Lipid Metabolism Practice Questions

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: 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: How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?

8 acetyl CoA. ***8 acetyl CoA*** - Palmitic acid is a **16-carbon saturated fatty acid (C16:0)**. During β-oxidation, each cycle cleaves two carbons as **acetyl CoA**. - The formula for acetyl CoA produced is **n/2**, where n = number of carbons. For palmitic acid: 16/2 = **8 acetyl CoA molecules**. - Alternatively: Palmitic acid undergoes **7 cycles of β-oxidation** [(n/2) - 1 = 7], each producing 1 acetyl CoA (7 total), plus the final 2-carbon fragment forming the 8th acetyl CoA. *3 acetyl CoA* - This number is too low for a 16-carbon fatty acid. **Short-chain fatty acids** would produce fewer acetyl CoA molecules. - This value corresponds to β-oxidation of a **6-carbon fatty acid** (hexanoic acid), not palmitic acid. *6 acetyl CoA* - This number is also too low for a 16-carbon fatty acid. - This quantity would be produced from a **12-carbon fatty acid** (lauric acid), not palmitic acid. *16 Acetyl CoA* - This number is too high and would incorrectly imply that each carbon forms an acetyl CoA independently. - Sixteen acetyl CoA molecules would be produced from a **32-carbon fatty acid**, which is extremely rare in biological systems.

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**.

Q: Epinephrine increases free fatty acid levels by causing which of the following?

Increasing lipolysis. ***Increasing lipolysis*** - Epinephrine activates **hormone-sensitive lipase** in adipose tissue through a **cAMP-dependent mechanism**, leading to the breakdown of stored triglycerides into free fatty acids and glycerol. - This process, known as **lipolysis**, directly increases the release of free fatty acids into the bloodstream. *Increased fatty acid synthesis* - **Fatty acid synthesis** is a process that builds fatty acids, which would decrease, not increase, free fatty acid levels in the blood. - Epinephrine's primary action is to mobilize energy reserves, which involves breaking down stored fats rather than synthesizing new ones. *Increasing cholesterol catabolism* - While cholesterol metabolism is important, epinephrine does not directly or significantly increase **cholesterol catabolism** as a primary mechanism for raising free fatty acid levels. - The catabolism of cholesterol primarily involves its conversion to bile acids and steroid hormones, which is distinct from fatty acid release. *None of the options* - This option is incorrect because increasing lipolysis is a direct and well-established mechanism by which epinephrine raises free fatty acid levels.

Q: What is the role of colipase in fat digestion?

Assists pancreatic lipase in fat digestion. ***Assists pancreatic lipase in fat digestion*** - Colipase **binds to pancreatic lipase** and the **lipid-water interface** of the fat droplet, providing a conformational change that enables lipase to access and hydrolyze triglycerides. - It also prevents bile salts from inactivating pancreatic lipase, ensuring efficient **fat emulsification and digestion**. *Is secreted in an inactive form* - Colipase is secreted as **procolipase** by the pancreas, which is then activated by **trypsin** in the duodenum. - While correct, this option describes its activation rather than its primary role in fat digestion. *Encoded by the gene CLPS* - The gene **CLPS** indeed encodes for colipase, but this is a genetic detail rather than its functional role in the digestive process. - Knowledge of the encoding gene is not directly relevant to understanding its biochemical function in fat digestion. *Is secreted by pancreatic cells* - Colipase is indeed synthesized and secreted by the **pancreas** into the small intestine. - This statement is true but describes the **origin** of colipase, not its specific functional role in fat digestion.

Question 1

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 2

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

Accepted Answer

APOE4

Answer

APOE3

Answer

APOE2

Answer

APOB

Question 3

How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?

Accepted Answer

8 acetyl CoA

Answer

3 acetyl CoA

Answer

16 Acetyl CoA

Answer

6 acetyl CoA

Question 4

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 5

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 6

Rate limiting enzyme in bile acid synthesis?

Accepted Answer

7α-hydroxylase

Answer

Desmolase

Answer

21α-hydroxylase

Answer

12α-hydroxylase

Question 7

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

Accepted Answer

Chylomicrons

Answer

LDL

Answer

HDL

Answer

VLDL

Question 8

Bile acids consist of all of the following except -

Accepted Answer

Bilirubin

Answer

Lithocholic acid

Answer

Deoxycholic acid

Answer

Chenodeoxycholic acid

Question 9

Epinephrine increases free fatty acid levels by causing which of the following?

Accepted Answer

Increasing lipolysis

Answer

Increasing fatty acid synthesis

Answer

Increasing cholesterol catabolism

Answer

None of the options

Question 10

What is the role of colipase in fat digestion?

Accepted Answer

Assists pancreatic lipase in fat digestion

Answer

Encoded by the gene CLPS

Answer

Is secreted in an inactive form

Answer

Is secreted by pancreatic cells

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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