What is the role of nonsense codons in protein synthesis?
Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?
Which molecule serves as a key link between carbohydrate metabolism and fatty acid synthesis?
Which carbohydrate is primarily metabolized by Aldolase-B?
Which vitamin is involved in one-carbon transfer reactions?
Which of the following statements about chylomicrons is true?
Which enzyme is primarily responsible for the fat metabolism in adipose tissue?
Enzyme deficient in Type I Hyperlipidemia?
Taurine is biosynthesized from which amino acid?
Selenocysteine is associated with ?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 111: What is the role of nonsense codons in protein synthesis?
- A. Elongation of the polypeptide chain
- B. Pre-translational modification of proteins
- C. Initiation of protein synthesis
- D. Termination of protein synthesis (Correct Answer)
Explanation: ***Termination of protein synthesis*** - **Nonsense codons**, also known as **stop codons** (UAA, UAG, UGA), signal the end of translation. - When a ribosome encounters a nonsense codon, it binds **release factors** instead of an aminoacyl-tRNA, leading to the dissociation of the polypeptide chain. *Elongation of the polypeptide chain* - **Elongation** involves the sequential addition of amino acids to the growing polypeptide chain, guided by sense codons. - Nonsense codons do not code for any amino acid and thus do not contribute to chain elongation. *Pre-translational modification of proteins* - **Pre-translational modifications** refer to events like protein folding and disulfide bond formation that occur as the polypeptide is being synthesized. - Nonsense codons are involved in halting the synthesis, not in modifying the protein. *Initiation of protein synthesis* - **Initiation** of protein synthesis begins at the **start codon** (AUG), which codes for methionine. - Nonsense codons are distinct from the start codon and fulfill a different role in the translation process.
Question 112: Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?
- A. Cortisol
- B. Glucagon
- C. Insulin (Correct Answer)
- D. Growth hormone
Explanation: ***Insulin*** - **Insulin** is an anabolic hormone that promotes glucose utilization and opposes **gluconeogenesis**. - While insulin does inhibit hepatic glucose production, it primarily acts by **repressing PEPCK (phosphoenolpyruvate carboxykinase)**, the rate-limiting enzyme of gluconeogenesis, rather than directly repressing pyruvate carboxylase biosynthesis. - **Note:** Modern biochemistry emphasizes that insulin's main transcriptional target in gluconeogenesis is **PEPCK**, not pyruvate carboxylase. However, this was the expected answer for **NEET-2012**, reflecting the understanding at that time. - Insulin also promotes dephosphorylation and inactivation of gluconeogenic enzymes and enhances glucose uptake and glycolysis. *Glucagon* - **Glucagon** is a catabolic hormone that **activates** enzymes involved in **gluconeogenesis** and glycogenolysis to raise blood glucose levels. - It would **increase**, not repress, the biosynthesis and activity of gluconeogenic enzymes including **pyruvate carboxylase**. *Cortisol* - **Cortisol** is a glucocorticoid hormone that **stimulates gluconeogenesis** in the liver as part of the stress response. - It typically **upregulates** the synthesis and activity of gluconeogenic enzymes like **pyruvate carboxylase** and **PEPCK**. *Growth hormone* - **Growth hormone** generally **increases insulin resistance** and can have a **diabetogenic effect**, promoting glucose production rather than repressing gluconeogenic enzymes. - It does not directly repress gluconeogenic enzyme biosynthesis; its metabolic effects favor lipolysis and protein synthesis.
Question 113: Which molecule serves as a key link between carbohydrate metabolism and fatty acid synthesis?
- A. Glucose-6-phosphate
- B. Acetyl-CoA
- C. Citrate (Correct Answer)
- D. Succinyl-CoA
Explanation: ***Citrate*** - **Citrate** is the crucial molecule that links carbohydrate metabolism to fatty acid synthesis via the **citrate-malate shuttle** - In the fed state, excess **acetyl-CoA** (derived from glucose metabolism via glycolysis and pyruvate dehydrogenase) condenses with oxaloacetate to form citrate in the mitochondria - **Citrate** is then transported from mitochondria to the cytosol, where **ATP-citrate lyase** cleaves it to regenerate **acetyl-CoA** and **oxaloacetate** for fatty acid synthesis - This is the primary mechanism for transporting acetyl-CoA equivalents from mitochondria (where glucose is oxidized) to the cytosol (where fatty acids are synthesized) - Citrate also acts as an **allosteric activator** of **acetyl-CoA carboxylase**, the rate-limiting enzyme of fatty acid synthesis *Glucose-6-phosphate* - While **glucose-6-phosphate** is a key intermediate in glycolysis and gluconeogenesis, it is not the molecule that directly links carbohydrate breakdown to fatty acid synthesis - It is several steps removed from the generation of cytosolic acetyl-CoA needed for fatty acid synthesis *Acetyl-CoA* - **Acetyl-CoA** is the direct precursor for fatty acid synthesis - However, acetyl-CoA generated in mitochondria from glucose oxidation **cannot directly cross the mitochondrial membrane** - It must be transported as citrate, making citrate the actual linking molecule between the two compartments *Succinyl-CoA* - **Succinyl-CoA** is a Krebs cycle intermediate involved in heme synthesis and propionate metabolism - It is not involved in transporting acetyl units from mitochondria to cytosol for fatty acid synthesis
Question 114: Which carbohydrate is primarily metabolized by Aldolase-B?
- A. Galactose
- B. Fructose (Correct Answer)
- C. Sucrose
- D. None of the options
Explanation: ***Fructose*** - **Aldolase B** is a key enzyme in the liver responsible for the metabolism of **fructose**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - A deficiency in **Aldolase B** leads to **hereditary fructose intolerance**, causing an accumulation of **fructose-1-phosphate** after fructose ingestion. *Galactose* - **Galactose** is primarily metabolized by enzymes in the **Leloir pathway**, including **galactokinase** and **galactose-1-phosphate uridylyltransferase**. - **Aldolase B** plays no significant role in the metabolism of galactose. *Sucrose* - **Sucrose** is a disaccharide composed of **glucose** and **fructose**. - It is first broken down by **sucrase** in the small intestine into its constituent monosaccharides before they are metabolized further. *None of the options* - This option is incorrect because **fructose** is indeed a carbohydrate primarily metabolized by Aldolase-B. - The enzyme's specific role in fructose metabolism is well-established.
Question 115: Which vitamin is involved in one-carbon transfer reactions?
- A. Folic acid (Correct Answer)
- B. Pantothenic acid
- C. Niacin
- D. Thiamine
Explanation: ***Folic acid*** - **Folic acid (Vitamin B9)** is a crucial coenzyme in the form of **tetrahydrofolate (THF)**, which acts as a carrier of **one-carbon units**. - These one-carbon units are essential for various metabolic pathways, including the synthesis of **purines**, **thymidylate**, and the metabolism of several **amino acids**. *Pantothenic acid* - **Pantothenic acid (Vitamin B5)** is a precursor to **Coenzyme A (CoA)**, which plays a central role in fatty acid metabolism and the **Krebs cycle**, not one-carbon transfers. - CoA is involved in transferring **acetyl groups**, not one-carbon units. *Niacin* - **Niacin (Vitamin B3)** is a component of **NAD+** and **NADP+**, which are vital coenzymes in **redox reactions** (electron transfer), not one-carbon metabolism. - It functions primarily in **energy metabolism** as an electron carrier. *Thiamine* - **Thiamine (Vitamin B1)** is a coenzyme for **dehydrogenase reactions** and **transketolase** in the **pentose phosphate pathway**, which are involved in carbohydrate metabolism. - It does not directly participate in one-carbon transfer reactions.
Question 116: Which of the following statements about chylomicrons is true?
- A. Chylomicrons are unrelated to triglyceride transport.
- B. Chylomicrons primarily contain cholesterol.
- C. Chylomicrons primarily contain triglycerides (TGs). (Correct Answer)
- D. Chylomicrons do not primarily contain triglycerides.
Explanation: ***Chylomicrons primarily contain triglycerides (TGs)*** - **Chylomicrons** are the largest and least dense lipoproteins, primarily responsible for transporting **dietary triglycerides** absorbed from the intestine to peripheral tissues. - They are synthesized in the **enterocytes** of the small intestine and released into the lymphatic system. - Approximately **85-90%** of a chylomicron's mass is composed of **triglycerides**, making them the primary carriers of exogenous fats. *Chylomicrons primarily contain cholesterol* - While chylomicrons do contain some **cholesterol**, it is a minor component (~3-5%) compared to their predominant content, which is **triglycerides**. - Lipoproteins like **LDL** and **HDL** are primarily responsible for cholesterol transport. *Chylomicrons are unrelated to triglyceride transport* - This statement is incorrect; chylomicrons are fundamentally involved in the **transport of dietary triglycerides** from the intestines to various tissues in the body. - After lipoprotein lipase acts on chylomicrons in peripheral tissues, triglycerides are hydrolyzed and fatty acids are taken up by tissues. *Chylomicrons do not primarily contain triglycerides* - This statement directly contradicts the main function and composition of chylomicrons, which are **rich in triglycerides**. - Without triglycerides as their primary content, chylomicrons would not be able to fulfill their physiological role in lipid transport.
Question 117: Which enzyme is primarily responsible for the fat metabolism in adipose tissue?
- A. Lipoprotein lipase
- B. Hormone-sensitive lipase (Correct Answer)
- C. Acid lipase
- D. Acid maltase
Explanation: ***Hormone-sensitive lipase*** - This enzyme is crucial for the **mobilization of stored triglycerides** in adipose tissue by hydrolyzing them into fatty acids and glycerol. - Its activity is stimulated by hormones like **epinephrine** and **norepinephrine** and inhibited by insulin, reflecting its role in regulating fat release during energy demand. *Lipoprotein lipase* - This enzyme is primarily located on the **endothelial surface of capillaries** in various tissues, including adipose tissue, muscle, and heart. - Its main role is to clear **triglyceride-rich lipoproteins** like chylomicrons and VLDL from the bloodstream, facilitating the uptake of fatty acids into cells for storage or energy, rather than direct fat metabolism within the adipose cell. *Acid lipase* - **Lysosomal acid lipase** functions within lysosomes to break down cholesterol esters and triglycerides that are taken up by cells. - Its primary role is in the degradation of lipids within the **lysosomal compartments**, not in the primary process of fat mobilization from adipose tissue stores. *Acid maltase* - Also known as **alpha-glucosidase**, this enzyme is a lysosomal enzyme responsible for breaking down glycogen into glucose. - Its function is related to **glycogen metabolism** and has no direct role in fat metabolism in adipose tissue.
Question 118: Enzyme deficient in Type I Hyperlipidemia?
- A. HMG CoA reductase
- B. Lipoprotein lipase (Correct Answer)
- C. Peroxidase
- D. Cholesterol acyl transferase
Explanation: ***Lipoprotein lipase*** - **Type I hyperlipidemia**, also known as **familial hyperchylomicronemia**, is characterized by a deficiency in **lipoprotein lipase (LPL)**. - LPL is crucial for hydrolyzing triglycerides in **chylomicrons** and **VLDLs** into fatty acids and glycerol, allowing their uptake by tissues. *HMG CoA reductase* - This enzyme is involved in the **rate-limiting step of cholesterol synthesis** in the liver. - While it plays a role in lipid metabolism, its deficiency is not characteristic of **Type I hyperlipidemia**. *Peroxidase* - **Peroxidase** is an enzyme involved in various oxidative reactions, including the breakdown of **hydrogen peroxide**. - It is not directly involved in the metabolism of **chylomicrons** or **triglycerides**, and its deficiency is unrelated to hyperlipidemia. *Cholesterol acyl transferase* - This enzyme, often referring to **lecithin-cholesterol acyltransferase (LCAT)** or **acyl-CoA:cholesterol acyltransferase (ACAT)**, is involved in **cholesterol esterification**. - While important for cholesterol transport and storage, its deficiency is not the primary cause of **Type I hyperlipidemia**, which is marked by severe **chylomicronemia**.
Question 119: Taurine is biosynthesized from which amino acid?
- A. Cysteine (Correct Answer)
- B. Valine
- C. Arginine
- D. Leucine
Explanation: ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.
Question 120: Selenocysteine is associated with ?
- A. Carbonic anhydrase
- B. Catalase
- C. Transferase
- D. Deiodinase (Correct Answer)
Explanation: ***Deiodinase*** - Selenocysteine is a critical component of **iodothyronine deiodinases**, a family of enzymes that regulate **thyroid hormone metabolism**. - These enzymes catalyze the removal of iodine from thyroid hormones, converting **thyroxine (T4)** into the more active **triiodothyronine (T3)** or inactive forms. *Carbonic anhydrase* - This enzyme contains **zinc** as its essential metal cofactor and is involved in the interconversion of **carbon dioxide** and **bicarbonate**. - Its primary role is in pH regulation and CO2 transport, without any direct association with selenocysteine. *Catalase* - Catalase is an enzyme primarily found in **peroxisomes** and contains **iron-porphyrin** groups as its prosthetic group. - Its function is to convert **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. *Transferase* - Transferases are a broad class of enzymes that catalyze the transfer of **functional groups** (e.g., methyl, glucose) from one molecule to another. - While essential for many metabolic processes, there is no inherent association of the general class of transferases with selenocysteine.