Lipid Metabolism Practice Questions

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.

Q: Which among the following is a primary bile acid?

Chenodeoxycholic acid. ***Chenodeoxycholic acid*** - **Chenodeoxycholic acid** is one of the two primary bile acids synthesized in the **liver** from **cholesterol**. - The other primary bile acid is **cholic acid**. *Deoxycholic acid* - **Deoxycholic acid** is a **secondary bile acid**, formed from **cholic acid** by bacterial action in the gut. - It is not directly synthesized in the liver. *Lithocholic acid* - **Lithocholic acid** is also a **secondary bile acid**, derived from **chenodeoxycholic acid** through bacterial dehydroxylation in the intestine. - Due to its low solubility, it is considered **toxic** and is efficiently excreted. *None of the options* - This option is incorrect because **chenodeoxycholic acid** is indeed a primary bile acid. - The other common primary bile acid, **cholic acid**, was not listed but is also synthesized directly in the liver.

Q: A child with intellectual disability presenting with hoarseness, frequent dermatitis, and skeletal deformities was diagnosed to have Farber's disease. Which enzyme is deficient in this child?

Ceramidase. ***Correct Answer: Ceramidase*** - Farber's disease, also known as **Farber lipogranulomatosis** or **acid ceramidase deficiency**, is caused by a deficiency in the enzyme **acid ceramidase** - This enzyme is crucial for the breakdown of **ceramide**, leading to its accumulation in various tissues - Accumulation results in the characteristic symptoms: **hoarseness** (from laryngeal involvement), **dermatitis** (subcutaneous nodules), and **skeletal deformities** (joint contractures) - This is a rare autosomal recessive lysosomal storage disorder *Incorrect: Arylsulfatase A* - Deficiency causes **metachromatic leukodystrophy**, a lysosomal storage disorder affecting the nervous system - Presents with progressive neurological deterioration, peripheral neuropathy, and developmental regression - Does not present with the specific constellation of **hoarseness**, **dermatitis**, and **skeletal deformities** seen in Farber's disease *Incorrect: Beta-Glucosidase* - Deficiency in **beta-glucosidase** (glucocerebrosidase) causes **Gaucher disease** - Characterized by **hepatosplenomegaly**, **anemia**, **thrombocytopenia**, and **bone complications** (bone pain, fractures) - The clinical picture differs from Farber's disease presentation *Incorrect: Sphingomyelinase* - Deficiency causes **Niemann-Pick disease** (Types A and B) - Characterized by **sphingomyelin accumulation**, **hepatosplenomegaly**, and **neurodegenerative symptoms** (Type A) - Symptoms differ from the **hoarseness**, **dermatitis**, and **skeletal deformities** characteristic of Farber's disease

Q: Essential fatty acids are helpful in controlling which of the following?

Atherosclerosis. ***Atherosclerosis*** - **Essential fatty acids**, particularly omega-3 fatty acids, help reduce **triglyceride levels** and have **anti-inflammatory** properties, both of which are beneficial in preventing and controlling atherosclerosis. - They also contribute to improving **endothelial function** and reducing **platelet aggregation**, decreasing the risk of plaque formation. *Nephritis* - While some **omega-3 fatty acids** might have general anti-inflammatory effects, there is no strong evidence to suggest they are a primary treatment or control measure for **nephritis** as a specific condition. - **Nephritis** involves inflammation of the kidneys, and its management typically focuses on addressing underlying causes like autoimmune diseases or infections. *Diabetes Mellitus* - **Essential fatty acids** do not directly control **blood glucose levels**, which is the primary challenge in **diabetes mellitus**. - Their role in diabetes management is indirect, mainly through improving **lipid profiles** and reducing cardiovascular risk, but not controlling the core metabolic dysfunction. *Oedema* - **Oedema** (swelling) is primarily related to fluid retention and imbalances in fluid regulation, often due to issues with the heart, kidneys, or lymphatic system. - **Essential fatty acids** do not have a direct mechanism to control **fluid retention** or resolve **oedema**.

Q: End point of fatty acid synthesis is the formation of

Palmitic acid. ***Palmitic acid*** - **Palmitic acid** (palmitate) is the primary 16-carbon saturated fatty acid that is synthesized **de novo** in fatty acid synthesis. - All other fatty acids are typically derived from palmitic acid through elongation and desaturation reactions. *Stearic acid* - **Stearic acid** is an 18-carbon saturated fatty acid, which is formed by the **elongation** of palmitic acid, not as the initial end-product of de novo synthesis. - This elongation primarily occurs in the **endoplasmic reticulum**. *Oleic acid* - **Oleic acid** is an 18-carbon **monounsaturated fatty acid** formed by the desaturation of stearic acid, making it a derivative rather than the initial end-product of de novo synthesis. - The double bond is introduced by an enzyme called **stearoyl-CoA desaturase**. *Linoleic acid* - **Linoleic acid** is an 18-carbon **polyunsaturated fatty acid** that is an **essential fatty acid**, meaning it cannot be synthesized by the human body and must be obtained from the diet. - It is not produced through de novo fatty acid synthesis in humans.

Q: Enzyme deficiency in Farber disease is:

Ceramidase. ***Ceramidase*** - Farber disease, also known as **Farber lipogranulomatosis** or **Farber's disease**, is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **acid ceramidase**. - This deficiency leads to the accumulation of **ceramide** in various tissues, causing granulomatous lesions, joint deformities, and neurological symptoms. *Arylsulfatase A* - Deficiency of **arylsulfatase A** is associated with **metachromatic leukodystrophy**, a lysosomal storage disorder affecting the nervous system. - This enzyme is crucial for the breakdown of **sulfatides**, not ceramide. *Sphingomyelinase* - A deficiency in **sphingomyelinase** is the hallmark of **Niemann-Pick disease**, specifically types A and B. - This enzyme is responsible for breaking down **sphingomyelin**, leading to its accumulation in organs like the liver, spleen, and brain. *Hexosaminidase A* - A deficiency in **hexosaminidase A** is the cause of **Tay-Sachs disease**, a severe neurodegenerative disorder. - This enzyme is essential for the metabolism of **GM2 ganglioside**, which accumulates in neurons in affected individuals.

Q: What is the major metabolic pathway for saturated fatty acids in the mitochondria?

beta-oxidation. ***beta-oxidation*** - **Beta-oxidation** is the primary metabolic pathway for **saturated fatty acids** in the **mitochondria**, progressively breaking them down into **acetyl-CoA** units. - Each cycle of beta-oxidation shortens the fatty acid chain by two carbons, producing **FADH2** and **NADH** for ATP synthesis in the electron transport chain. *a-oxidation* - **Alpha-oxidation** is a minor pathway primarily used for degrading **branched-chain fatty acids**, such as **phytanic acid**, in peroxisomes. - It removes one carbon at a time from the carboxyl end of the fatty acid, rather than two carbons like beta-oxidation. *ω-oxidation* - **Omega-oxidation** is a minor pathway of fatty acid metabolism that occurs in the **endoplasmic reticulum** and primarily targets **medium-chain fatty acids**. - It introduces a hydroxyl group at the omega (ω) carbon, the farthest carbon from the carboxyl group, which is then oxidized to a carboxyl group. *None of the above* - This option is incorrect because **beta-oxidation** is indeed the major metabolic pathway for saturated fatty acids in the mitochondria.

Q: An 8-month-old baby girl had normal growth and development for the first few months, but then progressively deteriorated with deafness, blindness, atrophied muscles, inability to swallow, and seizures. Early on in the diagnosis of the child, it was noticed that a cherry-red macula was present in both eyes. Considering the child in the above case, measurement of which one of the following would enable one to determine whether the mutation was in the hexA or hexB gene?

Globoside. ***Globoside*** - A mutation in the **hexA** gene leads to **Tay-Sachs disease** (deficient **HexA**), characterized by the accumulation of **GM2 ganglioside only**. - A mutation in the **hexB** gene leads to **Sandhoff disease** (deficient **HexB**), which results in the accumulation of **both GM2 ganglioside and globoside**, as **HexB** is crucial for the breakdown of **globosides**. - **Measuring globoside levels would differentiate between these two conditions** - elevated in Sandhoff, normal in Tay-Sachs. *GM1* - **GM1 ganglioside** accumulates in **GM1 gangliosidosis**, caused by a deficiency in **beta-galactosidase**, not hexosaminidase A or B. - Clinical features of **GM1 gangliosidosis** overlap but differ from Tay-Sachs or Sandhoff, and it's not directly affected by **hexA** or **hexB** mutations. *GM2* - **GM2 ganglioside** accumulates in **both Tay-Sachs disease** (due to **HexA** deficiency) and **Sandhoff disease** (due to **HexB** deficiency). - Measuring **GM2** alone would confirm a **hexosaminidase deficiency** but **would not differentiate** between a **hexA** (Tay-Sachs) or **hexB** (Sandhoff) mutation, as it is elevated in both conditions. *Glucocerebroside* - **Glucocerebroside** accumulates in **Gaucher disease**, which is caused by a deficiency in the enzyme **glucocerebrosidase**. - This condition has a distinct clinical presentation and is not related to mutations in the **hexA** or **hexB** genes.

Q: Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?

Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase. **Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase** - **Enoyl CoA isomerase** is necessary to convert *cis* double bonds to *trans* double bonds at the 3,4 position, which allows the beta-oxidation enzymes to continue processing the fatty acid. - **2,4 Dienoyl CoA reductase** is required to reduce *cis-2, cis-4* or *trans-2, cis-4* dienoyl intermediates into a single *trans-3* enoyl CoA, which can then be isomerized by enoyl CoA isomerase. *Dienoyl CoA isomerase and Enoyl CoA isomerase* - This option is incorrect because **Dienoyl CoA isomerase** is not a commonly recognized single enzyme directly involved in PUFA beta-oxidation in the way described. The key is to reduce a diene, which reductase does. - While **Enoyl CoA isomerase** is crucial, pairing it with another isomerase in this context does not fully address the reduction step needed for certain PUFAs. *Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase* - This option incorrectly names **Dienoyl CoA isomerase** as one of the two main required enzymes. A 2,4 Dienoyl CoA reductase does exist. - While **2,4 Dienoyl CoA reductase** is essential, the other enzyme should be Enoyl CoA isomerase to handle the initial *cis* to *trans* isomerizations. *Enoyl CoA isomerase and Enoyl CoA reductase* - This option is incorrect because **Enoyl CoA reductase** without the "2,4" prefix generally refers to the enzyme involved in fatty acid synthesis, not beta-oxidation of PUFAs. - **Enoyl CoA isomerase** is correctly identified, but the other enzyme specifically for PUFA oxidation is the **2,4 Dienoyl CoA reductase**.

Question 1

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 2

All are polyunsaturated fatty acids except:

Accepted Answer

Palmitic acid

Answer

Linoleic acid

Answer

Linolenic acid

Answer

Arachidonic acid

Question 3

Which among the following is a primary bile acid?

Accepted Answer

Chenodeoxycholic acid

Answer

Deoxycholic acid

Answer

Lithocholic acid

Answer

None of the options

Question 4

A child with intellectual disability presenting with hoarseness, frequent dermatitis, and skeletal deformities was diagnosed to have Farber's disease. Which enzyme is deficient in this child?

Accepted Answer

Ceramidase

Answer

Arylsulfatase A

Answer

Beta-Glucosidase

Answer

Sphingomyelinase

Question 5

Essential fatty acids are helpful in controlling which of the following?

Accepted Answer

Atherosclerosis

Answer

Nephritis

Answer

Diabetes Mellitus

Answer

Oedema

Question 6

End point of fatty acid synthesis is the formation of

Accepted Answer

Palmitic acid

Answer

Stearic acid

Answer

Oleic acid

Answer

Linoleic acid

Question 7

Enzyme deficiency in Farber disease is:

Accepted Answer

Ceramidase

Answer

Arylsulfatase A

Answer

Sphingomyelinase

Answer

Hexosaminidase A

Question 8

What is the major metabolic pathway for saturated fatty acids in the mitochondria?

Accepted Answer

beta-oxidation

Answer

α-oxidation

Answer

ω-oxidation

Answer

None of the above

Question 9

An 8-month-old baby girl had normal growth and development for the first few months, but then progressively deteriorated with deafness, blindness, atrophied muscles, inability to swallow, and seizures. Early on in the diagnosis of the child, it was noticed that a cherry-red macula was present in both eyes. Considering the child in the above case, measurement of which one of the following would enable one to determine whether the mutation was in the hexA or hexB gene?

Accepted Answer

Globoside

Answer

GM1

Answer

GM2

Answer

Glucocerebroside

Question 10

Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?

Accepted Answer

Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase

Answer

Dienoyl CoA isomerase and Enoyl CoA isomerase

Answer

Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase

Answer

Enoyl CoA isomerase and Enoyl CoA reductase

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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