Lipid Metabolism Practice Questions

Q: Liver produces ketones but cannot use it due to the deficiency of which of the following enzyme?

Thiophorase. ***Thiophorase*** - The liver lacks **thiophorase (succinyl-CoA:3-ketoacid CoA transferase)**, which is crucial for converting **acetoacetate** to **acetoacetyl-CoA**. - This enzyme deficiency prevents the liver from utilizing ketones as an energy source, even though it is a primary site for their production. *Alkaline phosphatase* - **Alkaline phosphatase** is a non-specific enzyme found in various tissues, including bone, liver, and intestine. - Its primary role is to **hydrolyze phosphate esters**, and it is not directly involved in ketone metabolism. *Alanine transaminase* - **Alanine transaminase (ALT)** is a liver enzyme primarily involved in **amino acid metabolism**, specifically in the transfer of an amino group from alanine to α-ketoglutarate. - It plays no direct role in the synthesis or utilization of ketone bodies. *Thiolase* - **Thiolase** is an enzyme involved in both the synthesis and breakdown of ketone bodies. - It converts **two acetyl-CoA molecules into acetoacetyl-CoA** during ketogenesis and also cleaves acetoacetyl-CoA into two acetyl-CoA molecules during ketolysis in extrahepatic tissues.

Q: BARTH syndrome is caused by deficiency of?

Cardiolipin. ***Cardiolipin*** - **BARTH syndrome** is a rare, X-linked genetic disorder caused by mutations in the **TAZ gene**, which encodes for the enzyme **tafazzin**. - **Tafazzin** is crucial for the remodeling of **cardiolipin**, a phospholipid essential for mitochondrial membrane integrity and function. Deficiency of properly remodeled (mature) cardiolipin leads to the characteristic cardiomyopathy, skeletal myopathy, neutropenia, and growth delay seen in BARTH syndrome. *Glycolipids* - **Glycolipids** are lipids with a carbohydrate attached, important for cell recognition and signaling. - Their deficiency or abnormal metabolism is associated with conditions like **glycosphingolipidoses** (e.g., Gaucher disease, Fabry disease), not BARTH syndrome. *Sphingomyelin* - **Sphingomyelin** is a type of sphingolipid found in animal cell membranes, particularly in the myelin sheath. - Its deficiency or accumulation is linked to **Niemann-Pick disease**, which presents with hepatosplenomegaly and neurodegeneration, distinct from BARTH syndrome. *Cerebroside* - **Cerebrosides** are a type of glycosphingolipid found in the myelin sheath and nerve cell membranes. - Disorders involving cerebroside metabolism include **Krabbe disease** (globoid cell leukodystrophy) and **Gaucher disease**, which are pathologically distinct from BARTH syndrome.

Q: Energy reserve of the body is:

Triglyceride. ***Triglyceride*** - **Triglycerides** are the primary form of **energy storage** in the body, stored predominantly in **adipose tissue**. - They provide a highly concentrated and long-term energy reserve, crucial for periods of fasting or increased energy demand. *Fibres* - **Dietary fibers** are indigestible plant-derived carbohydrates that contribute to digestive health but do not serve as a direct energy reserve for the body. - They are important for maintaining gut motility and regulating blood sugar, but are not metabolized for energy. *Carbohydrate* - **Carbohydrates** (primarily **glycogen**) serve as the **body's most immediate energy source**, stored in the liver and muscles. - However, glycogen stores are limited and are depleted much faster than triglyceride stores during prolonged energy needs. *Proteins* - **Proteins** have numerous structural and functional roles in the body, but are typically used for energy only in cases of **prolonged starvation** when carbohydrate and fat reserves are severely depleted. - Their primary function is not energy storage, and their breakdown for energy can lead to **muscle wasting** and compromise essential body functions.

Q: Which of the following is cardio protective?

HDL. ***HDL*** - **High-density lipoprotein (HDL)** is known as "good cholesterol" because it helps remove **excess cholesterol** from the body and transport it back to the liver for excretion. - This process, called **reverse cholesterol transport**, helps prevent the buildup of plaque in arteries, thereby reducing the risk of **atherosclerosis** and cardiovascular disease. *CHYLOMICRON* - **Chylomicrons** are responsible for transporting **dietary triglycerides** from the intestines to various tissues. - While essential for nutrient absorption, elevated chylomicron levels can contribute to **hypertriglyceridemia**, which is a risk factor for cardiovascular disease and pancreatitis. *VLDL* - **Very low-density lipoprotein (VLDL)** primarily transports **endogenously synthesized triglycerides** from the liver to peripheral tissues. - High levels of VLDL are considered a **risk factor for atherosclerosis** as they can be metabolized into LDL, contributing to plaque formation. *LDL* - **Low-density lipoprotein (LDL)** is often referred to as "bad cholesterol" because it deposits cholesterol in the walls of arteries. - This deposition leads to the formation of **atherosclerotic plaque**, which can narrow arteries and increase the risk of heart attacks and strokes.

Q: Essential fatty acid:

Linoleic acid. ***Linoleic acid*** - **Linoleic acid** is an **omega-6 fatty acid** that is considered essential because the human body cannot synthesize it and must obtain it through diet. - It is a precursor for other important fatty acids like **arachidonic acid**, which are involved in inflammation and blood clotting. *Citric acid* - **Citric acid** is an organic acid found in citrus fruits and is a key intermediate in the **Krebs cycle** (citric acid cycle), a central metabolic pathway, but it is not a fatty acid. - It is readily synthesized by the body and is therefore not considered an essential nutrient. *Palmitic acid* - **Palmitic acid** is a **saturated fatty acid** with 16 carbon atoms, which is the most common fatty acid in animals and plants. - It can be synthesized by the human body from excess carbohydrates and proteins, hence it is not an essential fatty acid. *Stearic acid* - **Stearic acid** is another common **saturated fatty acid** with 18 carbon atoms, found in various animal and plant fats. - Like palmitic acid, it can be endogenously synthesized by the body and is not considered essential.

Q: Colipase is found in?

Pancreatic juice. ***Pancreatic juice*** - **Colipase** is a co-enzyme secreted by the **pancreas** that is essential for the activity of pancreatic lipase. - It helps **pancreatic lipase** anchor to the surface of fat globules, preventing its inactivation by bile salts and facilitating fat digestion. *Saliva* - Saliva primarily contains **alpha-amylase** (ptyalin) for carbohydrate digestion and **lingual lipase** for initial fat digestion. - It does not contain colipase. *Bile* - **Bile** is produced by the liver and stored in the gallbladder, primarily aiding in fat emulsification. - It contains **bile salts**, cholesterol, bilirubin, and phospholipids but not digestive enzymes like colipase. *Succus entericus* - **Succus entericus**, or intestinal juice, is secreted by the small intestine and contains enzymes like **sucrase**, **maltase**, **lactase**, and peptidases. - It does not contain colipase, which is specifically a pancreatic enzyme.

Q: Apoprotein for chylomicron remnants:

Apo E. ***Apo E*** - **Apolipoprotein E** (**Apo E**) is crucial for the uptake of **chylomicron remnants** and **VLDL remnants** (IDL) by the liver via the **LDL receptor-related protein 1 (LRP1)**. - It acts as a **ligand** for this receptor, facilitating the clearance of dietary fats from the circulation. *Apo A2* - **Apo A2** is a primary apolipoprotein of **high-density lipoprotein (HDL)**, not chylomicron remnants. - Its exact function is not fully understood, but it may modulate the activity of **hepatic lipase**. *Apo C1* - **Apo C1** is found on **chylomicrons**, **VLDL**, and **HDL**, but it is not the primary apoprotein responsible for the uptake of chylomicron remnants by the liver. - It is known to **activate lecithin-cholesterol acyltransferase (LCAT)** and may inhibit **cholesterol ester transfer protein (CETP)**. *Apo A1* - **Apo A1** is the most abundant apolipoprotein in **HDL** and is essential for its structure and function. - It is a potent **activator of LCAT**, an enzyme that esterifies cholesterol in HDL, which is key for **reverse cholesterol transport**.

Q: Pregnenolone is not in the biosynthetic pathway of which substance?

1,25(OH)2D. ***1,25(OH)2D*** - **1,25-dihydroxyvitamin D (Calcitriol)** is the active form of **vitamin D**, synthesized from **cholesterol** in a pathway involving the skin, liver, and kidneys, not directly from pregnenolone. - Its synthesis begins with 7-dehydrocholesterol in the skin, which is converted to **cholecalciferol** by UV light, then hydroxylated in the liver and kidneys. *Cortisol* - **Pregnenolone** is a direct precursor in the synthesis of **cortisol**, as it is converted to progesterone, which then proceeds through various hydroxylations to form cortisol. - The adrenal cortex enzymes convert pregnenolone through a series of steps to produce **glucocorticoids** like cortisol. *DHEA* - **Dehydroepiandrosterone (DHEA)** is synthesized directly from **pregnenolone** by the enzyme 17α-hydroxylase/17,20-lyase (CYP17A1). - Pregnenolone is the initial steroid intermediate that can be shunted towards androgen synthesis, bypassing progesterone. *Estrogen* - **Estrogen** synthesis involves **pregnenolone** as a precursor, which is converted to **DHEA** and then to androstenedione, which is aromatized to estrone and subsequently estradiol. - The pathway from pregnenolone involves multiple steps including the formation of **androgens** as intermediates, which are then converted to estrogens.

Q: What is the role of insulin in lipid metabolism?

Activates lipoprotein lipase. ***Activates lipoprotein lipase*** - Insulin stimulates **lipoprotein lipase (LPL)** activity, leading to the uptake of **triglycerides** from chylomicrons and VLDL into adipocytes for storage. - This action promotes **lipid storage** in adipose tissue after a meal. *Increase lipolysis* - Insulin actually **inhibits lipolysis**, preventing the breakdown of stored triglycerides into free fatty acids. - The primary effect of insulin on lipid metabolism is to promote **lipid storage**, not breakdown. *Activate hormone sensitive lipase* - Insulin **inhibits hormone-sensitive lipase (HSL)**, which is responsible for breaking down stored triglycerides. - Inhibition of HSL is a key mechanism by which insulin prevents the release of **free fatty acids** from adipose tissue. *All of the options* - This option is incorrect because insulin **inhibits lipolysis** and **inactivates hormone-sensitive lipase**, directly contradicting two of the other statements. - Insulin's primary role in lipid metabolism is to **promote storage** and inhibit the breakdown and release of fats.

Q: All are required for conversion of progesterone to estrogen except

11-hydroxylase. ***11-hydroxylase*** - **11-hydroxylase** is involved in the synthesis of **cortisol** and **aldosterone** in the adrenal glands, converting 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone. - It is **not involved** in the metabolic pathway for converting progesterone to estrogen. - This enzyme functions in the **glucocorticoid and mineralocorticoid pathways**, which are distinct from the sex steroid pathway. *Isomerase* - **3β-hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase** is required for converting **pregnenolone to progesterone**, which occurs upstream before progesterone is formed. - Once progesterone is already synthesized, this isomerase is **not required** for the subsequent conversion of progesterone to estrogen. - The pathway from progesterone to estrogen proceeds via hydroxylation, lyase cleavage, and aromatization—not isomerization. *Reductase* - **17β-hydroxysteroid dehydrogenase** (which has reductase activity) is involved in the conversion of **androstenedione to testosterone** in the androgen synthesis pathway. - Since androgens serve as immediate precursors for estrogen synthesis, this reductase activity is **involved** in the overall pathway from progesterone to estrogen. - However, **5α-reductase** (testosterone → DHT) is not part of the progesterone-to-estrogen pathway. *Lyase* - **17,20-lyase** (part of the CYP17A1 enzyme complex) is **critical** for converting **17-hydroxyprogesterone to androstenedione** by cleaving the C17-C20 bond. - These **androgens** (androstenedione and testosterone) then serve as direct precursors for **estrogen synthesis** via the **aromatase enzyme** (CYP19A1). - Lyase is absolutely required in the progesterone-to-estrogen pathway.

Question 1

Liver produces ketones but cannot use it due to the deficiency of which of the following enzyme?

Accepted Answer

Thiophorase

Answer

Alkaline phosphatase

Answer

Alanine transaminase

Answer

Thiolase

Question 2

BARTH syndrome is caused by deficiency of?

Accepted Answer

Cardiolipin

Answer

Glycolipids

Answer

Sphingomyelin

Answer

Cerebroside

Question 3

Energy reserve of the body is:

Accepted Answer

Triglyceride

Answer

Fibres

Answer

Carbohydrate

Answer

Proteins

Question 4

Which of the following is cardio protective?

Accepted Answer

HDL

Answer

Chylomicron

Answer

VLDL

Answer

LDL

Question 5

Essential fatty acid:

Accepted Answer

Linoleic acid

Answer

Citric acid

Answer

Palmitic acid

Answer

Stearic acid

Question 6

Colipase is found in?

Accepted Answer

Pancreatic juice

Answer

Saliva

Answer

Bile

Answer

Succus entericus

Question 7

Apoprotein for chylomicron remnants:

Accepted Answer

Apo E

Answer

Apo A2

Answer

Apo C1

Answer

Apo A1

Question 8

Pregnenolone is not in the biosynthetic pathway of which substance?

Accepted Answer

1,25(OH)2D

Answer

Cortisol

Answer

DHEA

Answer

Estrogen

Question 9

What is the role of insulin in lipid metabolism?

Accepted Answer

Activates lipoprotein lipase

Answer

Increases lipolysis

Answer

Activates hormone sensitive lipase

Answer

All of the options

Question 10

All are required for conversion of progesterone to estrogen except

Accepted Answer

11-hydroxylase

Answer

Isomerase

Answer

Reductase

Answer

Lyase

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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