Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 681: Which of the following is NOT an enzyme involved in fatty acid synthesis?

A. Ketoacyl synthase
B. Enoyl reductase
C. Acetyl Co-A carboxylase
D. Acetoacetyl-CoA (Correct Answer)

Explanation: ***Acetoacetyl-CoA*** - **Acetoacetyl-CoA** is an intermediate compound, not an enzyme. - It is a **ketone body precursor** and also an intermediate in fatty acid synthesis and degradation. - It serves as a **substrate** for various enzymes but does not catalyze any reaction itself. *Acetyl Co-A carboxylase* - **Acetyl Co-A carboxylase** is the key regulatory enzyme in fatty acid synthesis, catalyzing the **carboxylation of acetyl-CoA** to form malonyl-CoA. - This reaction is the **rate-limiting step** and the **committed step** in fatty acid synthesis. - It requires **biotin** as a cofactor. *Ketoacyl synthase* - **Ketoacyl synthase** (beta-ketoacyl-ACP synthase) is a core catalytic domain of the fatty acid synthase complex. - It catalyzes the **condensation reaction** between an acyl group and malonyl-ACP, releasing CO₂. - This forms a **beta-ketoacyl-ACP intermediate**. *Enoyl reductase* - **Enoyl reductase** (enoyl-ACP reductase) is an enzyme domain in the fatty acid synthase complex. - It catalyzes the **final reduction step**, converting trans-enoyl-ACP to saturated acyl-ACP. - This reaction uses **NADPH** as the reducing agent.

Question 682: Which of the following statements about adiponectin is incorrect?

A. Secreted by adipose tissue
B. Increases FFA oxidation
C. Lowers glucose
D. Positive Correlation with BMI (Correct Answer)

Explanation: ***Positive Correlation with BMI (INCORRECT STATEMENT)*** - Adiponectin levels are **inversely correlated with BMI**, NOT positively correlated; as BMI increases, adiponectin levels generally decrease. - This inverse relationship is significant because lower adiponectin levels are associated with increased insulin resistance and **metabolic syndrome**. - This statement is **false**, making it the correct answer to this question. *Secreted by adipose tissue (Correct statement)* - Adiponectin is a **hormone primarily secreted by adipocytes** (fat cells). - It plays a crucial role in regulating glucose and lipid metabolism, and its secretion is altered in conditions like obesity. - This statement is **true**. *Lowers glucose (Correct statement)* - Adiponectin **enhances insulin sensitivity** in peripheral tissues like skeletal muscle and liver, leading to increased glucose uptake and utilization. - This action helps to **lower blood glucose levels** and improve glycemic control. - This statement is **true**. *Increases FFA oxidation (Correct statement)* - Adiponectin **promotes fatty acid oxidation** in muscle and liver, reducing intracellular lipid accumulation. - By increasing fatty acid burning, it helps to **decrease circulating free fatty acid (FFA) levels**, which can contribute to insulin resistance if elevated. - This statement is **true**.

Question 683: What is the role of colipase in fat digestion?

A. Encoded by the gene CLPS
B. Assists pancreatic lipase in fat digestion (Correct Answer)
C. Is secreted in an inactive form
D. Is secreted by pancreatic cells

Explanation: ***Assists pancreatic lipase in fat digestion*** - Colipase **binds to pancreatic lipase** and the **lipid-water interface** of the fat droplet, providing a conformational change that enables lipase to access and hydrolyze triglycerides. - It also prevents bile salts from inactivating pancreatic lipase, ensuring efficient **fat emulsification and digestion**. *Is secreted in an inactive form* - Colipase is secreted as **procolipase** by the pancreas, which is then activated by **trypsin** in the duodenum. - While correct, this option describes its activation rather than its primary role in fat digestion. *Encoded by the gene CLPS* - The gene **CLPS** indeed encodes for colipase, but this is a genetic detail rather than its functional role in the digestive process. - Knowledge of the encoding gene is not directly relevant to understanding its biochemical function in fat digestion. *Is secreted by pancreatic cells* - Colipase is indeed synthesized and secreted by the **pancreas** into the small intestine. - This statement is true but describes the **origin** of colipase, not its specific functional role in fat digestion.

Question 684: Which of the following stimulates Acetyl CoA Carboxylase?

A. Starvation
B. Glucagon
C. Citrate (Correct Answer)
D. None of the options

Explanation: ***Citrate*** - **Citrate** is an allosteric activator of **Acetyl-CoA Carboxylase (ACC)**, indicating abundant energy and precursor availability for fatty acid synthesis. - This activation promotes the conversion of **Acetyl-CoA** to **Malonyl-CoA**, the committed step in **fatty acid synthesis**. *Starvation* - **Starvation** leads to energy deficit, which generally **inhibits** anabolic processes like fatty acid synthesis. - In this state, enzymes involved in anabolic pathways are often downregulated or inhibited to conserve energy. *Glucagon* - **Glucagon** is a hormone that signals low blood glucose and promotes catabolic processes such as **glycogenolysis** and **gluconeogenesis**. - It **inhibits** fatty acid synthesis by phosphorylating and inactivating **Acetyl-CoA Carboxylase**, thus opposing citrate's activating effect. *None of the options* - **Citrate** is a known stimulator of Acetyl CoA Carboxylase. - This option is incorrect because there is a correct answer among the choices.

Question 685: Cholesterol is not a precursor for the synthesis of which of the following?

A. Testosterone
B. Lipocortin (Correct Answer)
C. Cortisol
D. Aldosterone

Explanation: ***Lipocortin*** - **Lipocortin** (also known as annexin A1) is a **protein**, not a steroid compound - It is synthesized via **protein translation** from mRNA, not from cholesterol - Mediates anti-inflammatory effects of glucocorticoids and is involved in cell growth regulation - **Key point:** Only steroids and bile acids are derived from cholesterol, not proteins *Testosterone* - **Testosterone** is an androgen (male sex hormone) **synthesized from cholesterol** - Cholesterol → Pregnenolone → DHEA → Androstenedione → Testosterone - Produced in the gonads (Leydig cells) and adrenal glands *Cortisol* - **Cortisol** is a glucocorticoid hormone **derived from cholesterol** - Cholesterol → Pregnenolone → 17-hydroxypregnenolone → Cortisol - Synthesized in the zona fasciculata of the adrenal cortex *Aldosterone* - **Aldosterone** is a mineralocorticoid hormone **synthesized from cholesterol** - Cholesterol → Pregnenolone → Progesterone → Aldosterone - Produced in the zona glomerulosa of the adrenal cortex

Question 686: What is the effect of moderate alcohol consumption on lipid profiles in dyslipidemia?

A. Decreased HDL levels
B. Increased HDL levels (Correct Answer)
C. Increased triglyceride levels
D. Decreased LDL levels

Explanation: ***Increased HDL levels*** - Moderate alcohol consumption is known to **increase high-density lipoprotein (HDL) cholesterol levels**, which is often considered beneficial for cardiovascular health. - This effect is thought to be mediated by alcohol's influence on **hepatic lipoprotein metabolism**, leading to enhanced HDL production and reduced catabolism. *Decreased HDL levels* - This is incorrect, as multiple studies have consistently shown that **moderate alcohol consumption** tends to elevate, rather than decrease, HDL cholesterol. - Low HDL levels are associated with increased cardiovascular risk, making this effect an undesirable outcome that is not typical of moderate drinking. *Increased triglyceride levels* - While heavy or chronic alcohol consumption can lead to **increased triglyceride levels**, moderate intake typically has a neutral or only slightly elevated effect, if any, often overshadowed by the HDL increase. - Significant hypertriglyceridemia is a concern with **excessive alcohol use**, not usually with moderate consumption in healthy individuals. *Decreased LDL levels* - Moderate alcohol consumption generally has **little to no significant effect** on **low-density lipoprotein (LDL) cholesterol levels**, often referred to as "bad" cholesterol. - While HDL increases are observed, alcohol does not effectively lower LDL, which is a primary target in the management of dyslipidemia.

Question 687: What is the primary effect of moderate alcohol consumption on cholesterol levels?

A. Total cholesterol
B. Low-Density Lipoprotein (LDL)
C. Very Low-Density Lipoprotein (VLDL)
D. High-Density Lipoprotein (HDL) (Correct Answer)

Explanation: ***High-Density Lipoprotein (HDL)*** - Moderate alcohol consumption is known to **increase HDL cholesterol** levels. - HDL cholesterol helps in the **reverse cholesterol transport**, removing excess cholesterol from tissues and transporting it back to the liver for excretion. *Total cholesterol* - The effect of moderate alcohol on **total cholesterol** is less consistent and may vary, as it is a sum of HDL, LDL, and 20% of VLDL. - While HDL increases, other components might remain unchanged or show minimal variation, thus not making it the primary and direct effect. *Low-Density Lipoprotein (LDL)* - Moderate alcohol consumption generally has **little to no significant effect** on **LDL cholesterol** levels. - Some studies suggest a slight decrease or no change, but it is not the primary lipid affected. *Very Low-Density Lipoprotein (VLDL)* - There is generally **no significant direct effect** of moderate alcohol consumption on **VLDL cholesterol** levels. - Excessive alcohol intake, however, can elevate triglycerides, which are the main component of VLDL particles.

Question 688: What is essential for the transfer of fatty acid across the mitochondrial membrane?

A. Creatinine
B. Carnitine (Correct Answer)
C. Biotin
D. Creatine

Explanation: ***Carnitine*** - **Carnitine** is crucial for transporting **long-chain fatty acids** into the mitochondrial matrix for **beta-oxidation**. - It forms **acylcarnitine** by esterifying with fatty acids, allowing passage through the inner mitochondrial membrane via the **carnitine-acylcarnitine translocase**. *Creatinine* - **Creatinine** is a waste product formed from the breakdown of **creatine phosphate** in muscles and is excreted by the kidneys. - It serves as a marker for **kidney function** and has no role in fatty acid transport. *Biotin* - **Biotin** is a vitamin cofactor essential for **carboxylase enzymes**, including acetyl-CoA carboxylase in **fatty acid synthesis**. - While involved in lipid metabolism, it plays no role in the transport of fatty acids across mitochondrial membranes. *Creatine* - **Creatine** is a nitrogenous organic acid that helps supply energy to cells, primarily muscle, by facilitating the regeneration of **ATP**. - It plays no direct role in the facilitated transport of fatty acids across the mitochondrial membrane.

Question 689: Which of the following statements about ketone bodies is false?

A. Synthesized in mitochondria
B. Synthesized in liver
C. HMG CoA reductase is the rate-limiting enzyme (Correct Answer)
D. Acetoacetate is primary ketone body

Explanation: ***HMG CoA reductase is the rate-limiting enzyme*** - This statement is **false** because **HMG-CoA synthase**, not HMG-CoA reductase, is the **rate-limiting enzyme in ketogenesis**. - **HMG-CoA reductase** is the rate-limiting enzyme in **cholesterol synthesis**, a completely different metabolic pathway. *Acetoacetate is primary ketone body* - **Acetoacetate** is indeed considered the **primary ketone body**, as **β-hydroxybutyrate** is derived from it and **acetone** is a spontaneous breakdown product of acetoacetate. - It is the first ketone body formed during the synthesis pathway. *Synthesized in mitochondria* - Ketone bodies are synthesized in the **mitochondrial matrix** of liver cells. - This location allows for the efficient use of **acetyl-CoA** generated from fatty acid oxidation. *Synthesized in liver* - The liver is the **primary site of ketogenesis**, where fatty acids are converted into ketone bodies. - This process is crucial for providing alternative fuel to extrahepatic tissues during periods of fasting or prolonged starvation.

Question 690: What is the primary reason for the detergent action of bile salts?

A. Hydrophobic properties
B. Acts as a zwitterion
C. Amphipathic nature (Correct Answer)
D. None of the options

Explanation: ***Amphipathic nature*** - Bile salts are **amphipathic molecules**, meaning they have both **hydrophilic (water-loving)** and **hydrophobic (water-fearing)** regions. - This dual nature allows them to emulsify fats by surrounding lipid droplets with their hydrophobic ends dissolving in the fat and their hydrophilic ends facing the aqueous environment, stabilizing the emulsion. *Hydrophobic properties* - While bile salts do possess **hydrophobic regions**, these alone are not sufficient for detergent action. - The ability to interact with both oil and water phases simultaneously is crucial for their role in **emulsification**. *Acts as a zwitterion* - A zwitterion is a molecule with both a **positive and negative charge**, but an overall neutral charge. - This property is not the primary mechanism behind the **detergent action** of bile salts, which relies more on their ability to solubilize fats. *None of the options* - The **amphipathic nature** is indeed the primary reason for the detergent action; therefore, this option is incorrect.

Practice by Chapter

Lipid Classification and Chemistry

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Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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