Lipid Metabolism Practice Questions

Q: Which of the following is a member of the ω9 family of fatty acids?

Oleic Acid. **Explanation:** Fatty acids are classified into families (ω3, ω6, ω7, or ω9) based on the position of the first double bond relative to the methyl (omega) end of the carbon chain. **Why Oleic Acid is Correct:** **Oleic Acid (18:1; Δ9)** is the most common monounsaturated fatty acid (MUFA) in the human body. Counting from the methyl end (omega carbon), the first (and only) double bond occurs at the 9th carbon. Therefore, it belongs to the **ω9 family**. It is synthesized in the body from Stearic acid by the enzyme Δ9-desaturase. **Analysis of Incorrect Options:** * **Timnodonic Acid (20:5; ω3):** Also known as Eicosapentaenoic acid (EPA). It is a polyunsaturated fatty acid (PUFA) found in fish oil and belongs to the **ω3 family**. * **Cervonic Acid (22:6; ω3):** Also known as Docosahexaenoic acid (DHA). It is critical for retinal and brain development and belongs to the **ω3 family**. * **Arachidonic Acid (20:4; ω6):** A precursor for prostaglandins and leukotrienes. Since the first double bond from the methyl end is at carbon 6, it belongs to the **ω6 family**. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Humans lack Δ12 and Δ15 desaturases; thus, **Linoleic acid (ω6)** and **Linolenic acid (ω3)** must be obtained from the diet. * **Arachidonic acid** becomes "conditionally essential" only if its precursor, Linoleic acid, is deficient in the diet. * **EFA Deficiency:** Characterized by scaly dermatitis (phrynoderma or toad skin), poor wound healing, and fatty liver.

Q: Beta-glucosidase is defective in which disease?

Gaucher's disease. **Explanation:** **1. Why Gaucher's Disease is Correct:** Gaucher’s disease is the most common **Lysosomal Storage Disorder (LSD)**. It is caused by a deficiency of the enzyme **Acid Beta-glucosidase** (also known as **Glucocerebrosidase**). Under normal conditions, this enzyme breaks down glucocerebroside into glucose and ceramide. When defective, glucocerebroside accumulates within the lysosomes of macrophages, transforming them into characteristic **"Gaucher cells"** (described as having a "wrinkled tissue paper" appearance). **2. Why the Other Options are Incorrect:** * **Tay-Sachs Disease:** This is caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. It is clinically characterized by a cherry-red spot on the macula and progressive neurodegeneration. * **Galactosaemia:** This is a disorder of carbohydrate metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. It is not a lysosomal storage disease. * **Diabetes Mellitus:** This is a metabolic disorder characterized by hyperglycemia due to insulin deficiency or resistance, unrelated to lysosomal enzyme defects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Pathognomonic macrophages with fibrillary cytoplasm (wrinkled tissue paper appearance) found in the bone marrow, liver, and spleen. * **Clinical Triad:** Hepatosplenomegaly, bone involvement (Erlenmeyer flask deformity of the femur, bone crises), and pancytopenia. * **Biochemical Marker:** Elevated levels of **Serum Acid Phosphatase** (Tartrate-resistant) are often seen. * **Treatment:** Enzyme Replacement Therapy (ERT) with recombinant glucocerebrosidase (Imiglucerase) is the gold standard.

Q: Cholesteryl ester transfer protein transports cholesterol from HDL to which of the following?

VLDL. **Explanation:** **Cholesteryl Ester Transfer Protein (CETP)** plays a pivotal role in the "Reverse Cholesterol Transport" pathway. Its primary function is to facilitate the exchange of lipids between high-density lipoproteins (HDL) and triglyceride-rich lipoproteins. **Why VLDL is the Correct Answer:** CETP mediates a reciprocal exchange where it collects **cholesteryl esters** from HDL and transfers them to **VLDL** (and to a lesser extent, LDL). In exchange, VLDL provides **triglycerides** to HDL. This process allows VLDL to be enriched with cholesterol, eventually being metabolized into LDL, while HDL becomes enriched with triglycerides (which are later hydrolyzed by hepatic lipase). **Analysis of Incorrect Options:** * **B. IDL & C. LDL:** While CETP can transfer cholesteryl esters to these lipoproteins, **VLDL** is the primary and initial acceptor in the endogenous pathway. In the context of NEET-PG, VLDL is the standard textbook answer for the primary site of exchange. * **D. Chylomicrons:** While CETP can interact with chylomicrons (exogenous pathway), the major flux of cholesterol exchange in the fasting state occurs between HDL and VLDL. **High-Yield Clinical Pearls for NEET-PG:** * **Reverse Cholesterol Transport:** This is the process of moving cholesterol from peripheral tissues back to the liver via HDL. CETP is a key regulator of this. * **CETP Inhibition:** Drugs that inhibit CETP (e.g., Anacetrapib) significantly increase HDL-C levels and decrease LDL-C, though their clinical utility in reducing cardiovascular events is still a subject of research. * **Atherogenic Profile:** High CETP activity can be pro-atherogenic as it lowers "good" HDL-cholesterol and increases "bad" VLDL/LDL-cholesterol.

Q: Insulin increases the activity of which of the following enzymes?

HMG-CoA reductase. **Explanation:** **HMG-CoA reductase** is the rate-limiting and key regulatory enzyme of cholesterol biosynthesis. Insulin, a hormone secreted in the well-fed state, promotes anabolic processes, including cholesterol synthesis. It increases the activity of HMG-CoA reductase through **dephosphorylation** (via protein phosphatase-1) and by inducing gene expression. This ensures that when energy and substrates (Acetyl-CoA) are abundant, the body synthesizes cholesterol. **Analysis of Incorrect Options:** * **HMG-CoA lyase:** This enzyme is involved in **ketogenesis** (the breakdown of fats into ketone bodies) and leucine catabolism. Ketogenesis occurs during fasting or starvation when insulin levels are low and glucagon is high. * **HMG-CoA synthase:** While this enzyme participates in both cholesterol synthesis (cytosolic isoform) and ketogenesis (mitochondrial isoform), it is not the primary rate-limiting step regulated by insulin in the same manner as the reductase. * **Thiolase:** This enzyme catalyzes the initial condensation of two Acetyl-CoA molecules to Acetoacetyl-CoA. It is a reversible enzyme involved in both the synthesis and breakdown pathways and is not the primary target for insulin-mediated hormonal regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Statins:** These are competitive inhibitors of HMG-CoA reductase, used to treat hypercholesterolemia. * **Hormonal Regulation:** HMG-CoA reductase is **activated by Insulin** and Thyroxine, but **inhibited by Glucagon** and Epinephrine (via phosphorylation). * **Subcellular Location:** Cholesterol synthesis occurs in the **cytosol and ER**, whereas ketogenesis occurs in the **mitochondria**. * **AMPK:** High AMP levels (low energy) inhibit HMG-CoA reductase to conserve energy.

Q: A broad beta band in electrophoretic pattern of lipoproteins is characteristic of which type of hyperlipoproteinemia?

Type III hyperlipoproteinemia. ### Explanation **Type III Hyperlipoproteinemia**, also known as **Dysbetalipoproteinemia** or Remnant Removal Disease, is characterized by the accumulation of **IDL (Intermediate-Density Lipoprotein)** and **Chylomicron remnants**. 1. **Why Type III is correct:** In normal electrophoresis, VLDL (pre-beta) and LDL (beta) migrate separately. In Type III, the accumulating IDL (which has a density between VLDL and LDL) creates a single, continuous, wide band that spans the pre-beta and beta regions. This is classically referred to as the **"Broad Beta Band."** It is caused by a genetic deficiency in **Apolipoprotein E (Apo E)**, specifically the E2/E2 phenotype, which prevents the liver from recognizing and clearing remnants. 2. **Why other options are incorrect:** * **Type I:** Characterized by high Chylomicrons due to Lipoprotein Lipase (LPL) or Apo C-II deficiency. Electrophoresis shows a heavy band at the origin. * **Type IIa:** Characterized by high LDL. Electrophoresis shows an intense, sharp **Beta band**. * **Type IV:** Characterized by high VLDL. Electrophoresis shows an intense **Pre-beta band**. ### High-Yield Clinical Pearls for NEET-PG: * **Clinical Hallmark:** Pathognomonic **Palmar Xanthomas** (orange-yellow discoloration of palmar creases) and Tuberoeruptive xanthomas. * **Genetic Defect:** Homozygosity for **Apo E2** isoform (which has low affinity for the LDL receptor). * **Lipid Profile:** Simultaneous elevation of both Cholesterol and Triglycerides (often in a 1:1 ratio). * **Treatment:** Fibrates are highly effective as they increase fatty acid oxidation and remnant clearance.

Q: What is the main lipid component of LDL?

Cholesterol. **Explanation:** The correct answer is **Cholesterol**. Lipoproteins are classified based on their density and the specific ratio of lipids (triacylglycerols, cholesterol, and phospholipids) to proteins (apolipoproteins) they contain. **Why Cholesterol is correct:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol in the blood. It is formed from the metabolism of VLDL (via IDL). As VLDL loses triacylglycerols through the action of lipoprotein lipase, the relative concentration of cholesterol increases. Approximately **50% of the mass of an LDL particle is cholesterol** (primarily cholesterol esters), making it the dominant lipid component. Its physiological role is to transport cholesterol from the liver to peripheral tissues. **Why the other options are incorrect:** * **A. Triacylglycerol:** This is the main lipid component of **Chylomicrons** (dietary) and **VLDL** (endogenous). By the time these particles transition to LDL, most triacylglycerols have been hydrolyzed. * **C. Phospholipids:** While present in the outer shell of all lipoproteins to maintain solubility, they are never the "main" lipid component by weight compared to core lipids like TAGs or cholesterol. * **D. Free fatty acids:** These are not transported within lipoproteins; instead, they circulate in the blood bound to **Albumin**. **High-Yield Clinical Pearls for NEET-PG:** * **"Bad Cholesterol":** LDL is termed "bad" because high levels are strongly associated with atherosclerosis and Coronary Artery Disease (CAD). * **Apolipoprotein:** The characteristic apoprotein of LDL is **Apo B-100**. * **LDL Uptake:** LDL is taken up by cells via LDL receptors (Apo B-100 receptors). A defect in these receptors leads to **Type IIa Familial Hypercholesterolemia**. * **Friedewald Formula:** LDL = [Total Cholesterol] – [HDL] – [TG/5] (Note: This formula is invalid if TG > 400 mg/dL).

Q: In the well-fed state, which of the following inhibits Carnitine Palmitoyltransferase 1 (CPT1) on the outer membrane of mitochondria?

Malonyl CoA. ### Explanation **1. Why Malonyl CoA is Correct:** In the **well-fed state**, high levels of glucose lead to an increase in insulin. Insulin promotes fatty acid synthesis by activating **Acetyl CoA Carboxylase (ACC)**, which converts Acetyl CoA into **Malonyl CoA**. Malonyl CoA serves as a critical regulatory molecule; it acts as a potent **allosteric inhibitor of Carnitine Palmitoyltransferase 1 (CPT1)**. By inhibiting CPT1, Malonyl CoA prevents the entry of long-chain fatty acids into the mitochondria for beta-oxidation. This ensures that fatty acid synthesis and fatty acid breakdown do not occur simultaneously (preventing a "futile cycle"). **2. Why the Other Options are Incorrect:** * **B. Acetyl CoA:** While it is a precursor for Malonyl CoA, it does not directly inhibit CPT1. Its primary role in the fed state is to provide the carbon skeleton for lipogenesis. * **C. ADP:** ADP is a marker of low energy status. In a well-fed state, ATP levels are high and ADP levels are low. ADP generally stimulates catabolic pathways (like the TCA cycle), not the inhibition of fatty acid transport. * **D. Glucose:** While glucose availability triggers the hormonal shift (insulin) that leads to CPT1 inhibition, glucose itself does not interact with the CPT1 enzyme. **3. Clinical Pearls & High-Yield Facts:** * **Rate-Limiting Step:** CPT1 is the rate-limiting enzyme for **Beta-oxidation**. * **Location:** CPT1 is located on the **outer** mitochondrial membrane, while CPT2 is on the **inner** membrane. * **Hormonal Control:** Glucagon decreases Malonyl CoA levels (by inhibiting ACC), thereby relieving the inhibition on CPT1 and stimulating fatty acid oxidation during fasting. * **Carnitine Shuttle:** This process is essential for long-chain fatty acids; however, short and medium-chain fatty acids can bypass this shuttle and enter the mitochondria directly.

Q: Colipase is:

Encoded by the gene CLPS. **Explanation:** **Colipase** is a small protein co-enzyme required for the optimal activity of pancreatic lipase. It is essential for fat digestion in the small intestine. 1. **Why Option D is correct:** Colipase is indeed a protein product of the **CLPS gene** located on chromosome 6. It functions by binding to the C-terminal domain of pancreatic lipase, anchoring it to the surface of lipid droplets. 2. **Why Option A is incorrect:** Colipase is synthesized and secreted by the **pancreatic acinar cells**, not the oxyntic (parietal) cells of the stomach (which secrete HCl and Intrinsic Factor). 3. **Why Option B is incorrect:** It is secreted as an inactive precursor called **procolipase**. It must be activated (cleaved) in the intestinal lumen by **trypsin** to become functional colipase. 4. **Why Option C is incorrect:** Colipase specifically assists **pancreatic lipase**, not gastric lipase. Its primary role is to prevent the inhibitory effect of bile salts. Bile salts normally displace lipase from the fat droplet; colipase "clears" a path through the bile salts, allowing pancreatic lipase to bind to its substrate. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Colipase shifts the pH optimum of pancreatic lipase from 8.0–9.0 back to 6.0–7.0, which is the actual pH of the duodenum. * **Ratio:** Pancreatic lipase and procolipase are secreted by the pancreas in a **1:1 ratio**. * **Clinical Significance:** In cases of severe pancreatic insufficiency, fat malabsorption (steatorrhea) occurs because lipase cannot function without its co-factor in the presence of bile salts.

Q: CoA and Acyl carrier protein (ACP) are similar in that they

both contain pantothenic acid. **Explanation:** The correct answer is **B: both contain pantothenic acid.** **1. Why the correct answer is right:** Both Coenzyme A (CoA) and Acyl Carrier Protein (ACP) serve as carriers of acyl groups during lipid metabolism. The functional core of both molecules is **4'-phosphopantetheine**, which is derived from **Pantothenic acid (Vitamin B5)**. This moiety contains a terminal sulfhydryl (-SH) group that forms a high-energy thioester bond with fatty acid chains, allowing them to be activated and processed by enzymes. **2. Why the incorrect options are wrong:** * **Option A:** While CoA is found in both the mitochondria (for Beta-oxidation and TCA cycle) and the cytosol, **ACP is primarily located in the cytosol** as part of the Fatty Acid Synthase (FAS) multienzyme complex. * **Option C:** CoA is used in fatty acid **degradation** (Beta-oxidation), but ACP is exclusively used in fatty acid **synthesis** (lipogenesis). * **Option D:** Activation of fatty acids (conversion to Acyl-CoA) specifically requires **CoA** and the enzyme Thiokinase. ACP does not activate free fatty acids; it holds the growing chain during the elongation steps of synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B5 Deficiency:** Rare, but can lead to "Burning Foot Syndrome." * **Fatty Acid Synthase (FAS) Complex:** In eukaryotes, this is a dimer of two identical polypeptides, each containing seven enzyme activities and one ACP domain. * **The "Shuttle" Concept:** Remember that CoA is the carrier for **catabolism** (breakdown), while ACP is the carrier for **anabolism** (synthesis). * **Key Difference:** In CoA, the phosphopantetheine is linked to adenosine diphosphate; in ACP, it is linked to a serine residue of the protein.

Q: Fatty acids are oxidized by all the following tissues, except?

Brain. ### Explanation The correct answer is **Brain (Option C)**. **Why the Brain cannot oxidize Fatty Acids:** Although the brain has a high energy requirement, it cannot utilize long-chain fatty acids as a primary fuel source. This is due to two main reasons: 1. **Blood-Brain Barrier (BBB):** Large, albumin-bound fatty acids cannot effectively cross the BBB. 2. **Enzymatic Limitation:** Neurons have relatively low levels of the enzymes required for **$\beta$-oxidation**. Furthermore, $\beta$-oxidation is a slow process and generates reactive oxygen species (ROS), which can cause oxidative stress in the delicate neural environment. Instead, the brain relies on **glucose** (primary) and **ketone bodies** (during prolonged fasting). **Analysis of Incorrect Options:** * **Liver (A):** The liver is the primary site for fatty acid metabolism. It oxidizes fatty acids to generate ATP and provides the acetyl-CoA necessary for ketogenesis. * **Adipose Tissue (B):** While adipose tissue primarily stores triacylglycerols, it contains mitochondria and can oxidize fatty acids to meet its own basal energy needs. * **Skeletal Muscle (D):** At rest and during low-to-moderate intensity exercise, skeletal muscle prefers fatty acids as its major fuel source via aerobic $\beta$-oxidation. **High-Yield NEET-PG Pearls:** * **Erythrocytes (RBCs):** Like the brain, RBCs also **cannot** oxidize fatty acids because they lack mitochondria. * **Ketone Bodies:** While the brain cannot use fatty acids, it can use ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) during starvation. However, the **liver** cannot use ketone bodies because it lacks the enzyme **thiophorase** (succinyl-CoA:3-ketoacid CoA transferase). * **Heart Muscle:** The myocardium is highly dependent on fatty acid oxidation for energy (approx. 60-80% of its requirement).

Question 1

Which of the following is a member of the ω9 family of fatty acids?

Accepted Answer

Oleic Acid

Answer

Timnodonic Acid

Answer

Cervonic Acid

Answer

Arachidonic Acid

Question 2

Beta-glucosidase is defective in which disease?

Accepted Answer

Gaucher's disease

Answer

Tay-Sachs disease

Answer

Galactosaemia

Answer

Diabetes Mellitus

Question 3

Cholesteryl ester transfer protein transports cholesterol from HDL to which of the following?

Accepted Answer

VLDL

Answer

IDL

Answer

LDL

Answer

Chylomicrons

Question 4

Insulin increases the activity of which of the following enzymes?

Accepted Answer

HMG-CoA reductase

Answer

HMG-CoA lyase

Answer

HMG-CoA synthase

Answer

Thiolase

Question 5

A broad beta band in electrophoretic pattern of lipoproteins is characteristic of which type of hyperlipoproteinemia?

Accepted Answer

Type III hyperlipoproteinemia

Answer

Type I hyperlipoproteinemia

Answer

Type IIa hyperlipoproteinemia

Answer

Type IV hyperlipoproteinemia

Question 6

What is the main lipid component of LDL?

Accepted Answer

Cholesterol

Answer

Triacylglycerol

Answer

Phospholipids

Answer

Free fatty acids

Question 7

In the well-fed state, which of the following inhibits Carnitine Palmitoyltransferase 1 (CPT1) on the outer membrane of mitochondria?

Accepted Answer

Malonyl CoA

Answer

Acetyl CoA

Answer

ADP

Answer

Glucose

Question 8

Colipase is:

Accepted Answer

Encoded by the gene CLPS

Answer

Secreted by oxyntic cells

Answer

Secreted in the active form

Answer

Helps gastric lipase

Question 9

CoA and Acyl carrier protein (ACP) are similar in that they

Accepted Answer

both contain pantothenic acid

Answer

are both present in mitochondria

Answer

are both used in fatty acid degradation

Answer

are both required for activation of fatty acids

Question 10

Fatty acids are oxidized by all the following tissues, except?

Accepted Answer

Brain

Answer

Liver

Answer

Adipose tissue

Answer

Skeletal muscle

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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