Lipid Metabolism Practice Questions

Q: Which of the following helps in the transport of fatty acids across the inner mitochondrial membrane?

Carnitine. ***Carnitine*** - **Carnitine** plays a crucial role in transporting long-chain fatty acids from the **cytosol** into the **mitochondrial matrix** for beta-oxidation. - It acts as a shuttling molecule, forming **acylcarnitine** which can cross the inner mitochondrial membrane via the **carnitine-acylcarnitine translocase**. *Acyl carrier protein* - **Acyl carrier protein (ACP)** is primarily involved in **fatty acid synthesis** in the cytoplasm, not in the transport of fatty acids into mitochondria for degradation. - It carries acyl groups during the elongation reactions of fatty acid synthesis. *Lecithin-cholesterol acyltransferase* - **Lecithin-cholesterol acyltransferase (LCAT)** is an enzyme found in plasma that catalyzes the formation of **cholesterol esters**, which are then transported by lipoproteins. - It is involved in **cholesterol metabolism** and reverse cholesterol transport, not in the mitochondrial transport of fatty acids. *Carnitine and albumin* - While **carnitine** is essential for mitochondrial fatty acid transport, **albumin** transports fatty acids in the blood plasma, from adipose tissue to other tissues. - Albumin does not transport fatty acids across the inner mitochondrial membrane; its role is extra-mitochondrial and related to systemic transport.

Q: In Retinitis pigmentosa, which fatty acid is found to be decreased in the retina?

Docosahexaenoic acid. ***Docosahexaenoic acid*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in the **photoreceptor outer segments** of the retina. - Its reduced levels are linked to photoreceptor dysfunction and degeneration seen in conditions like **Retinitis Pigmentosa**, influencing membrane fluidity and visual function. - DHA comprises up to **50% of the fatty acids** in photoreceptor membranes and is essential for maintaining retinal structure and function. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 fatty acid** primarily involved in **inflammatory pathways** and cellular signaling. - While present in the retina, its decrease is not typically associated with the pathogenesis of **Retinitis Pigmentosa**. *Linoleic acid* - **Linoleic acid** is an **omega-6 essential fatty acid** and a precursor to **arachidonic acid**, but it is not a direct structural component of photoreceptor membranes. - Its levels are not specifically decreased in **Retinitis Pigmentosa** as a primary factor. *Palmitic acid* - **Palmitic acid** is a **saturated fatty acid** present in retinal membranes but not specifically concentrated in photoreceptors. - Its levels are not characteristically decreased in **Retinitis Pigmentosa**, unlike the omega-3 fatty acid DHA.

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

APOE4. ***APOE4*** - The **APOE4 allele** is the strongest genetic risk factor for **late-onset Alzheimer's disease (AD)**, significantly increasing the risk and lowering the age of onset. - Individuals with one copy of APOE4 have a 2-3 times higher risk, while those with two copies have an 8-12 times higher risk of developing AD compared to those with APOE3. *APOE3* - **APOE3** is the most common allele and is considered a **neutral risk factor** for Alzheimer's disease, serving as the reference for risk comparison. - It plays a normal role in **lipid metabolism** and **cholesterol transport** in the brain. *APOE2* - The **APOE2 allele** is associated with a **reduced risk** of Alzheimer's disease. - It is believed to be **protective** against AD, possibly by **improving amyloid-beta clearance** or reducing neuroinflammation. *APOB* - **Apolipoprotein B (APOB)** is primarily involved in the assembly and secretion of **chylomicrons** and **very-low-density lipoproteins (VLDL)** in the liver and intestine. - While important for lipid metabolism, it is **not directly implicated** as a genetic risk factor for Alzheimer's disease in the same way APOE variants are.

Q: In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?

Chylomicrons. ***Chylomicrons*** - **Lipoprotein lipase (LPL)** is essential for the breakdown of **triglycerides** within chylomicrons, which are absorbed after a fatty meal. - In LPL deficiency, **chylomicron clearance** from the bloodstream is impaired, leading to their accumulation and elevated levels. *High-Density Lipoprotein (HDL)* - HDL particles are primarily involved in **reverse cholesterol transport** and are not directly affected by LPL deficiency in terms of their synthesis or initial concentration post-meal. - While LPL activity can influence HDL metabolism, a deficiency would not directly cause an acute increase in HDL levels following a fatty meal. *Very Low-Density Lipoprotein (VLDL)* - VLDL is synthesized in the liver to transport **endogenous triglycerides**, rather than dietary triglycerides. - While prolonged LPL deficiency can indirectly affect VLDL metabolism, it is not the primary lipoprotein that acutely accumulates in response to a fatty meal in this condition. *Low-Density Lipoprotein (LDL)* - LDL is formed from the catabolism of VLDL and **intermediate-density lipoprotein (IDL)**. - In LPL deficiency, the initial step of **triglyceride hydrolysis** for chylomicrons is impaired, making it unlikely for LDL to increase immediately after a fatty meal.

Q: Bile acids are synthesized from ?

Cholesterol. ***Cholesterol*** - **Bile acids** are derivatives of **cholesterol**, synthesized in the liver through a multi-step enzymatic pathway. - The conversion of cholesterol to bile acids is a primary mechanism for the excretion and transport of cholesterol from the body. *Heme* - **Heme** is a component of hemoglobin and myoglobin, primarily involved in oxygen transport and storage. - Its degradation product is **bilirubin**, which forms part of bile but is distinct from bile acids. *Ribulose* - **Ribulose** is a 5-carbon sugar, playing a key role in the **pentose phosphate pathway** and the **Calvin cycle** in photosynthesis. - It is not a precursor for bile acid synthesis. *Arachidonic acid* - **Arachidonic acid** is a polyunsaturated fatty acid that serves as a precursor for **eicosanoids** (prostaglandins, thromboxanes, and leukotrienes). - These molecules are involved in inflammation and immune responses but are unrelated to bile acid synthesis.

Q: Which protein does the domain of plasminogen resemble?

Apolipoprotein (a) (a lipoprotein). ***Apolipoprotein (a) (a lipoprotein)*** - **Plasminogen** and **apolipoprotein (a)** share structural homology, specifically due to the presence of **kringle domains**. - This structural similarity suggests a potential for apolipoprotein (a) to **interfere with plasminogen’s fibrinolytic activity**, contributing to **atherosclerosis**. *Fibrinogen (a clotting protein)* - While plasmin acts on fibrinogen (and its derivative fibrin), its domain structure does not **resemble fibrinogen**. - **Fibrinogen** is a large, multi-domain glycoprotein crucial for **clot formation**, distinct from plasminogen's primarily **kringle-rich structure**. *LDL receptor (a lipid metabolism protein)* - The **LDL receptor** is involved in **cholesterol uptake** by cells and has structural features like ligand-binding repeats and epidermal growth factor (EGF) repeats. - Its domain structure is **not similar to plasminogen**, which is characterized by **kringle domains** and a protease domain. *Prothrombin (a coagulation protein)* - **Prothrombin** is a precursor to thrombin, featuring **gla domains**, kringle-like domains (though structurally distinct from plasminogen's), and a serine protease domain. - While both are involved in coagulation/fibrinolysis, their **overall domain arrangements and specific kringle structures differ** significantly.

Q: What primarily forms the core of chylomicrons?

Triglycerides. ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.

Q: Rate limiting enzyme in bile acid synthesis?

7α-hydroxylase. ***7α-hydroxylase*** - This enzyme, specifically **cholesterol 7α-hydroxylase**, catalyzes the first and rate-limiting step in the classic pathway of **bile acid synthesis**, converting cholesterol to 7α-hydroxycholesterol. - Its activity is tightly regulated, primarily by the availability of cholesterol and feedback inhibition by bile acids, making it a key control point. *Desmolase* - **Cholesterol desmolase** (CYP11A1) is the rate-limiting enzyme in **steroid hormone synthesis** in the adrenal glands, converting cholesterol to pregnenolone. - It is not involved in the committed steps of bile acid synthesis from cholesterol. *21α-hydroxylase* - **21α-hydroxylase** (CYP21A2) is crucial in the synthesis of **cortisol and aldosterone** from progesterone and 17-hydroxyprogesterone, respectively. - Deficiency in this enzyme is the most common cause of **congenital adrenal hyperplasia**, but it has no direct role in bile acid synthesis. *12α-hydroxylase* - **12α-hydroxylase** (CYP8B1) is an enzyme involved in the later steps of bile acid synthesis, specifically in the formation of **cholic acid** from 7α-hydroxy-4-cholesten-3-one. - While essential for synthesizing primary bile acids, it is not the *rate-limiting enzyme* for the overall pathway; 7α-hydroxylase holds that distinction.

Q: Transport of lipids from the intestine to other tissues is by -

Chylomicrons. ***Chylomicrons*** - **Chylomicrons** are the **largest lipoprotein particles** that transport **dietary (exogenous) lipids** from the **intestine** to peripheral tissues - They are synthesized in **intestinal enterocytes** after fat absorption and enter the bloodstream via the **lymphatic system (thoracic duct)** - They carry **triglycerides (85-95%), cholesterol, phospholipids, and fat-soluble vitamins** (A, D, E, K) - **Apolipoprotein B-48** is the characteristic structural protein of chylomicrons - After delivering triglycerides to tissues (via lipoprotein lipase), chylomicron remnants are taken up by the **liver** *LDL (Low-Density Lipoprotein)* - LDL transports **cholesterol from the liver to peripheral tissues** (not from intestine) - It carries **endogenous cholesterol**, not dietary lipids from the intestine - Often called "**bad cholesterol**" due to its role in atherosclerosis - Contains **Apolipoprotein B-100** *HDL (High-Density Lipoprotein)* - HDL performs **reverse cholesterol transport** - moving excess cholesterol from peripheral tissues **back to the liver** - It does **not transport lipids from the intestine** to tissues - Called "**good cholesterol**" for its protective cardiovascular role - Contains **Apolipoprotein A-I and A-II** *VLDL (Very-Low-Density Lipoprotein)* - VLDL is synthesized in the **liver** (not intestine) and transports **endogenous triglycerides** to peripheral tissues - It carries lipids **from the liver**, not from the intestine - VLDL is converted to IDL and then LDL after losing triglycerides - Contains **Apolipoprotein B-100**

Q: Bile acids consist of all of the following except -

Bilirubin. ***Bilirubin*** - **Bilirubin** is a pigment formed from the breakdown of **heme**, not a bile acid. - It is excreted in bile but does not aid in **lipid digestion** or **absorption**. *Lithocholic acid* - **Lithocholic acid** is a **secondary bile acid** formed in the colon by bacterial dehydroxylation of chenodeoxycholic acid. - It is still considered a bile acid, despite its secondary nature. *Deoxycholic acid* - **Deoxycholic acid** is a **secondary bile acid** formed by bacterial action on cholic acid in the colon. - Like other bile acids, it plays a role in **fat digestion** and **absorption**. *Chenodeoxycholic acid* - **Chenodeoxycholic acid** is a **primary bile acid** synthesized in the liver from cholesterol. - It is one of the main bile acids directly involved in **emulsifying dietary fats**.

Question 1

Which of the following helps in the transport of fatty acids across the inner mitochondrial membrane?

Accepted Answer

Carnitine

Answer

Acyl carrier protein

Answer

Lecithin-cholesterol acyltransferase

Answer

Carnitine and albumin

Question 2

In Retinitis pigmentosa, which fatty acid is found to be decreased in the retina?

Accepted Answer

Docosahexaenoic acid

Answer

Arachidonic

Answer

Palmitic acid

Answer

Linoleic acid

Question 3

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

Accepted Answer

APOE4

Answer

APOE3

Answer

APOE2

Answer

APOB

Question 4

In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?

Accepted Answer

Chylomicrons

Answer

High-Density Lipoprotein (HDL)

Answer

Very Low-Density Lipoprotein (VLDL)

Answer

Low-Density Lipoprotein (LDL)

Question 5

Bile acids are synthesized from ?

Accepted Answer

Cholesterol

Answer

Heme

Answer

Ribulose

Answer

Arachidonic acid

Question 6

Which protein does the domain of plasminogen resemble?

Accepted Answer

Apolipoprotein (a) (a lipoprotein)

Answer

Fibrinogen (a clotting protein)

Answer

LDL receptor (a lipid metabolism protein)

Answer

Prothrombin (a coagulation protein)

Question 7

What primarily forms the core of chylomicrons?

Accepted Answer

Triglycerides

Answer

Triglycerides and Cholesterol together

Answer

Free fatty acids

Answer

Triglyceride, Cholesterol and Phospholipids

Question 8

Rate limiting enzyme in bile acid synthesis?

Accepted Answer

7α-hydroxylase

Answer

Desmolase

Answer

21α-hydroxylase

Answer

12α-hydroxylase

Question 9

Transport of lipids from the intestine to other tissues is by -

Accepted Answer

Chylomicrons

Answer

LDL

Answer

HDL

Answer

VLDL

Question 10

Bile acids consist of all of the following except -

Accepted Answer

Bilirubin

Answer

Lithocholic acid

Answer

Deoxycholic acid

Answer

Chenodeoxycholic acid

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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