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: 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: 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: 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**.

Q: Which of the following enzymes is not part of the fatty acid synthase complex?

Acetyl-CoA carboxylase. ***Acetyl-CoA carboxylase*** - **Acetyl-CoA carboxylase (ACC)** is a crucial enzyme in fatty acid synthesis, catalyzing the committed and rate-limiting step of converting **acetyl-CoA to malonyl-CoA**. - While essential for providing the substrates for fatty acid synthase, ACC is a **separate, distinct enzyme** and not structurally part of the fatty acid synthase complex itself. *Ketoacyl reductase* - **Ketoacyl reductase** is an integral enzymatic domain of the fatty acid synthase complex. - It catalyzes the **first reduction step** in the fatty acid synthesis cycle, converting a $\beta$-ketoacyl group to a $\beta$-hydroxyacyl group using NADPH. *Enoyl reductase* - **Enoyl reductase** is an intrinsic enzymatic domain of the fatty acid synthase complex. - It catalyzes the **second reduction step**, converting a trans- $\alpha$, $\beta$-enoyl group to a saturated acyl group using NADPH. *Ketoacyl synthase* - **Ketoacyl synthase (or $\beta$-ketoacyl-ACP synthase)** is a core enzymatic domain within the fatty acid synthase complex. - It catalyzes the **condensation reaction** between the growing acyl chain and malonyl-ACP, forming a $\beta$-ketoacyl-ACP.

Q: Which of the following is not an action of HDL?

Transfer of malate into mitochondria. ***Reverse transport of cholesterol to the liver*** - This statement is actually a primary and well-known function of **HDL**, also known as **reverse cholesterol transport**. - HDL collects excess cholesterol from peripheral tissues and transports it back to the liver for excretion or recycling. *Storage of apolipoproteins* - **HDL** functions as a **circulating reservoir** for apolipoproteins such as **apoC-II** and **apoE**, which it can donate to other lipoproteins. - This storage and transfer of apolipoproteins are crucial for the metabolism of other lipoproteins like chylomicrons and VLDL. *Transfer of malate into mitochondria* - The transfer of **malate into mitochondria** is a function of the **malate shuttle system**, a metabolic pathway involved in gluconeogenesis and energy production. - This process is not directly related to the functions of High-Density Lipoprotein (HDL) in lipid metabolism. *Esterification of cholesterol* - **HDL** contains and activates the enzyme **lecithin-cholesterol acyltransferase (LCAT)**, which esterifies cholesterol on its surface. - This process is essential for trapping cholesterol within the HDL particle, facilitating its transport and preventing its release back into peripheral tissues.

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

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

Accepted Answer

Atherosclerosis

Answer

Nephritis

Answer

Diabetes Mellitus

Answer

Oedema

Question 5

End point of fatty acid synthesis is the formation of

Accepted Answer

Palmitic acid

Answer

Stearic acid

Answer

Oleic acid

Answer

Linoleic acid

Question 6

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 7

In beta-oxidation of fatty acids, carnitine is required for:

Accepted Answer

Transport of long chain fatty acid to mitochondrial inner layer

Answer

Conversion of long chain fatty acids to short chain fatty acids

Answer

Transport of long chain fatty acid across the outer mitochondrial membrane

Answer

Conversion of short chain fatty acids to medium chain fatty acids

Question 8

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

Question 9

Which of the following enzymes is not part of the fatty acid synthase complex?

Accepted Answer

Acetyl-CoA carboxylase

Answer

Enoyl reductase

Answer

Ketoacyl synthase

Answer

Ketoacyl reductase

Question 10

Which of the following is not an action of HDL?

Accepted Answer

Transfer of malate into mitochondria

Answer

Esterification of cholesterol

Answer

Storage of apolipoproteins

Answer

Reverse transport of cholesterol to the liver

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?