Lipid Metabolism Practice Questions

Q: What is the primary mechanism of action of 5-α reductase?

Reduction of C4-C5 double bond. ***Reduction of C4-C5 double bond*** - 5-α reductase is a **NADPH-dependent reductase enzyme** that catalyzes the **reduction (saturation) of the C4-C5 double bond** in the A-ring of testosterone to form **dihydrotestosterone (DHT)**. - This reduction involves **adding two hydrogen atoms** across the double bond, converting it to a single bond with **5-α stereochemistry**. - DHT is a more potent androgen crucial for **prostate development, external genitalia formation, and male pattern baldness**, making 5-α reductase inhibitors (like finasteride) clinically important for treating benign prostatic hyperplasia and androgenetic alopecia. *Breakage of amide bond* - Breaking **amide bonds (C-N bonds with a carbonyl)** is the function of **proteases and amidases**, not reductases. - This process involves **hydrolysis** and is fundamental to protein degradation and peptide metabolism. *Breakage of C-N bond* - **Carbon-nitrogen bond cleavage** occurs in reactions like **deamination** (catalyzed by deaminases) or metabolism of nitrogenous compounds. - Reductases perform **electron transfer reactions**, not bond cleavage reactions. *Breakage of N-N bond* - **Nitrogen-nitrogen bond** cleavage is rare in human biochemistry and may occur in hydrazine metabolism or by specialized enzymes. - Steroid hormones do not contain N-N bonds, making this mechanism irrelevant to 5-α reductase function.

Q: Progesterone has how many carbons?

21. ***21*** - Progesterone is a **C21 steroid hormone**, meaning it has 21 carbon atoms in its complete structure. - The steroid nucleus contains **17 carbons** arranged in four fused rings (A, B, C, D), plus **two methyl groups** (C-18 and C-19) and a **2-carbon side chain** at position 17 (an acetyl group: -COCH₃). - This C21 structure classifies it as a **progestogen** along with other pregnancy-related hormones. *20* - No major physiologically significant steroid hormones contain exactly 20 carbons. - This count does not correspond to progesterone's actual molecular structure. *19* - **Androgens** such as testosterone and androstenedione are **C19 steroid hormones**, characterized by 19 carbon atoms. - These lack the 2-carbon side chain at C-17 that is present in progesterone, having only a keto or hydroxyl group at that position. *18* - **Estrogens** (e.g., estradiol, estrone) are **C18 steroid hormones**, characterized by an aromatic A-ring and 18 carbon atoms. - They lack both the C-19 methyl group and the C-17 side chain, resulting in fewer carbons than progesterone.

Q: Bile salts undergo conjugation for enhanced solubility:

After conjugation with taurine and glycine. ***After conjugation with taurine and glycine*** - This statement accurately describes the most common conjugation pathway for bile acids, increasing their **amphipathic properties** and solubility. - Conjugation with these amino acids forms **bile salts** (e.g., glycocholate, taurocholate), which are essential for **micelle formation** and fat digestion. - This is the primary mechanism by which bile acids become bile salts with enhanced solubility. *After conjugation with betaglucuronic acid* - While bile acids do undergo conjugation for increased solubility, they are primarily conjugated with glycine or taurine, not beta-glucuronic acid. - Conjugation with beta-glucuronic acid is a common detoxification pathway for many xenobiotics and bilirubin, but not the primary method for bile acids. *After conjugation with derived proteins* - Bile salts are primarily steroid derivatives and are not conjugated with derived proteins. - The purpose of conjugation is to increase hydrophilicity, which proteins would not achieve in this context. *After conjugation with lysine* - Lysine is an amino acid but is not involved in the conjugation of bile acids. - Bile acid conjugation specifically uses the amino acids glycine and taurine.

Q: One of the following has the least density:

Chylomicron. **Chylomicron** - Chylomicrons are the **largest** and **least dense** of all lipoproteins due to their very high triglyceride content. - Their primary function is to transport **dietary lipids** from the intestines to other parts of the body. *HDL* - **High-density lipoprotein (HDL)** is the **most dense** lipoprotein due to its high protein-to-lipid ratio. - It is responsible for **reverse cholesterol transport**, removing excess cholesterol from tissues. *VLDL* - **Very low-density lipoprotein (VLDL)** is less dense than LDL and HDL, but **denser than chylomicrons**. - Its main role is to transport **endogenously synthesized triglycerides** from the liver to peripheral tissues. *LDL* - **Low-density lipoprotein (LDL)** is denser than VLDL and chylomicrons but less dense than HDL. - It primarily transports **cholesterol** from the liver to peripheral cells, and elevated levels are associated with increased cardiovascular risk.

Q: Active metabolite in the synthesis of fatty acids is:

Malonyl CoA. ***Malonyl CoA*** - **Malonyl CoA** is the immediate **two-carbon donor** in fatty acid synthesis, formed from acetyl CoA and bicarbonate. - It adds **two-carbon units** to the growing fatty acid chain during each cycle of synthesis, making it the primary active metabolic form in this process. *Stearate (an end product of fatty acid synthesis)* - **Stearate** is a **saturated fatty acid end product** (C18:0) of fatty acid synthesis, not an active metabolite that directly participates in the elongation process. - While it is a result of fatty acid synthesis, it does not serve as a building block for further elongation in the manner of malonyl CoA. *Acetyl CoA (a precursor in fatty acid synthesis)* - **Acetyl CoA** is the **initial precursor** for fatty acid synthesis, which is then carboxylated to form malonyl CoA. - It is not the *active* two-carbon donor during the elongation steps of fatty acid synthesis itself, but rather the substrate for malonyl CoA synthesis. *Palmitate (an end product of fatty acid synthesis)* - **Palmitate** is the **primary 16-carbon saturated fatty acid** and is the usual end product of *de novo* fatty acid synthesis in humans. - Like stearate, it is an end product and does not serve as an active metabolic intermediate for chain elongation during the synthesis process itself.

Q: Which apolipoprotein is most strongly associated with increased risk of Alzheimer's disease?

APOE4. APOE4 - The APOE4 allele is the strongest genetic risk factor for late-onset Alzheimer's disease, increasing risk and lowering the age of onset in a gene-dose dependent manner. - Individuals with one copy of APOE4 have a 2-3 times increased risk, and those with two copies have an 8-12 times increased risk compared to those with APOE3. APOE3 - APOE3 is the most common allele and is considered a neutral risk factor for Alzheimer's disease. - It plays a normal role in lipid metabolism and brain function without significantly increasing or decreasing AD risk. APOC3 - APOC3 is involved in triglyceride metabolism and is primarily linked to cardiovascular disease risk, not Alzheimer's disease. - While it modulates lipid levels, it does not have a direct, strong genetic association with AD pathogenesis. APOE2 - APOE2 is associated with a reduced risk of Alzheimer's disease, offering a protective effect against its development. - It is thought to improve the clearance of amyloid-beta (A̠) plaques, a hallmark of AD pathology.

Q: In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Lecithin. ***Lecithin*** - **Lecithin** (also known as **phosphatidylcholine**) is the primary component of **surfactant** in the lungs, which reduces surface tension and prevents alveolar collapse. - In **preterm babies**, insufficient production of lecithin due to immature lung development leads to **respiratory distress syndrome (RDS)**. *Cardiolipin* - **Cardiolipin** is a major phospholipid found in the **inner mitochondrial membrane**, crucial for oxidative phosphorylation. - Deficiency is associated with mitochondrial disorders like **Barth syndrome**, not primary respiratory distress. *Sphingomyelin* - **Sphingomyelin** is a significant component of **cell membranes** and **myelin sheaths**, important for nerve insulation. - While present in the lungs, its primary role is not in surface tension reduction, and its deficiency is not directly linked to RDS. *Phosphatidylinositol* - **Phosphatidylinositol** is a precursor for various **signaling molecules** and plays a role in cell membrane structure. - While involved in cellular processes, it is not the critical surfactant component whose deficiency causes RDS.

Q: Type-I hyperlipoproteinemia is caused by deficiency of:-

Lipoprotein lipase. ***Lipoprotein lipase*** - **Type I hyperlipoproteinemia**, also known as **familial lipoprotein lipase deficiency**, is caused by a genetic defect leading to **deficiency or defect in lipoprotein lipase (LPL)** or its cofactor **apolipoprotein C-II**. - LPL is crucial for the **hydrolysis of triglycerides** in chylomicrons and VLDL at the capillary endothelium. - This enzymatic deficiency leads to **massive accumulation of chylomicrons** and severe hypertriglyceridemia (often >1000 mg/dL). - Clinical features include **eruptive xanthomas, lipemia retinalis, hepatosplenomegaly**, and **recurrent pancreatitis**. *Elevated triglycerides in plasma* - This is indeed the **most prominent laboratory finding** in Type I hyperlipoproteinemia, with triglyceride levels often exceeding 1000-2000 mg/dL. - However, this is the **consequence/manifestation** of the LPL deficiency, not the underlying cause. - The question asks what causes Type I hyperlipoproteinemia, which is the enzyme deficiency itself. *Elevated LDL* - Type I hyperlipoproteinemia typically has **normal or even reduced LDL levels**. - **Elevated LDL** is characteristic of **Type IIa hyperlipoproteinemia (familial hypercholesterolemia)**, which involves defects in LDL receptor or ApoB-100. - Type I primarily affects **chylomicron metabolism**, not LDL. *Elevated cholesterol* - Cholesterol levels are typically **normal or only mildly elevated** in Type I hyperlipoproteinemia. - The triglyceride elevation is disproportionately massive compared to any cholesterol elevation. - Significant isolated cholesterol elevation points to Type IIa or IIb dyslipidemias.

Q: Retinitis pigmentosa is associated with deficiency of:

DHA. ***DHA*** - **Docosahexaenoic acid (DHA)** is the major polyunsaturated fatty acid in the **retinal photoreceptor outer segments** and is crucial for their function. - Deficiency in DHA has been linked to several retinal degeneration disorders, including **retinitis pigmentosa**, suggesting its importance in maintaining retinal health. *Timnodonic acid* - This is an older term for **eicosapentaenoic acid (EPA)**, which is an omega-3 fatty acid. - While EPA is beneficial for overall health, it is **not the primary fatty acid** associated with the direct structural and functional health of retinal photoreceptors as DHA is. *Eicosa pentaenoic acid* - **Eicosapentaenoic acid (EPA)** is an omega-3 fatty acid found in fish oil, known for its anti-inflammatory properties. - While important for general health, EPA is **not as abundant or critical for retinal structure and function** as DHA. *Arachidonic acid* - **Arachidonic acid (AA)** is an omega-6 fatty acid found in cell membranes and is a precursor to pro-inflammatory mediators. - While present in the retina, AA is generally **not associated with a protective or causative role in retinitis pigmentosa** in the same way DHA is.

Q: In case of LPL deficiency, which of the following will increase after a fat rich diet?

Chylomicron. ***Chylomicron*** - **LPL (lipoprotein lipase)** is crucial for the breakdown of **chylomicrons** and VLDL. A deficiency leads to an accumulation of undigested chylomicrons in the bloodstream after a fat-rich meal. - **Chylomicrons** transport dietary triglycerides from the intestines to tissues. Without LPL, these triglycerides remain packaged in chylomicrons. *LDL* - **LDL (low-density lipoprotein)** levels are not directly increased by a short-term fat-rich diet in the context of LPL deficiency. LDL primarily carries cholesterol and is formed from VLDL remnants, a process that is also impaired by LPL deficiency indirectly. - While chronic LPL deficiency can affect overall lipid metabolism, the immediate post-meal increase is not in LDL but in triglyceride-rich lipoproteins. *HDL* - **HDL (high-density lipoprotein)** is involved in reverse cholesterol transport and is generally not directly increased after a fat-rich diet, especially in LPL deficiency. - In fact, severe hypertriglyceridemia, often seen in LPL deficiency, can sometimes lead to lower HDL levels due to altered lipid exchange. *Lipoprotein (a)* - **Lipoprotein (a)**, or Lp(a), is a genetically determined lipoprotein similar to LDL but with an added apolipoprotein (a) and its levels are not acutely affected by dietary fat intake or LPL deficiency. - Lp(a) levels are determined primarily by genetic factors and do not participate in the post-prandial handling of dietary fats.

Question 1

What is the primary mechanism of action of 5-α reductase?

Accepted Answer

Reduction of C4-C5 double bond

Answer

Breakage of amide bond

Answer

Breakage of C-N bond

Answer

Breakage of N-N bond

Question 2

Progesterone has how many carbons?

Accepted Answer

21

Answer

20

Answer

19

Answer

18

Question 3

Bile salts undergo conjugation for enhanced solubility:

Accepted Answer

After conjugation with taurine and glycine

Answer

After conjugation with derived proteins

Answer

After conjugation with lysine

Answer

After conjugation with betaglucuronic acid

Question 4

One of the following has the least density:

Accepted Answer

Chylomicron

Answer

HDL

Answer

VLDL

Answer

LDL

Question 5

Active metabolite in the synthesis of fatty acids is:

Accepted Answer

Malonyl CoA

Answer

Stearate

Answer

Acetyl CoA

Answer

Palmitate

Question 6

Which apolipoprotein is most strongly associated with increased risk of Alzheimer's disease?

Accepted Answer

APOE4

Answer

APOE3

Answer

APOC3

Answer

APOE2

Question 7

In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Accepted Answer

Lecithin

Answer

Cardiolipin

Answer

Sphingomyelin

Answer

Phosphatidylinositol

Question 8

Type-I hyperlipoproteinemia is caused by deficiency of:-

Accepted Answer

Lipoprotein lipase

Answer

Elevated triglycerides in plasma

Answer

Elevated LDL

Answer

Elevated cholesterol

Question 9

Retinitis pigmentosa is associated with deficiency of:

Accepted Answer

DHA

Answer

Timnodonic acid

Answer

Eicosa pentaenoic acid

Answer

Arachidonic acid

Question 10

In case of LPL deficiency, which of the following will increase after a fat rich diet?

Accepted Answer

Chylomicron

Answer

LDL

Answer

HDL

Answer

Lipoprotein (a)

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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