins and lipids (e.g., VLDL assembly) rather than the primary synthesis of fatty acids. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin** as a cofactor). * **Reducing equivalent:** **NADPH** is essential for lipogenesis, primarily supplied by the Hexose Monophosphate (HMP) Shunt. * **End product:** The primary product of the FAS complex is **Palmitate** (a 16-carbon saturated fatty acid). * **Hormonal Regulation:** Lipogenesis is stimulated by **Insulin** (well-fed state) and inhibited by Glucagon and Epinephrine. * **Mnemonic:** "S"ynthesis occurs in the "S"ol (Cytosol); "B"reakdown occurs in the "B"ody (Mitochondrial body).

Q: Which of the following is NOT a product of the oxygenase pathway?

LT6. **Explanation:** The question focuses on the metabolism of **Arachidonic Acid**, a 20-carbon polyunsaturated fatty acid. Arachidonic acid is metabolized via two primary pathways: the **Cyclooxygenase (COX) pathway** and the **Lipoxygenase (LOX) pathway**. **1. Why LT6 is the correct answer:** Leukotrienes (LTs) are products of the **Lipoxygenase (LOX)** pathway, not the oxygenase (COX) pathway. Furthermore, the number in the subscript of a leukotriene (e.g., LTA4, LTB4) denotes the number of double bonds in the molecule. Since arachidonic acid has only 4 double bonds, the standard leukotrienes produced from it belong to the **"4-series"** (LT4). There is no naturally occurring **LT6** derived from the standard human oxygenase pathways, making it the correct "incorrect" option. **2. Why the other options are incorrect:** * **PGE2, PGD2, and PGF2:** These are all **Prostaglandins**. Prostaglandins, along with Thromboxanes and Prostacyclins, are collectively known as prostanoids. They are synthesized via the **Cyclooxygenase (COX) pathway** (also known as the Prostaglandin Endoperoxide Synthase pathway). Since the question asks for products of the oxygenase (COX) pathway, these are all valid products and thus incorrect choices for the "NOT" criteria. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2**. * **Inhibition:** Corticosteroids inhibit Phospholipase A2, while NSAIDs (like Aspirin) specifically inhibit the COX pathway. * **LOX Pathway:** Leads to Leukotrienes (involved in asthma and anaphylaxis). Zileuton inhibits LOX, while Montelukast blocks leukotriene receptors. * **Prostacyclin (PGI2):** Produced by vascular endothelium; causes vasodilation and inhibits platelet aggregation. * **Thromboxane (TXA2):** Produced by platelets; causes vasoconstriction and promotes platelet aggregation.

Q: All of the following inhibit hormone-dependent lipase, EXCEPT:

Adrenaline. **Explanation:** The enzyme **Hormone-Sensitive Lipase (HSL)** is the rate-limiting enzyme for lipolysis in adipose tissue. It catalyzes the breakdown of stored triacylglycerols into free fatty acids and glycerol. Its activity is regulated via **cAMP-dependent phosphorylation**. **1. Why Adrenaline is the Correct Answer:** Adrenaline (and Noradrenaline) acts via **$\beta_3$-adrenergic receptors** to activate Adenylate Cyclase. This increases intracellular cAMP levels, which activates Protein Kinase A (PKA). PKA then phosphorylates and **activates** HSL. Therefore, Adrenaline is a potent **stimulator**, not an inhibitor, of HSL. **2. Why the other options are incorrect (Inhibitors of HSL):** * **Insulin:** The most significant inhibitor of HSL. It activates a phosphatase that dephosphorylates (inactivates) HSL and promotes the degradation of cAMP via phosphodiesterase. * **Prostaglandin E1 (PGE1):** Inhibits Adenylate Cyclase, leading to decreased cAMP levels and reduced HSL activity. * **Nicotinic Acid (Niacin):** Potently inhibits lipolysis in adipose tissue by inhibiting Adenylate Cyclase, which is why it is used clinically to lower plasma free fatty acids. **Clinical Pearls for NEET-PG:** * **HSL vs. LPL:** Do not confuse Hormone-Sensitive Lipase (adipose tissue; mobilizes fat) with **Lipoprotein Lipase** (capillary endothelium; clears chylomicrons/VLDL). * **Glucagon & ACTH:** Like Adrenaline, these hormones also stimulate HSL. * **Caffeine/Theophylline:** These inhibit phosphodiesterase, maintaining high cAMP levels and thus **stimulating** HSL activity (promoting lipolysis).

Q: Conversion of stearic acid to oleic acid is catalyzed by which enzyme?

D9 desaturase. **Explanation:** The conversion of **stearic acid** (a 18-carbon saturated fatty acid, 18:0) to **oleic acid** (a 18-carbon monounsaturated fatty acid, 18:1; Δ9) is a process of desaturation. **1. Why Option C is Correct:** In humans, the synthesis of monounsaturated fatty acids is catalyzed by the **Fatty Acid Desaturase system** (specifically **Δ9-desaturase**), located in the endoplasmic reticulum. This enzyme introduces a double bond between **carbons 9 and 10** of the fatty acid chain. Stearic acid is the primary substrate for this enzyme, resulting in the formation of oleic acid. This reaction requires molecular oxygen, NADH, and cytochrome b5. **2. Why Other Options are Incorrect:** * **Δ3 and Δ6 desaturases (Options A & B):** These enzymes are involved in the further desaturation of polyunsaturated fatty acids (PUFAs) like linoleic and α-linolenic acid to form longer-chain PUFAs (e.g., arachidonic acid). They do not act directly on saturated stearic acid to form oleic acid. * **Δ12 desaturase (Option D):** This enzyme introduces a double bond at the 12th position. **Humans lack Δ12 and Δ15 desaturases**, which is why linoleic (18:2; Δ9,12) and linolenic (18:3; Δ9,12,15) acids cannot be synthesized endogenously and are classified as **Essential Fatty Acids (EFA)**. **Clinical Pearls & High-Yield Facts:** * **Essential Fatty Acids:** Humans cannot introduce double bonds beyond the Δ9 position. Therefore, any fatty acid with a double bond at Δ12 or Δ15 must be obtained from the diet. * **Non-essentiality of Oleic Acid:** Since we possess Δ9-desaturase, oleic acid (Omega-9) is considered a non-essential fatty acid. * **Enzyme Components:** The desaturase system is a multienzyme complex consisting of **Desaturase, Cytochrome b5, and Cytochrome b5 reductase.**

Q: Delayed clotting in abetalipoproteinemia is due to:

Reduced clotting factor synthesis. **Explanation:** **Abetalipoproteinemia** is an autosomal recessive disorder caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)**. This defect prevents the assembly and secretion of ApoB-containing lipoproteins (Chylomicrons, VLDL, and LDL). **Why the correct answer is right:** The hallmark of abetalipoproteinemia is severe **fat malabsorption**. Because dietary fats cannot be packaged into chylomicrons, fat-soluble vitamins (A, D, E, and K) are not absorbed from the intestine. **Vitamin K** is a vital cofactor for the γ-carboxylation of clotting factors **II, VII, IX, and X**. A deficiency in Vitamin K leads to the production of non-functional clotting factors, resulting in a bleeding diathesis and delayed clotting. **Why incorrect options are wrong:** * **A & C (Decreased VLDL/Chylomicrons):** While these are characteristic biochemical findings in abetalipoproteinemia, they are the *cause* of malabsorption, not the direct mechanism for delayed clotting. Clotting is a protein-mediated process, not a lipid-mediated one. * **D (Decreased fatty acid synthesis):** De novo fatty acid synthesis occurs primarily in the liver and is not the primary defect in this condition. **Clinical Pearls for NEET-PG:** * **Blood Smear:** Look for **Acanthocytes** (spur cells) due to altered membrane lipid composition. * **Neurological symptoms:** Ataxia and retinitis pigmentosa occur due to severe **Vitamin E** deficiency. * **Biopsy:** Intestinal biopsy shows **lipid-laden enterocytes** (vacuoles) because triglycerides are trapped and cannot be exported. * **Labs:** Extremely low levels of Cholesterol and Triglycerides; absent ApoB-48 and ApoB-100.

Q: What is produced with the help of NADP in extramitochondrial sites?

Steroids. **Explanation:** The correct answer is **Steroids**. The synthesis of fatty acids and steroids (cholesterol) occurs in the **cytosol** (extramitochondrial site) and requires **NADPH** as a crucial reducing equivalent. **Why Steroids?** Steroidogenesis involves the conversion of Acetyl-CoA to cholesterol and subsequently to various steroid hormones. This process requires NADPH, which is primarily generated via the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). NADPH provides the electrons necessary for the reductive biosynthetic reactions and the action of the cytochrome P450 monooxygenase system involved in steroid hydroxylation. **Analysis of Incorrect Options:** * **A. Glycogen:** Glycogen synthesis (Glycogenesis) occurs in the cytosol but requires **UTP** (Uridine triphosphate) and ATP, not NADPH. * **C. Ketone bodies:** Ketogenesis occurs exclusively within the **mitochondrial matrix** of hepatocytes. It does not require NADPH; rather, it is a pathway used to export energy when glucose is low. **High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The HMP shunt (via G6PD enzyme) is the major source. Another important source is the **Malic enzyme**, which converts malate to pyruvate in the cytosol. * **Key NADPH-dependent processes:** 1. Reductive biosynthesis (Fatty acids, Cholesterol, Steroids). 2. Maintenance of reduced **Glutathione** (protecting RBCs from oxidative stress). 3. Phagocytosis by WBCs (Respiratory burst via NADPH oxidase). 4. Nitric Oxide (NO) synthesis. * **Location Tip:** Remember "Fatty Acid synthesis and Steroid synthesis stay together in the Cytosol and both need NADPH."

Q: Which of the following is not synthesized from mevalonate?

Ubiquitin. **Explanation:** The synthesis of cholesterol involves the **Mevalonate pathway**, which produces several vital isoprenoid intermediates. The question asks which substance is *not* derived from this pathway. **Why Ubiquitin is the correct answer:** **Ubiquitin** is a small, highly conserved **regulatory protein** found in almost all eukaryotic tissues. Its primary function is to mark proteins for degradation via the proteasome (the Ubiquitin-Proteasome Pathway). It is synthesized through protein translation (amino acids), not the lipid-based mevalonate pathway. **Note:** Do not confuse **Ubiquitin** with **Ubiquinone** (Coenzyme Q). Ubiquinone *is* synthesized from mevalonate. **Why the other options are incorrect:** The mevalonate pathway follows this sequence: Mevalonate → Isopentenyl pyrophosphate (IPP) → **Geranyl pyrophosphate (GPP)** → **Farnesyl pyrophosphate (FPP)** → Squalene → Cholesterol. * **Geranyl pyrophosphate (Option B):** A 10-carbon intermediate in the pathway. * **Farnesyl pyrophosphate (Option A):** A 15-carbon intermediate used for cholesterol synthesis and the prenylation of proteins (like Ras). * **Dolichol (Option C):** A specialized lipid synthesized from farnesyl pyrophosphate. It is essential for the N-linked glycosylation of proteins in the endoplasmic reticulum. **High-Yield Clinical Pearls for NEET-PG:** * **HMG-CoA Reductase:** The rate-limiting enzyme of this pathway, inhibited by **Statins**. * **Statin-induced Myopathy:** May be caused by the depletion of **Ubiquinone (CoQ10)**, a side product of the mevalonate pathway required for the mitochondrial electron transport chain. * **Dolichol deficiency:** Leads to Congenital Disorders of Glycosylation (CDG). * **Protein Prenylation:** FPP and GPP are used to anchor proteins (e.g., Ras G-protein) to cell membranes.

Question 1

Lingual lipase is secreted by which of the following glands?

Accepted Answer

Ebner's glands

Answer

Parotid glands

Answer

Sublingual glands

Answer

None of the above

Question 2

PUFA consumption is associated with which of the following changes in serum lipid profile?

Accepted Answer

Lowering of serum cholesterol and lowering of LDL cholesterol

Answer

Lowering of serum cholesterol and a rise in LDL cholesterol

Answer

A rise in serum cholesterol and a rise in LDL cholesterol

Answer

A rise in serum cholesterol and lowering of LDL cholesterol

Question 3

Lipogenesis occurs in which cellular compartment?

Accepted Answer

Cytosol

Answer

Endoplasmic reticulum

Answer

Golgi body

Answer

Mitochondria

Question 4

Which of the following is NOT a product of the oxygenase pathway?

Accepted Answer

LT6

Answer

PGE2

Answer

PG D2

Answer

PGF2

Question 5

All of the following inhibit hormone-dependent lipase, EXCEPT:

Accepted Answer

Adrenaline

Answer

Insulin

Answer

Prostaglandin E1

Answer

Nicotinic acid

Question 6

In which of the following enzyme deficiencies is steatorrhea associated with increased triglycerides?

Accepted Answer

Lipoprotein lipase

Answer

Pancreatic lipase

Answer

Serum lipase

Answer

Acetyl CoA carboxylase

Question 7

Conversion of stearic acid to oleic acid is catalyzed by which enzyme?

Accepted Answer

D9 desaturase

Answer

D3 desaturase

Answer

D6 desaturase

Answer

D12 desaturase

Question 8

Delayed clotting in abetalipoproteinemia is due to:

Accepted Answer

Reduced clotting factor synthesis

Answer

A decreased VLDL

Answer

Decreased chylomicrons

Answer

Decreased fatty acid synthesis

Question 9

What is produced with the help of NADP in extramitochondrial sites?

Accepted Answer

Steroids

Answer

Glycogen

Answer

Ketone bodies

Answer

None of the above

Question 10

Which of the following is not synthesized from mevalonate?

Accepted Answer

Ubiquitin

Answer

Farnesyl pyrophosphate

Answer

Geranyl pyrophosphate

Answer

Dolichol

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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