Lipid Metabolism Practice Questions

Q: Which of the following is NOT an enzyme involved in fatty acid synthesis?

Acetoacetyl-CoA. ***Acetoacetyl-CoA*** - **Acetoacetyl-CoA** is an intermediate compound, not an enzyme. - It is a **ketone body precursor** and also an intermediate in fatty acid synthesis and degradation. - It serves as a **substrate** for various enzymes but does not catalyze any reaction itself. *Acetyl Co-A carboxylase* - **Acetyl Co-A carboxylase** is the key regulatory enzyme in fatty acid synthesis, catalyzing the **carboxylation of acetyl-CoA** to form malonyl-CoA. - This reaction is the **rate-limiting step** and the **committed step** in fatty acid synthesis. - It requires **biotin** as a cofactor. *Ketoacyl synthase* - **Ketoacyl synthase** (beta-ketoacyl-ACP synthase) is a core catalytic domain of the fatty acid synthase complex. - It catalyzes the **condensation reaction** between an acyl group and malonyl-ACP, releasing CO₂. - This forms a **beta-ketoacyl-ACP intermediate**. *Enoyl reductase* - **Enoyl reductase** (enoyl-ACP reductase) is an enzyme domain in the fatty acid synthase complex. - It catalyzes the **final reduction step**, converting trans-enoyl-ACP to saturated acyl-ACP. - This reaction uses **NADPH** as the reducing agent.

Q: What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?

SR-BI. ***SR-BI*** - **Scavenger Receptor class B type 1 (SR-BI)** is the primary receptor responsible for selective uptake of **cholesteryl esters** from HDL into cells, particularly the liver and steroidogenic tissues. - Unlike other lipoprotein receptors, SR-BI mediates the **selective transfer** of cholesterol without internalizing the entire HDL particle. *LDLR* - The **Low-Density Lipoprotein Receptor (LDLR)** is the primary receptor for **LDL** and very low-density lipoprotein (VLDL) remnants, mediating their endocytosis and degradation. - While it plays a crucial role in cholesterol metabolism, its main function is related to the uptake of **LDL cholesterol**, not HDL. *HDLR* - **HDLR** is not a recognized receptor in cholesterol metabolism. - This term may be a distracter created by combining HDL with the common receptor nomenclature. *SR-82* - **SR-82** is not a recognized receptor involved in cholesterol metabolism. - Similar to HDLR, this is a distracter term.

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: HDL is called good cholesterol because -

Removes cholesterol from peripheral tissues. ***Removes cholesterol from peripheral tissues*** - **High-density lipoprotein (HDL)** is known as "good cholesterol" due to its role in **reverse cholesterol transport**, a process where it collects excess cholesterol from peripheral cells and tissues. - This action helps to prevent the accumulation of cholesterol in arteries, thereby reducing the risk of **atherosclerosis** and cardiovascular disease. - HDL then transports this cholesterol to the liver for excretion via bile, completing the protective cycle. *Increases cholesterol delivery to peripheral tissues* - This is actually the opposite of HDL's function and describes the role of **LDL (low-density lipoprotein)**, which is considered "bad cholesterol." - LDL delivers cholesterol to peripheral tissues, and excess LDL can lead to **atherosclerotic plaque formation**. *Stimulates cholesterol synthesis in the liver* - HDL does not directly stimulate cholesterol synthesis in the liver; rather, its role is primarily in **cholesterol efflux** from cells and transport. - The liver's cholesterol synthesis is regulated by various factors, including dietary intake and cellular cholesterol levels via the **SREBP pathway**, but HDL does not upregulate hepatic cholesterol synthesis. *Activates enzymes that break down triglycerides* - While HDL does activate **LCAT (lecithin-cholesterol acyltransferase)** for cholesterol esterification, its primary "good" function is not the breakdown of triglycerides. - **Lipoprotein lipase (LPL)** is the primary enzyme responsible for triglyceride breakdown in lipoproteins like VLDL and chylomicrons.

Q: Cholesterol is not a precursor for the synthesis of which of the following?

Lipocortin. ***Lipocortin*** - **Lipocortin** (also known as annexin A1) is a **protein**, not a steroid compound - It is synthesized via **protein translation** from mRNA, not from cholesterol - Mediates anti-inflammatory effects of glucocorticoids and is involved in cell growth regulation - **Key point:** Only steroids and bile acids are derived from cholesterol, not proteins *Testosterone* - **Testosterone** is an androgen (male sex hormone) **synthesized from cholesterol** - Cholesterol → Pregnenolone → DHEA → Androstenedione → Testosterone - Produced in the gonads (Leydig cells) and adrenal glands *Cortisol* - **Cortisol** is a glucocorticoid hormone **derived from cholesterol** - Cholesterol → Pregnenolone → 17-hydroxypregnenolone → Cortisol - Synthesized in the zona fasciculata of the adrenal cortex *Aldosterone* - **Aldosterone** is a mineralocorticoid hormone **synthesized from cholesterol** - Cholesterol → Pregnenolone → Progesterone → Aldosterone - Produced in the zona glomerulosa of the adrenal cortex

Q: Which of the following statements about adiponectin is incorrect?

Positive Correlation with BMI. ***Positive Correlation with BMI (INCORRECT STATEMENT)*** - Adiponectin levels are **inversely correlated with BMI**, NOT positively correlated; as BMI increases, adiponectin levels generally decrease. - This inverse relationship is significant because lower adiponectin levels are associated with increased insulin resistance and **metabolic syndrome**. - This statement is **false**, making it the correct answer to this question. *Secreted by adipose tissue (Correct statement)* - Adiponectin is a **hormone primarily secreted by adipocytes** (fat cells). - It plays a crucial role in regulating glucose and lipid metabolism, and its secretion is altered in conditions like obesity. - This statement is **true**. *Lowers glucose (Correct statement)* - Adiponectin **enhances insulin sensitivity** in peripheral tissues like skeletal muscle and liver, leading to increased glucose uptake and utilization. - This action helps to **lower blood glucose levels** and improve glycemic control. - This statement is **true**. *Increases FFA oxidation (Correct statement)* - Adiponectin **promotes fatty acid oxidation** in muscle and liver, reducing intracellular lipid accumulation. - By increasing fatty acid burning, it helps to **decrease circulating free fatty acid (FFA) levels**, which can contribute to insulin resistance if elevated. - This statement is **true**.

Q: Which of the following stimulates Acetyl CoA Carboxylase?

Citrate. ***Citrate*** - **Citrate** is an allosteric activator of **Acetyl-CoA Carboxylase (ACC)**, indicating abundant energy and precursor availability for fatty acid synthesis. - This activation promotes the conversion of **Acetyl-CoA** to **Malonyl-CoA**, the committed step in **fatty acid synthesis**. *Starvation* - **Starvation** leads to energy deficit, which generally **inhibits** anabolic processes like fatty acid synthesis. - In this state, enzymes involved in anabolic pathways are often downregulated or inhibited to conserve energy. *Glucagon* - **Glucagon** is a hormone that signals low blood glucose and promotes catabolic processes such as **glycogenolysis** and **gluconeogenesis**. - It **inhibits** fatty acid synthesis by phosphorylating and inactivating **Acetyl-CoA Carboxylase**, thus opposing citrate's activating effect. *None of the options* - **Citrate** is a known stimulator of Acetyl CoA Carboxylase. - This option is incorrect because there is a correct answer among the choices.

Q: Apo B48 is synthesized in -

Intestine. ***Intestine*** - **Apo B48** is a truncated form of apolipoprotein B-100, uniquely synthesized in the **intestine** through RNA editing. - It is a crucial structural component of **chylomicrons**, which are lipoprotein particles responsible for transporting exogenous dietary lipids from the intestine to other tissues. *Liver* - The liver primarily synthesizes **Apo B100**, which is a full-length apolipoprotein B and a major component of VLDL, IDL, and LDL. - It does not produce Apo B48. *Kidney* - The kidneys are involved in filtering waste products and regulating fluid balance, but they do not play a role in the synthesis of apolipoproteins like Apo B48. - Kidney cells are not equipped with the specific machinery for Apo B mRNA editing. *RBCs* - Red blood cells (RBCs) are primarily responsible for oxygen transport and lack a nucleus and most organelles, including those required for protein synthesis. - Therefore, RBCs cannot synthesize proteins such as Apo B48.

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.

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.

Question 1

Which of the following is NOT an enzyme involved in fatty acid synthesis?

Accepted Answer

Acetoacetyl-CoA

Answer

Ketoacyl synthase

Answer

Enoyl reductase

Answer

Acetyl Co-A carboxylase

Question 2

What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?

Accepted Answer

SR-BI

Answer

LDLR

Answer

HDLR

Answer

SR-82

Question 3

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 4

HDL is called good cholesterol because -

Accepted Answer

Removes cholesterol from peripheral tissues

Answer

Increases cholesterol delivery to peripheral tissues

Answer

Stimulates cholesterol synthesis in the liver

Answer

Activates enzymes that break down triglycerides

Question 5

Cholesterol is not a precursor for the synthesis of which of the following?

Accepted Answer

Lipocortin

Answer

Testosterone

Answer

Cortisol

Answer

Aldosterone

Question 6

Which of the following statements about adiponectin is incorrect?

Accepted Answer

Positive Correlation with BMI

Answer

Secreted by adipose tissue

Answer

Increases FFA oxidation

Answer

Lowers glucose

Question 7

Which of the following stimulates Acetyl CoA Carboxylase?

Accepted Answer

Citrate

Answer

Starvation

Answer

Glucagon

Answer

None of the options

Question 8

Apo B48 is synthesized in -

Accepted Answer

Intestine

Answer

Liver

Answer

Kidney

Answer

RBCs

Question 9

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

Question 10

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

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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