Lipid Metabolism Practice Questions

Q: Insulin promotes lipogenesis by all except?

Inhibits PDH. ***Inhibits PDH*** - Insulin **activates pyruvate dehydrogenase (PDH)**, not inhibits it, converting pyruvate to acetyl-CoA for fatty acid synthesis. - Since insulin **does not inhibit PDH**, this is the correct answer to this EXCEPT question. - Inhibiting PDH would decrease lipogenesis, which is opposite to insulin's effect. *Increasing acetyl CoA carboxylase activity* - **Acetyl-CoA carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis. - Insulin **activates ACC** (by dephosphorylation), which is a key mechanism for promoting lipogenesis. *Increases the transport of glucose into the cell* - Insulin enhances glucose uptake into adipose tissue and muscle cells by promoting **GLUT4 transporter** translocation to the cell membrane. - This increased glucose provides substrate (via glycolysis to acetyl-CoA) for fatty acid synthesis. *Decreases intracellular cAMP level* - Insulin **lowers intracellular cAMP levels** by activating phosphodiesterase, which degrades cAMP. - Lower cAMP reduces **hormone-sensitive lipase (HSL)** activity, leading to decreased lipolysis and thereby favoring lipogenesis.

Q: A 37-year-old man in diabetic ketoacidosis has yellowish papules scattered over his trunk and extremities. A tube of plasma collected from the patient develops a turbid supernate. This is consistent with which type of hyperlipoproteinemia?

Type IV. Type IV Hyperlipoproteinemia - This patient presents with eruptive xanthomas (yellowish papules) and a turbid plasma supernate, which suggests significantly elevated very-low-density lipoprotein (VLDL) and chylomicrons. - In Type IV hyperlipoproteinemia, there is an overproduction or decreased clearance of VLDL, often exacerbated by underlying conditions like uncontrolled diabetes mellitus (such as in DKA) [1]. Type I Hyperlipoproteinemia - Characterized by a deficiency of lipoprotein lipase or its coenzyme apoC-II, leading to extremely high chylomicron levels. - While it presents with eruptive xanthomas and milky plasma, diabetes mellitus is not a primary driver of this specific type, and the VLDL component is less pronounced than in Type IV. Lipoprotein lipase expression is notably enhanced by insulin [2]. Type II Hyperlipoproteinemia - This type primarily involves elevated low-density lipoprotein (LDL), often due to defects in LDL receptors, leading to conditions like familial hypercholesterolemia. - It's mainly associated with tendinous xanthomas and corneal arcus, and the plasma, though sometimes turbid due to LDL, would not typically show the degree of chylomicron or VLDL elevation expected with eruptive xanthomas and DKA. Type III Hyperlipoproteinemia - This condition involves elevated chylomicron remnants and VLDL remnants due to a defective apoE, specifically apoE2/E2 homozygosity. - While it can manifest with eruptive xanthomas, it more classically presents with palmar xanthomas and tuberoeruptive xanthomas, and the turbid supernate would typically have a distinct 'creamy layer' after refrigeration, which is not emphasized here.

Q: Apolipoprotein A-I is found in which of the following lipoproteins?

HDL. ***HDL*** - **Apolipoprotein A-I (ApoA-I)** is the primary protein component of **high-density lipoprotein (HDL)**. - Its main roles are the activation of **lecithin-cholesterol acyltransferase (LCAT)** and to facilitate the transport of **cholesterol** from peripheral tissues back to the liver (**reverse cholesterol transport**). *VLDL* - **Very-low-density lipoprotein (VLDL)** primarily contains **apolipoprotein B-100 (ApoB-100)**, **ApoC-II**, and **ApoE**. - Its main function is the transport of **triglycerides** synthesized in the liver to peripheral tissues. *LDL* - **Low-density lipoprotein (LDL)** contains a single molecule of **apolipoprotein B-100 (ApoB-100)**. - LDL is responsible for delivering **cholesterol** from the liver to peripheral cells. *Chylomicrons* - **Chylomicrons** primarily contain **apolipoprotein B-48 (ApoB-48)**, **ApoC-II**, and **ApoE**. - They are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestines to peripheral tissues and the liver.

Q: All of the following are products of the cyclooxygenase pathway from arachidonic acid, except for:

LTD4. ***LTD4*** - Leukotriene D4 (**LTD4**) is a product of the **lipoxygenase pathway**, not the cyclooxygenase pathway, from arachidonic acid. - It plays a significant role in **bronchoconstriction** and inflammation, especially in asthma. *PGE2* - **Prostaglandin E2 (PGE2)** is a major product of the **cyclooxygenase (COX) pathway**. - It is involved in mediating **fever**, pain, and inflammation. *PGD2* - **Prostaglandin D2 (PGD2)** is also produced via the **cyclooxygenase (COX) pathway**. - It is primarily associated with allergic reactions, **bronchoconstriction**, and sleep regulation. *PGF2α* - **Prostaglandin F2 alpha (PGF2α)** is a product derived from arachidonic acid through the **cyclooxygenase (COX) pathway**. - It plays a role in **uterine contractions**, fertility, and vascular tone.

Q: Enzymes involved in polyunsaturated fatty acid synthesis are

Desaturase. ***Correct: Desaturase*** - **Desaturases** introduce double bonds into fatty acid chains, a key step in synthesizing **polyunsaturated fatty acids (PUFAs)**. - This process is essential for converting saturated and monounsaturated fatty acids into increasingly unsaturated forms. *Incorrect: Pyruvate carboxylase* - **Pyruvate carboxylase** is involved in **gluconeogenesis** and **anaplerosis** by converting pyruvate to oxaloacetate. - It does not play a direct role in the synthesis of **fatty acids**, especially polyunsaturated ones. *Incorrect: Acetyltransferase* - **Acetyltransferases** catalyze the transfer of an acetyl group from **acetyl-CoA** to another molecule. - While acetyl-CoA is a precursor for fatty acid synthesis, acetyltransferases themselves are not the enzymes responsible for introducing **double bonds** into fatty acid chains to make them polyunsaturated. *Incorrect: Acyltransferase* - **Acyltransferases** transfer an acyl group (a fatty acid chain) from one molecule to another, often involved in **lipid synthesis** and modification. - They are crucial for forming esters like triglycerides and phospholipids but do not introduce **double bonds** to create polyunsaturated fatty acids.

Q: Which cooking oil is known to have the highest percentage of polyunsaturated fatty acids?

Safflower oil. ***Safflower oil*** - Safflower oil contains one of the **highest concentrations of polyunsaturated fatty acids**, particularly **linoleic acid**, among commonly used cooking oils. - This composition makes it a popular choice for health-conscious consumers looking to increase their intake of beneficial fats. *Coconut oil* - Coconut oil is predominantly composed of **saturated fatty acids**, notably **lauric acid**, which makes it solid at room temperature. - Its high saturated fat content distinguishes it significantly from oils rich in polyunsaturated fats. *Butter* - Butter is a dairy product that is very high in **saturated animal fats** and **cholesterol**. - Its fatty acid profile is dominated by short- and medium-chain saturated fatty acids, making it distinct from oils rich in polyunsaturated fats. *Vanaspati* - Vanaspati is a **partially hydrogenated vegetable oil** that is high in **trans fatty acids** and saturated fats due to its processing. - While it originates from vegetable oils, hydrogenation alters its fatty acid structure, reducing polyunsaturated content and increasing less healthy fats.

Q: A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?

VLDL. ***VLDL*** - A low-fat diet with increased **carbohydrates** can lead to increased hepatic synthesis of triglycerides, which are then packaged into **VLDL** particles for transport from the liver. This is because excess carbohydrates can be converted to fatty acids and then to triglycerides in the liver. - The liver's increased triglyceride production, driven by abundant **glucose** from carbohydrates, directly corresponds to a rise in **VLDL** secretion to export these lipids. *Chylomicron* - **Chylomicrons** primarily transport **dietary fats** (exogenous triglycerides) absorbed from the intestine. - Switching to a low-fat diet would typically lead to a *decrease* in chylomicron production, as less dietary fat is available for absorption. *IDL* - **IDL** (Intermediate-Density Lipoprotein) is a remnant of **VLDL** metabolism, formed after VLDL loses some triglycerides. - While VLDL may increase, leading to *more* IDL formation, IDL itself is not the primary component that *increases* directly due to high carbohydrate intake; rather, the precursor **VLDL** is directly affected. *HDL* - **HDL** (High-Density Lipoprotein) is involved in **reverse cholesterol transport**, picking up excess cholesterol from peripheral tissues and returning it to the liver. - High carbohydrate intake, especially refined carbohydrates, can sometimes lead to a *decrease* in HDL levels, not an increase.

Q: The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is which one?

Chylomicrons. ***Chylomicrons*** - Chylomicrons are primarily responsible for transporting **exogenous (dietary) triglycerides** from the intestines to peripheral tissues. - They are the **largest and least dense** lipoproteins, composed of approximately 85-90% triacylglycerol by weight. *VLDL* - **Very Low-Density Lipoproteins (VLDL)** transport **endogenous (liver-synthesized) triglycerides** to peripheral tissues. - While rich in triglycerides (about 50% by weight), their percentage is significantly lower than that of chylomicrons. *HDL* - **High-Density Lipoproteins (HDL)** are primarily involved in **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. - They have the **highest percentage of protein** and the lowest percentage of triacylglycerol among the major lipoproteins. *LDL* - **Low-Density Lipoproteins (LDL)** are the primary transporters of **cholesterol** to peripheral tissues. - They are derived from VLDL and contain a high percentage of cholesterol esters, with a relatively low percentage of triacylglycerol.

Q: A person is diagnosed with familial type IIa hyperlipoproteinemia. What is the basic defect in this type of hyperlipoproteinemia?

Defective LDL receptor. ***Defective LDL receptor*** - **Familial hypercholesterolemia** (Type IIa hyperlipoproteinemia) is characterized by high levels of **LDL cholesterol** due to a genetic defect in the **LDL receptor** gene. - This defective receptor leads to impaired clearance of LDL particles from the bloodstream, resulting in their accumulation. *Lipoprotein lipase deficiency* - This defect is associated with **Type I hyperlipoproteinemia**, which is characterized by elevated **chylomicrons** and **triglycerides**, not primarily LDL cholesterol. - **Lipoprotein lipase (LPL)** is essential for the hydrolysis of triglycerides in chylomicrons and VLDL. *Abnormal activity of Apo E* - Variants of **Apolipoprotein E (Apo E)**, particularly Apo E2, are associated with **Type III hyperlipoproteinemia** (familial dysbetalipoproteinemia). - This condition involves increased levels of **chylomicron remnants** and **VLDL remnants** (IDL), not primarily isolated LDL elevation. *Overproduction of LDL* - While increased **LDL production** can contribute to elevated LDL levels, the primary genetic defect in familial type IIa hyperlipoproteinemia is strictly related to the impaired **clearance** of LDL due to a defective **LDL receptor**, rather than solely overproduction. - Many secondary causes of hypercholesterolemia can involve LDL overproduction, but Type IIa is specifically linked to the receptor defect.

Q: NADPH is required in which of the following cellular processes?

Lipogenesis. ***Lipogenesis*** - **NADPH** is critically required for anabolic processes such as **fatty acid synthesis** (lipogenesis), where it acts as a **reducing agent**. - It supplies the electrons necessary for the sequential reduction steps in the conversion of acetyl-CoA to fatty acids in the cytoplasm. *HMP shunt* - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is the primary cellular source of **NADPH**. - Therefore, it produces NADPH rather than requiring it as a substrate for its main function. *Gluconeogenesis* - **Gluconeogenesis** is the metabolic pathway that produces **glucose** from non-carbohydrate precursors. - This process primarily uses **ATP** and **GTP** as energy sources, and NADH (not NADPH) is involved in some reduction reactions. *Glycogenolysis* - **Glycogenolysis** is the breakdown of **glycogen** into glucose-6-phosphate and then glucose. - This catabolic process does not directly require **NADPH**; instead, it releases glucose for energy or other metabolic uses.

Question 1

Insulin promotes lipogenesis by all except?

Accepted Answer

Inhibits PDH

Answer

Increases the transport of glucose in to the cell

Answer

Increasing acetyl co A carboxylase activity

Answer

Decreases intracellular cAMP level

Question 2

A 37-year-old man in diabetic ketoacidosis has yellowish papules scattered over his trunk and extremities. A tube of plasma collected from the patient develops a turbid supernate. This is consistent with which type of hyperlipoproteinemia?

Accepted Answer

Type IV

Answer

Type I

Answer

Type II

Answer

Type III

Question 3

Apolipoprotein A-I is found in which of the following lipoproteins?

Accepted Answer

HDL

Answer

VLDL

Answer

LDL

Answer

Chylomicrons

Question 4

All of the following are products of the cyclooxygenase pathway from arachidonic acid, except for:

Accepted Answer

LTD4

Answer

PGE2

Answer

PGD2

Answer

PGF2α

Question 5

Enzymes involved in polyunsaturated fatty acid synthesis are

Accepted Answer

Desaturase

Answer

Acetyltransferase

Answer

Pyruvate carboxylase

Answer

Acyltransferase

Question 6

Which cooking oil is known to have the highest percentage of polyunsaturated fatty acids?

Accepted Answer

Safflower oil

Answer

Coconut oil

Answer

Butter

Answer

Vanaspati

Question 7

A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?

Accepted Answer

VLDL

Answer

Chylomicron

Answer

IDL

Answer

HDL

Question 8

The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is which one?

Accepted Answer

Chylomicrons

Answer

VLDL

Answer

HDL

Answer

LDL

Question 9

A person is diagnosed with familial type IIa hyperlipoproteinemia. What is the basic defect in this type of hyperlipoproteinemia?

Accepted Answer

Defective LDL receptor

Answer

Lipoprotein lipase deficiency

Answer

Abnormal activity of Apo E

Answer

Overproduction of LDL

Question 10

NADPH is required in which of the following cellular processes?

Accepted Answer

Lipogenesis

Answer

HMP shunt

Answer

Glycogenolysis

Answer

Gluconeogenesis

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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