Lipid Metabolism Practice Questions

Q: Acetyl-CoA is transported out of the mitochondria to serve as a substrate for fatty acid or cholesterol synthesis. Which of the following enzymes involved in this transport process provides NADPH required for these reductive biosynthesis reactions?

Malic enzyme. **Explanation:** Fatty acid synthesis occurs in the cytosol, but Acetyl-CoA is produced in the mitochondria. Since the inner mitochondrial membrane is impermeable to Acetyl-CoA, it condenses with oxaloacetate to form **Citrate**, which is transported out via the tricarboxylate transporter (the **Citrate Shuttle**). **Why Malic Enzyme is correct:** Once in the cytosol, citrate is cleaved back into Acetyl-CoA and oxaloacetate. The oxaloacetate is reduced to malate, which is then decarboxylated by **Malic Enzyme** to form pyruvate. This specific reaction reduces NADP+ to **NADPH**. This is a high-yield concept because NADPH is the essential reducing equivalent required for the reductive biosynthesis of fatty acids and cholesterol. **Analysis of Incorrect Options:** * **A. ATP-citrate lyase:** This enzyme cleaves Citrate into Acetyl-CoA and Oxaloacetate in the cytosol. While it is essential for the shuttle, it does not produce NADPH. * **B. Citrate synthase:** This is a mitochondrial enzyme that condenses Acetyl-CoA and Oxaloacetate to form Citrate. It is the first step of the TCA cycle and the shuttle, but it does not generate reducing power. * **C. Malate dehydrogenase:** The cytosolic version of this enzyme converts oxaloacetate to malate using NADH. It actually *consumes* reducing equivalents rather than producing NADPH. **High-Yield NEET-PG Pearls:** * **Sources of NADPH:** The two primary sources for fatty acid synthesis are the **Pentose Phosphate Pathway (HMP Shunt)** (major source) and the **Malic Enzyme** reaction (minor but significant source). * **Rate-limiting step:** The rate-limiting enzyme for fatty acid synthesis is **Acetyl-CoA Carboxylase (ACC)**, which requires Biotin. * **Location:** Remember "Link reaction/TCA = Mitochondria" vs. "Fatty acid synthesis = Cytosol." The Citrate Shuttle bridges this gap.

Q: The elongation of fatty acids occurs in which of the diagrammatic structures shown in the illustration?

Structure C. ***Structure C*** - Represents the **smooth endoplasmic reticulum (SER)**, which is the primary site of **fatty acid elongation** using **elongase enzymes** and **malonyl-CoA**. - The **ER elongation system** extends fatty acids beyond C16, particularly producing **long-chain fatty acids** essential for membrane synthesis. *Structure B* - Represents **mitochondria**, primarily involved in **beta-oxidation** for fatty acid breakdown, not elongation. - While minor elongation can occur here, it's not the main site and involves different enzyme systems. *Structure A* - Represents the **rough endoplasmic reticulum (RER)**, studded with ribosomes and primarily involved in **protein synthesis and processing**, not fatty acid elongation. - This compartment is distinct from the smooth ER and does not carry out the elongase reactions. *Structure D* - Represents the **Golgi apparatus**, which is involved in **protein modification, sorting, and vesicular transport**, not fatty acid elongation.

Q: Lithogenic bile has which of the following properties?

Decreased bile and cholesterol ratio. **Explanation:** The formation of gallstones (cholelithiasis) is primarily driven by the production of **lithogenic bile**. Bile is a complex fluid containing bile salts, phospholipids (lecithin), and cholesterol. Under normal physiological conditions, cholesterol—which is water-insoluble—is kept in a soluble state by being incorporated into mixed micelles formed by bile salts and phospholipids. **1. Why Option B is Correct:** Bile becomes lithogenic when it is **supersaturated with cholesterol**. This occurs due to either an absolute increase in cholesterol secretion or a relative decrease in bile salts and phospholipids. Therefore, a **decreased bile salt-to-cholesterol ratio** means there is insufficient "detergent" (bile salts) to keep the cholesterol in solution. This leads to the precipitation of cholesterol crystals, which eventually aggregate to form stones. **2. Why Incorrect Options are Wrong:** * **Option A:** An increased bile-to-cholesterol ratio would mean more bile salts are available to solubilize cholesterol, making the bile *less* likely to form stones (non-lithogenic). * **Option C:** An equal ratio does not account for the specific saturation index required to maintain solubility; supersaturation is the key driver. * **Option D:** Decreased cholesterol would actually decrease the risk of stone formation, as the bile would be undersaturated. **NEET-PG High-Yield Pearls:** * **The Solubility Triangle:** The stability of bile depends on the molar ratios of bile salts, lecithin, and cholesterol (Admirand-Small Triangle). * **Rate-Limiting Enzyme:** **7-alpha-hydroxylase** is the rate-limiting enzyme for bile acid synthesis. Its deficiency or inhibition leads to lithogenic bile. * **Risk Factors (The 4 F's):** Fat, Female, Fertile, and Forty. * **Statin Mechanism:** HMG-CoA reductase inhibitors (Statins) can reduce the lithogenicity of bile by decreasing biliary cholesterol secretion.

Q: Which protein, having structural homology with plasminogen, is responsible for myocardial infarction and stroke?

LP(a). **Explanation:** **Lipoprotein(a) [Lp(a)]** is the correct answer because it consists of an LDL-like particle containing **Apolipoprotein B-100** linked to a unique glycoprotein called **Apolipoprotein(a)**. 1. **Mechanism of Action:** Apo(a) shares significant **structural homology with Plasminogen**, the precursor to plasmin. Due to this similarity, Lp(a) competitively inhibits the binding of plasminogen to fibrin and endothelial cell receptors. This impairs fibrinolysis (clot breakdown) and promotes thrombosis. Furthermore, Lp(a) is highly pro-atherogenic as it deposits cholesterol into the arterial wall. This dual **pro-thrombotic and pro-atherogenic** nature significantly increases the risk of Myocardial Infarction (MI) and Stroke. **Analysis of Incorrect Options:** * **A. HDL:** Known as "good cholesterol," it mediates reverse cholesterol transport. High levels are cardioprotective, not causative of MI. * **C. LDL:** While the primary carrier of cholesterol and a major risk factor for atherosclerosis, it lacks the plasminogen-like Apo(a) component. * **D. Homocysteine:** High levels (Hyperhomocysteinemia) are a risk factor for vascular disease due to endothelial damage, but it is an amino acid derivative, not a protein with structural homology to plasminogen. **High-Yield Facts for NEET-PG:** * **Kringles:** The structural homology between Apo(a) and plasminogen lies in the **"Kringle IV"** repeats. * **Genetic Determinant:** Lp(a) levels are primarily genetically determined and are not significantly altered by diet or statins. * **Niacin:** Historically, Niacin was used to lower Lp(a), though its clinical benefit on outcomes remains debated. * **Clinical Marker:** Lp(a) is considered an independent risk factor for premature coronary artery disease (CAD).

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).

Question 1

How many ATPs are formed from the complete beta-oxidation of stearic acid?

Accepted Answer

147

Answer

7

Answer

18

Answer

56

Question 2

Acetyl-CoA is transported out of the mitochondria to serve as a substrate for fatty acid or cholesterol synthesis. Which of the following enzymes involved in this transport process provides NADPH required for these reductive biosynthesis reactions?

Accepted Answer

Malic enzyme

Answer

ATP-citrate lyase

Answer

Citrate synthase

Answer

Malate dehydrogenase

Question 3

What is the best marker for the prediction of coronary artery disease?

Accepted Answer

LDL/HDL ratio

Answer

Serum cholesterol

Answer

Cholesterol/Triglyceride ratio

Answer

Blood cholesterol

Question 4

The elongation of fatty acids occurs in which of the diagrammatic structures shown in the illustration?

Accepted Answer

Structure C

Answer

Structure D

Answer

Structure B

Answer

Structure A

Question 5

Lithogenic bile has which of the following properties?

Accepted Answer

Decreased bile and cholesterol ratio

Answer

Increased bile and cholesterol ratio

Answer

Equal bile and cholesterol ratio

Answer

Decreased cholesterol only

Question 6

Which protein, having structural homology with plasminogen, is responsible for myocardial infarction and stroke?

Accepted Answer

LP(a)

Answer

HDL

Answer

LDL

Answer

Homocysteine

Question 7

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

Accepted Answer

VLDL

Answer

IDL

Answer

LDL

Answer

Chylomicrons

Question 8

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 9

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 10

What is the main lipid component of LDL?

Accepted Answer

Cholesterol

Answer

Triacylglycerol

Answer

Phospholipids

Answer

Free fatty acids

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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