Lipid Metabolism Practice Questions

Q: What is the rate-controlling enzyme of fatty acid synthesis?

Acetyl-CoA carboxylase. ***Acetyl-CoA carboxylase*** - **Acetyl-CoA carboxylase (ACC)** catalyzes the committed step in fatty acid synthesis, converting **acetyl-CoA** to **malonyl-CoA**. - This enzyme is subject to both allosteric regulation (e.g., activation by **citrate** and inhibition by **long-chain fatty acyl-CoA**) and hormonal regulation (e.g., phosphorylation by glucagon and dephosphorylation by insulin). *Thioesterase* - **Thioesterase** is the enzyme responsible for releasing the completed fatty acid chain from the **fatty acid synthase complex**. - While essential for the termination of synthesis, it does not regulate the initiation or overall rate of the pathway. *Transacetylase* - **Transacetylase** (specifically, acetyl-CoA-ACP transacetylase and malonyl-CoA-ACP transacetylase) is involved in transferring acetyl and malonyl groups to the **acyl carrier protein (ACP)** within the fatty acid synthesis complex. - This is an intermediary step, but not the primary **rate-controlling** or committed step. *Ketoacyl synthase* - **Ketoacyl synthase (or β-ketoacyl-ACP synthase)** is responsible for condensing the growing acyl chain with malonyl-ACP, leading to the formation of a **β-ketoacyl-ACP**. - This is a crucial chain elongation step within the fatty acid synthase complex, but not the enzyme that controls the overall commitment to fatty acid synthesis.

Q: Chylomicron remnants are associated with ?

Apo-E. ***Apo-E*** - **Apolipoprotein E (Apo-E)** is a crucial apolipoprotein on the surface of chylomicron remnants, acting as a **ligand for the LDL receptor-related protein 1 (LRP1)** in the liver. - This binding facilitates the **hepatic uptake and clearance** of chylomicron remnants from circulation. *Apo-A* - **Apo-AI** is the primary apolipoprotein of **HDL** and plays a key role in reverse cholesterol transport by activating **lecithin-cholesterol acyltransferase (LCAT)**. - While chylomicrons *acquire* some Apo-AI from HDL, it is not the primary apolipoprotein defining their remnants' hepatic clearance. *Apo-C* - **Apo-CII** is a vital activator of **lipoprotein lipase (LPL)**, which metabolizes triglycerides in chylomicrons and VLDL. - **Apo-CIII** inhibits LPL and hinders receptor-mediated uptake, but **Apo-E** is the key for remnant recognition and uptake, not Apo-C in general. *Apo-B100* - **Apo-B100** is the main structural apolipoprotein of **LDL** and **VLDL**, serving as the ligand for the LDL receptor, mediating their hepatic uptake. - While chylomicrons have **Apo-B48**, which is a truncated form of Apo-B100, Apo-B100 itself is not found on chylomicron remnants.

Q: Lipogenesis is stimulated by?

Insulin. ***Insulin*** - **Insulin** is a key anabolic hormone that promotes the synthesis and storage of fat (lipogenesis) by increasing the uptake of glucose into adipose tissue and stimulating enzymes involved in fatty acid synthesis. - It enhances the conversion of excess carbohydrates into **triglycerides** for storage. *Glucagon* - **Glucagon** is a catabolic hormone that primarily promotes the breakdown of glycogen (glycogenolysis) and fat (lipolysis) to release glucose and fatty acids into the bloodstream, especially during fasting. - It generally **inhibits** lipogenesis and stimulates **gluconeogenesis**. *Epinephrine* - **Epinephrine** (adrenaline) is a stress hormone that promotes the breakdown of fat (lipolysis) to provide energy during acute stress or exercise. - It would **inhibit** lipogenesis, as its primary role is to mobilize energy stores. *Corticosteroids* - While **corticosteroids** can influence fat metabolism, their effect on lipogenesis is complex and often indirect. High levels can lead to fat redistribution (e.g., central obesity) rather than direct stimulation of overall lipogenesis. - Corticosteroids generally promote **lipolysis** in the extremities and can contribute to insulin resistance, which would hinder lipogenesis in some tissues.

Q: Which of the following statements about chylomicrons is true?

Chylomicrons primarily contain triglycerides (TGs).. ***Chylomicrons primarily contain triglycerides (TGs)*** - **Chylomicrons** are the largest and least dense lipoproteins, primarily responsible for transporting **dietary triglycerides** absorbed from the intestine to peripheral tissues. - They are synthesized in the **enterocytes** of the small intestine and released into the lymphatic system. - Approximately **85-90%** of a chylomicron's mass is composed of **triglycerides**, making them the primary carriers of exogenous fats. *Chylomicrons primarily contain cholesterol* - While chylomicrons do contain some **cholesterol**, it is a minor component (~3-5%) compared to their predominant content, which is **triglycerides**. - Lipoproteins like **LDL** and **HDL** are primarily responsible for cholesterol transport. *Chylomicrons are unrelated to triglyceride transport* - This statement is incorrect; chylomicrons are fundamentally involved in the **transport of dietary triglycerides** from the intestines to various tissues in the body. - After lipoprotein lipase acts on chylomicrons in peripheral tissues, triglycerides are hydrolyzed and fatty acids are taken up by tissues. *Chylomicrons do not primarily contain triglycerides* - This statement directly contradicts the main function and composition of chylomicrons, which are **rich in triglycerides**. - Without triglycerides as their primary content, chylomicrons would not be able to fulfill their physiological role in lipid transport.

Q: Which of the following enzymes uses citrate in fatty acid synthesis?

ATP citrate lyase. ***ATP citrate lyase*** - This enzyme is crucial for fatty acid synthesis, as it cleaves **citrate** in the cytoplasm to generate **acetyl-CoA** and oxaloacetate. - The acetyl-CoA produced is then used as the primary building block for **fatty acid synthesis**. *Aconitase* - This enzyme isomerizes **citrate** to isocitrate within the **Krebs cycle** (TCA cycle) in the mitochondria. - It does not directly participate in the cytosolic pathway of fatty acid synthesis. *Citrate synthase* - This enzyme synthesizes **citrate** from acetyl-CoA and oxaloacetate, initiating the **Krebs cycle** in the mitochondrial matrix. - It is involved in citrate formation, not its utilization for fatty acid synthesis in the cytoplasm. *Malic enzyme* - This enzyme converts **malate** to pyruvate, generating **NADPH** in the cytoplasm. - While NADPH is essential for fatty acid synthesis, malic enzyme does not directly use citrate.

Q: Enzyme deficient in Type I Hyperlipidemia?

Lipoprotein lipase. ***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**.

Q: Where does omega oxidation of fatty acids occur?

Endoplasmic Reticulum. ***Endoplasmic Reticulum*** - **Omega oxidation** of fatty acids occurs in the **endoplasmic reticulum (microsomes)** of liver and kidney cells. - This pathway involves **hydroxylation of the terminal omega carbon** by **cytochrome P450 enzymes** located in the smooth ER. - The omega carbon is then oxidized to a **carboxyl group**, forming a **dicarboxylic acid**. - This is a **minor pathway** that becomes important when **beta-oxidation is impaired** or for metabolism of **medium-chain fatty acids**. *Cytosol* - The cytosol is involved in **fatty acid synthesis**, not omega oxidation. - While some later steps of fatty acid metabolism occur in the cytosol, the initial hydroxylation step of omega oxidation requires ER-localized cytochrome P450 enzymes. *Mitochondria* - **Mitochondria** are the primary site for **beta-oxidation** of fatty acids, not omega oxidation. - Beta-oxidation sequentially removes **two-carbon units from the carboxyl end**, which is distinct from omega oxidation. - The dicarboxylic acids produced by omega oxidation may subsequently undergo beta-oxidation in mitochondria. *None of the options* - This option is incorrect because the endoplasmic reticulum is the correct cellular location for omega oxidation. - The ER contains the necessary cytochrome P450 enzymes for the hydroxylation reaction that initiates this pathway.

Q: Apo-E deficiency is seen in which of the following conditions?

Type III hyperlipoproteinemia. ***Type III hyperlipoproteinemia*** - This condition, also known as **familial dysbetalipoproteinemia** or **broad beta disease**, is characterized by a deficiency or abnormal function of **apolipoprotein E (apoE)**. - The deficiency in functional apoE impairs the clearance of **chylomicron remnants** and **intermediate-density lipoproteins (IDLs)** from the blood. *Type II hyperlipoproteinemia* - This condition primarily involves elevated **LDL cholesterol** and is often due to defects in the **LDL receptor** or mutations in **apoB-100**, not apoE deficiency. - It does not directly involve the impaired clearance of chylomicron remnants or IDLs. *Type I hyperlipoproteinemia* - Also known as **familial chylomicronemia syndrome**, this condition is characterized by severe elevation of **chylomicrons** and **triglycerides**. - It is caused by a deficiency of **lipoprotein lipase (LPL)** or its cofactor **apoC-II**, not apoE. *Type IV hyperlipoproteinemia* - This condition, also known as **familial hypertriglyceridemia**, is characterized by abnormally high levels of **very-low-density lipoproteins (VLDL)** and **triglycerides**. - It is typically caused by increased VLDL production or impaired VLDL clearance, but not directly due to an apoE deficiency.

Q: Number of ATP formed by the oxidation of one molecule of palmitic acid (16 carbon atoms):

106. ***106*** - Palmitic acid (16 carbon atoms) undergoes **7 cycles of beta-oxidation**. Each cycle produces 1 FADH2, 1 NADH, and 1 acetyl-CoA. - The 7 cycles yield **7 FADH2** (7 x 1.5 ATP = 10.5 ATP), **7 NADH** (7 x 2.5 ATP = 17.5 ATP), and **8 acetyl-CoA** (8 x 10 ATP = 80 ATP in the TCA cycle), totaling 108 ATP. Two ATP equivalents are used for activation, resulting in a net yield of **106 ATP**. *146* - This number is a common misconception, often arising from older calculations that used different ATP yields per NADH and FADH2 (e.g., 3 ATP for NADH and 2 ATP for FADH2). - Modern bioenergetics typically uses **2.5 ATP per NADH** and **1.5 ATP per FADH2** based on more precise proton pump stoichiometry. *135* - This value is not consistent with the current understanding of ATP yields from the complete oxidation of palmitic acid. - It might stem from errors in calculation or using an **incorrect number of beta-oxidation cycles** or ATP equivalents for coenzymes. *34* - This number is significantly **too low** for the complete oxidation of a 16-carbon fatty acid. - It is closer to the ATP yield from the complete oxidation of a **glucose molecule** (around 30-32 ATP), not a fatty acid.

Q: All are polyunsaturated fatty acids except:

Palmitic acid. ***Palmitic acid*** - **Palmitic acid** is a **saturated fatty acid**, meaning it has no double bonds in its carbon chain. - Its chemical structure is CH₃(CH₂)₁₄COOH, indicating it is **fully saturated** with hydrogen atoms. *Linoleic acid* - **Linoleic acid** is an **omega-6 polyunsaturated fatty acid** with two double bonds. - It is an **essential fatty acid**, meaning the human body cannot synthesize it and it must be obtained through diet. *Linolenic acid* - **Linolenic acid** typically refers to **alpha-linolenic acid (ALA)**, an **omega-3 polyunsaturated fatty acid** with three double bonds. - Like linoleic acid, ALA is an **essential fatty acid** crucial for various physiological functions. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 polyunsaturated fatty acid** with four double bonds. - It is a precursor for the synthesis of **eicosanoids**, such as prostaglandins and leukotrienes, which are important signaling molecules.

Question 1

What is the rate-controlling enzyme of fatty acid synthesis?

Accepted Answer

Acetyl-CoA carboxylase

Answer

Thioesterase

Answer

Transacetylase

Answer

Ketoacyl synthase

Question 2

Chylomicron remnants are associated with ?

Accepted Answer

Apo-E

Answer

Apo-C

Answer

Apo-A

Answer

Apo-B100

Question 3

Lipogenesis is stimulated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Epinephrine

Answer

Corticosteroids

Question 4

Which of the following statements about chylomicrons is true?

Accepted Answer

Chylomicrons primarily contain triglycerides (TGs).

Answer

Chylomicrons are unrelated to triglyceride transport.

Answer

Chylomicrons primarily contain cholesterol.

Answer

Chylomicrons do not primarily contain triglycerides.

Question 5

Which of the following enzymes uses citrate in fatty acid synthesis?

Accepted Answer

ATP citrate lyase

Answer

Aconitase

Answer

Malic enzyme

Answer

Citrate synthase

Question 6

Enzyme deficient in Type I Hyperlipidemia?

Accepted Answer

Lipoprotein lipase

Answer

HMG CoA reductase

Answer

Peroxidase

Answer

Cholesterol acyl transferase

Question 7

Where does omega oxidation of fatty acids occur?

Accepted Answer

Endoplasmic Reticulum

Answer

Cytosol

Answer

Mitochondria

Answer

None of the options

Question 8

Apo-E deficiency is seen in which of the following conditions?

Accepted Answer

Type III hyperlipoproteinemia

Answer

Type II hyperlipoproteinemia

Answer

Type I hyperlipoproteinemia

Answer

Type IV hyperlipoproteinemia

Question 9

Number of ATP formed by the oxidation of one molecule of palmitic acid (16 carbon atoms):

Accepted Answer

106

Answer

146

Answer

135

Answer

34

Question 10

All are polyunsaturated fatty acids except:

Accepted Answer

Palmitic acid

Answer

Linoleic acid

Answer

Linolenic acid

Answer

Arachidonic acid

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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