Cholesterol Metabolism and Biosynthesis — MCQs
Enzyme deficient in Type I Hyperlipidemia?
Statins act on:
Which of the following enzymes is not targeted by hypolipidemic drugs?
A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?
Rate limiting enzyme in bile acid synthesis?
What is the primary role of cholesterol in low-density lipoprotein (LDL) metabolism?
Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?
What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?
In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?
Which of the following statements about LDL is false?
Cholesterol Metabolism and Biosynthesis Indian Medical PG Practice Questions and MCQs
Question 1: Enzyme deficient in Type I Hyperlipidemia?
- A. HMG CoA reductase
- B. Lipoprotein lipase (Correct Answer)
- C. Peroxidase
- D. Cholesterol acyl transferase
Explanation: ***Lipoprotein lipase*** - **Type I hyperlipidemia**, also known as **familial hyperchylomicronemia**, is characterized by a deficiency in **lipoprotein lipase (LPL)**. - LPL is crucial for hydrolyzing triglycerides in **chylomicrons** and **VLDLs** into fatty acids and glycerol, allowing their uptake by tissues. *HMG CoA reductase* - This enzyme is involved in the **rate-limiting step of cholesterol synthesis** in the liver. - While it plays a role in lipid metabolism, its deficiency is not characteristic of **Type I hyperlipidemia**. *Peroxidase* - **Peroxidase** is an enzyme involved in various oxidative reactions, including the breakdown of **hydrogen peroxide**. - It is not directly involved in the metabolism of **chylomicrons** or **triglycerides**, and its deficiency is unrelated to hyperlipidemia. *Cholesterol acyl transferase* - This enzyme, often referring to **lecithin-cholesterol acyltransferase (LCAT)** or **acyl-CoA:cholesterol acyltransferase (ACAT)**, is involved in **cholesterol esterification**. - While important for cholesterol transport and storage, its deficiency is not the primary cause of **Type I hyperlipidemia**, which is marked by severe **chylomicronemia**.
Question 2: Statins act on:
- A. HMG CoA synthetase
- B. HMG CoA reductase (Correct Answer)
- C. HMG CoA hydratase
- D. Squalene epoxidase
Explanation: ***HMG CoA reductase*** - **HMG-CoA reductase inhibitors** (statins) are the most effective and widely used class of hypolipidemic agents [1, 2, 3]. - This enzyme is the **rate-limiting step** in cholesterol biosynthesis in the liver [1, 2, 3]. *HMG CoA synthetase* - HMG-CoA synthetase is involved in the synthesis of **HMG-CoA** from acetyl-CoA and acetoacetyl-CoA. - This enzyme precedes the HMG-CoA reductase step and is **not the primary target** for cholesterol-lowering drugs. *Squalene epoxidase* - Squalene epoxidase is an enzyme involved in the later stages of the **cholesterol synthesis pathway**, specifically in converting squalene to squalene epoxide. - While inhibition of this enzyme would reduce cholesterol synthesis, it is **not the main target** of current widely used hypolipidemic agents. *HMG CoA hydratase* - HMG-CoA hydratase (also known as HMG-CoA lyase) is involved in the breakdown of HMG-CoA into **acetyl-CoA and acetoacetate** in ketogenesis. - It is **not directly involved** in the main pathway of cholesterol synthesis that is targeted by current hypolipidemic drugs.
Question 3: Which of the following enzymes is not targeted by hypolipidemic drugs?
- A. HMG Co A reductase
- B. Lipoprotein lipase
- C. Acyl CoA, cholesterol acyl transferase 1
- D. Peripheral decarboxylase (Correct Answer)
Explanation: ***Peripheral decarboxylase*** - **Peripheral decarboxylase** (also known as DOPA decarboxylase) is involved in the synthesis of dopamine from L-DOPA and is a target for drugs used in **Parkinson's disease**, not hypolipidemic drugs. - Its inhibition by drugs like **carbidopa** or **benserazide** prevents the peripheral conversion of L-DOPA to dopamine, increasing L-DOPA availability for the brain. *HMG Co A reductase* - **HMG-CoA reductase** is the rate-limiting enzyme in cholesterol biosynthesis and is the primary target for **statins** (e.g., atorvastatin, simvastatin). - Statins effectively lower **LDL cholesterol** by inhibiting this enzyme, reducing endogenous cholesterol production. *Lipoprotein lipase* - **Lipoprotein lipase (LPL)** activity can be enhanced by certain hypolipidemic drugs, such as **fibrates**, which activate **PPAR-α**. - Increased LPL activity leads to enhanced hydrolysis of **triglycerides** from VLDL and chylomicrons, reducing triglyceride levels in plasma. *Acyl CoA, cholesterol acyl transferase 1* - **Acyl-CoA:cholesterol acyltransferase (ACAT) inhibitors** were developed as potential hypolipidemic agents to prevent cholesterol esterification and absorption. - While not widely used clinically due to efficacy and side effect profiles, **ACAT1** is involved in cholesterol esterification in the intestine and liver, making it a target for reducing cholesterol absorption.
Question 4: A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?
- A. Chylomicron
- B. IDL
- C. HDL
- D. VLDL (Correct Answer)
Explanation: ***VLDL*** - A low-fat diet with increased **carbohydrates** can lead to increased hepatic synthesis of triglycerides, which are then packaged into **VLDL** particles for transport from the liver. This is because excess carbohydrates can be converted to fatty acids and then to triglycerides in the liver. - The liver's increased triglyceride production, driven by abundant **glucose** from carbohydrates, directly corresponds to a rise in **VLDL** secretion to export these lipids. *Chylomicron* - **Chylomicrons** primarily transport **dietary fats** (exogenous triglycerides) absorbed from the intestine. - Switching to a low-fat diet would typically lead to a *decrease* in chylomicron production, as less dietary fat is available for absorption. *IDL* - **IDL** (Intermediate-Density Lipoprotein) is a remnant of **VLDL** metabolism, formed after VLDL loses some triglycerides. - While VLDL may increase, leading to *more* IDL formation, IDL itself is not the primary component that *increases* directly due to high carbohydrate intake; rather, the precursor **VLDL** is directly affected. *HDL* - **HDL** (High-Density Lipoprotein) is involved in **reverse cholesterol transport**, picking up excess cholesterol from peripheral tissues and returning it to the liver. - High carbohydrate intake, especially refined carbohydrates, can sometimes lead to a *decrease* in HDL levels, not an increase.
Question 5: Rate limiting enzyme in bile acid synthesis?
- A. Desmolase
- B. 21α-hydroxylase
- C. 7α-hydroxylase (Correct Answer)
- D. 12α-hydroxylase
Explanation: ***7α-hydroxylase*** - This enzyme, specifically **cholesterol 7α-hydroxylase**, catalyzes the first and rate-limiting step in the classic pathway of **bile acid synthesis**, converting cholesterol to 7α-hydroxycholesterol. - Its activity is tightly regulated, primarily by the availability of cholesterol and feedback inhibition by bile acids, making it a key control point. *Desmolase* - **Cholesterol desmolase** (CYP11A1) is the rate-limiting enzyme in **steroid hormone synthesis** in the adrenal glands, converting cholesterol to pregnenolone. - It is not involved in the committed steps of bile acid synthesis from cholesterol. *21α-hydroxylase* - **21α-hydroxylase** (CYP21A2) is crucial in the synthesis of **cortisol and aldosterone** from progesterone and 17-hydroxyprogesterone, respectively. - Deficiency in this enzyme is the most common cause of **congenital adrenal hyperplasia**, but it has no direct role in bile acid synthesis. *12α-hydroxylase* - **12α-hydroxylase** (CYP8B1) is an enzyme involved in the later steps of bile acid synthesis, specifically in the formation of **cholic acid** from 7α-hydroxy-4-cholesten-3-one. - While essential for synthesizing primary bile acids, it is not the *rate-limiting enzyme* for the overall pathway; 7α-hydroxylase holds that distinction.
Question 6: What is the primary role of cholesterol in low-density lipoprotein (LDL) metabolism?
- A. Cholesterol binds to receptors on cell membranes.
- B. Excess cholesterol in cells reduces the number of LDL receptors. (Correct Answer)
- C. Cholesterol regulates the activity of enzymes involved in cholesterol metabolism.
- D. Cholesterol in LDL is primarily involved in transporting cholesterol to tissues.
Explanation: ***Excess cholesterol in cells reduces the number of LDL receptors.*** - High intracellular **cholesterol levels** signal the cell to *downregulate* the production of **LDL receptors** via the **SREBP-2 pathway**. - This negative feedback mechanism prevents excessive accumulation of cholesterol within cells and maintains cellular **cholesterol homeostasis**. - This is the primary regulatory mechanism specifically related to **LDL receptor-mediated metabolism**. *Cholesterol binds to receptors on cell membranes.* - It is actually the **LDL particle**, specifically its **apolipoprotein B-100 (apoB-100)** component, that binds to the **LDL receptors** on cell membranes. - While cholesterol is the cargo within LDL, it does not directly bind to the receptors itself. *Cholesterol regulates the activity of enzymes involved in cholesterol metabolism.* - While **intracellular cholesterol levels** do regulate various enzymes (e.g., **HMG-CoA reductase** via SREBP-2, and **ACAT**), this describes cholesterol's broader role in **cholesterol synthesis regulation** rather than specifically in **LDL metabolism**. - The question asks specifically about cholesterol's role in **LDL metabolism**, which refers to the receptor-mediated pathway and its regulation. *Cholesterol in LDL is primarily involved in transporting cholesterol to tissues.* - This statement describes the *function of LDL itself*, which is to transport cholesterol to peripheral tissues. - However, the question asks for the **primary role of cholesterol *in* LDL metabolism**, referring to its regulatory effects on the LDL receptor pathway rather than its transport function.
Question 7: Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?
- A. Gluconeogenesis
- B. TCA (Correct Answer)
- C. HMP pathway
- D. Glycolysis
Explanation: ***TCA (Tricarboxylic Acid Cycle)*** - The **TCA cycle** (also called Krebs cycle or citric acid cycle) is the **final common oxidative pathway** where all three macronutrients converge - **Carbohydrates** → Pyruvate → **Acetyl-CoA** (via pyruvate dehydrogenase) - **Lipids** → Fatty acids → **Acetyl-CoA** (via beta-oxidation) - **Proteins** → Amino acids → **Acetyl-CoA or TCA intermediates** (via deamination/transamination) - Complete oxidation of acetyl-CoA occurs in the TCA cycle, producing **NADH, FADH2, and GTP** for energy production *Gluconeogenesis* - This is a **biosynthetic pathway** that synthesizes glucose from non-carbohydrate precursors (lactate, glycerol, amino acids) - It is an **anabolic process**, not the catabolic final common pathway for energy production from all macronutrients *Glycolysis* - **Carbohydrate-specific pathway** that converts glucose to pyruvate - It is only the initial breakdown pathway for carbohydrates, not the common pathway where lipids and proteins also converge - Pyruvate from glycolysis must enter TCA cycle for complete oxidation *HMP pathway (Pentose Phosphate Pathway)* - Parallel pathway to glycolysis that generates **NADPH** (for biosynthesis and antioxidant defense) and **ribose-5-phosphate** (for nucleotide synthesis) - Processes only **glucose-6-phosphate** from carbohydrate metabolism - Not involved in lipid or protein metabolism integration
Question 8: What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?
- A. SR-BI (Correct Answer)
- B. LDLR
- C. HDLR
- D. SR-82
Explanation: ***SR-BI*** - **Scavenger Receptor class B type 1 (SR-BI)** is the primary receptor responsible for selective uptake of **cholesteryl esters** from HDL into cells, particularly the liver and steroidogenic tissues. - Unlike other lipoprotein receptors, SR-BI mediates the **selective transfer** of cholesterol without internalizing the entire HDL particle. *LDLR* - The **Low-Density Lipoprotein Receptor (LDLR)** is the primary receptor for **LDL** and very low-density lipoprotein (VLDL) remnants, mediating their endocytosis and degradation. - While it plays a crucial role in cholesterol metabolism, its main function is related to the uptake of **LDL cholesterol**, not HDL. *HDLR* - **HDLR** is not a recognized receptor in cholesterol metabolism. - This term may be a distracter created by combining HDL with the common receptor nomenclature. *SR-82* - **SR-82** is not a recognized receptor involved in cholesterol metabolism. - Similar to HDLR, this is a distracter term.
Question 9: In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?
- A. Chylomicron (Correct Answer)
- B. LDL
- C. HDL
- D. Apo-A
Explanation: ***Chylomicron*** - Lipoprotein lipase (LPL) is essential for the breakdown of **chylomicrons** in the bloodstream. A deficiency in LPL means chylomicrons cannot be cleared effectively. - After a **fatty meal**, the body absorbs dietary fats as chylomicrons. Without functional LPL, these chylomicrons accumulate in the plasma, leading to **marked elevation** of chylomicron levels. - This results in **lipemic (milky) serum**, a characteristic finding in Type I hyperlipoproteinemia. *LDL* - **LDL (Low-Density Lipoprotein)** levels are primarily affected by the metabolism of VLDL (Very Low-Density Lipoprotein), which is a separate pathway from chylomicron metabolism. - LPL deficiency specifically impacts chylomicron clearance, not directly causing an increase in LDL. In fact, LDL may be normal or even low in severe hypertriglyceridemia. *HDL* - **HDL (High-Density Lipoprotein)** is involved in reverse cholesterol transport and is typically **decreased** (not increased) in LPL deficiency. - During normal lipolysis by LPL, surface components from chylomicrons are transferred to HDL. In LPL deficiency, this process is impaired, leading to **reduced HDL levels**. *Apo-A* - **Apolipoprotein A-I (Apo-AI)** is the primary apolipoprotein found on HDL particles and is crucial for HDL formation and function. - Since HDL levels are decreased in LPL deficiency, Apo-AI levels would also be decreased (not increased) following a fatty meal.
Question 10: Which of the following statements about LDL is false?
- A. More dense than chylomicron
- B. Transports maximum amount of lipid (Correct Answer)
- C. Contains maximum cholesterol
- D. Smaller than VLDL
Explanation: ***Transports maximum amount of lipid*** - This statement is false because **chylomicrons**, not LDL, are primarily responsible for transporting the **maximum amount of dietary lipids** (triglycerides) from the intestines to various tissues. - While LDL does transport lipids, its primary role is to deliver **cholesterol** to cells, and it contains a lower proportion of triglyceride compared to chylomicrons and VLDL. *More dense than chylomicron* - This statement is true; **LDL is denser than chylomicrons** because it has a higher protein-to-lipid ratio. - **Chylomicrons** are the least dense lipoproteins due to their very high triglyceride content. *Smaller than VLDL* - This statement is true; **LDL is smaller than VLDL** (Very Low-Density Lipoprotein). - VLDL particles are larger and contain more triglycerides, which are gradually removed, leading to the formation of smaller LDL particles. *Contains maximum cholesterol* - This statement is true; **LDL contains the highest proportion of cholesterol** (specifically, **cholesterol esters**) among the lipoproteins. - This characteristic makes LDL the primary carrier for delivering cholesterol to peripheral tissues.
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