Increased uric acid levels are seen in which glycogen storage disease ?
What are isoenzymes?
Which hormone inhibits hormone-sensitive lipase?
Which of the following is required for fatty acid synthesis ?
What is the cofactor required for the enzyme xanthine oxidase?
NADH via glycerophosphate shunt makes how many ATP?
Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?
What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
What is the role of nonsense codons in protein synthesis?
Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 31: Increased uric acid levels are seen in which glycogen storage disease ?
- A. Type I (Von Gierke's disease) (Correct Answer)
- B. Type II (Pompe disease)
- C. Type IV (Andersen disease)
- D. Type III (Cori disease)
Explanation: ***Type I (Von Gierke's disease)*** - In **Von Gierke's disease**, the deficiency of **glucose-6-phosphatase** leads to accumulation of glucose-6-phosphate in hepatocytes. - **Hyperuricemia** occurs due to: 1. **Increased purine degradation** - Metabolic stress leads to accelerated ATP breakdown and increased uric acid production 2. **Decreased renal excretion** - Lactic acidosis (from G6P → pyruvate → lactate) competitively inhibits uric acid secretion in renal tubules 3. **Enhanced purine synthesis** - Increased availability of ribose-5-phosphate from pentose phosphate pathway - Classic triad: **Hepatomegaly, hypoglycemia, and lactic acidosis with hyperuricemia** *Type II (Pompe disease)* - Caused by a deficiency of **acid alpha-glucosidase** (acid maltase), leading to glycogen accumulation in **lysosomes**. - Primarily affects the **heart**, **muscles**, and **liver**, but does not cause hyperuricemia. *Type IV (Andersen disease)* - Results from a deficiency of **glycogen branching enzyme**, leading to the formation of abnormal glycogen with long, unbranched chains. - Primarily affects the **liver** and **spleen**, causing cirrhosis and hepatic failure, but not hyperuricemia. *Type III (Cori disease)* - Caused by a deficiency of **glycogen debranching enzyme** (amylo-1,6-glucosidase), leading to abnormal accumulation of glycogen with short outer branches. - Presents with hepatomegaly, hypoglycemia, and muscle weakness, but **hyperuricemia is not a characteristic feature**.
Question 32: What are isoenzymes?
- A. Physically same forms of different enzymes
- B. Forms of same enzyme that catalyze different reactions
- C. Forms of different enzyme that catalyze same reactions
- D. Physically distinct forms of the same enzyme (Correct Answer)
Explanation: ***Physically distinct forms of the same enzyme*** - Isoenzymes are **multiple forms of an enzyme** that catalyze the **same reaction** but differ in their **physical or biochemical properties**, such as electrophoretic mobility, optimal pH, or kinetic parameters. - These differences usually arise from **genetic variations** (different genes encoding isoforms) or **post-translational modifications** (e.g., phosphorylation, glycosylation). *Physically same forms of different enzymes* - This statement is incorrect as isoenzymes are forms of the **same enzyme**, not different enzymes. - While different enzymes can catalyze similar reactions in certain pathways, they are not referred to as isoenzymes if they are structurally identical. *Forms of same enzyme that catalyze different reactions* - This describes enzymes with **broad substrate specificity** or those that act on different substrates but are not necessarily isoenzymes. - Isoenzymes specifically catalyze the **same chemical reaction**, but they may do so with different efficiencies or under different regulatory controls. *Forms of different enzyme that catalyze same reactions* - This describes a scenario where different enzymes might exhibit **catalytic promiscuity** or broad specificity, but not isoenzymes. - Isoenzymes are always derived from the **same parent enzyme** and catalyze the identical reaction.
Question 33: Which hormone inhibits hormone-sensitive lipase?
- A. Insulin (Correct Answer)
- B. GH
- C. ACTH
- D. Thyroid hormone
Explanation: ***Insulin*** - **Insulin** is a key anabolic hormone that promotes energy storage and inhibits catabolic processes, including the breakdown of triglycerides. - It directly inhibits **hormone-sensitive lipase (HSL)** activity, thus reducing the release of free fatty acids from adipose tissue. *Thyroid hormone* - **Thyroid hormones** (T3 and T4) generally promote catabolism and increase metabolic rate, including the mobilization of lipids. - They tend to **stimulate rather than inhibit** hormone-sensitive lipase expression and activity. *GH* - **Growth hormone (GH)** has lipolytic effects, meaning it promotes the breakdown of fats to provide energy. - GH typically **stimulates HSL activity** and increases the release of free fatty acids from adipocytes. *ACTH* - **Adrenocorticotropic hormone (ACTH)** primarily stimulates the adrenal cortex to produce cortisol. - **Cortisol** can have lipolytic effects in certain contexts and does not directly inhibit HSL; instead, catecholamines act as direct stimulators of HSL.
Question 34: Which of the following is required for fatty acid synthesis ?
- A. NADPH (Correct Answer)
- B. NADH
- C. FADH₂
- D. None of the options
Explanation: ***NADPH*** - **NADPH** is crucial for fatty acid synthesis, providing the **reducing power** needed for the successive reduction steps. - The enzymes involved, such as **fatty acid synthase**, utilize **NADPH** for the conversion of keto groups to hydroxyl groups and then to saturated methylene groups. *NADH* - **NADH** plays a primary role in **oxidative phosphorylation** and the electron transport chain to generate ATP. - It is generally produced during **catabolic reactions** and is not primarily used as a reducing agent in anabolic processes like fatty acid synthesis. *FADH* - **FADH2** (reduced form of FAD, not FADH) is a coenzyme involved in redox reactions, particularly in the **Krebs cycle** and beta-oxidation of fatty acids. - Like NADH, it is mostly involved in **catabolic processes** that generate energy, rather than anabolic processes requiring reducing equivalents for synthesis. *None of the options* - This option is incorrect because **NADPH** is indeed required for fatty acid synthesis, serving as the essential reducing agent. - The other coenzymes mentioned (NADH, FADH) have different metabolic roles, primarily in energy production rather than biosynthesis.
Question 35: What is the cofactor required for the enzyme xanthine oxidase?
- A. Selenium
- B. Zinc
- C. Molybdenum (Correct Answer)
- D. Magnesium
Explanation: ***Molybdenum*** - **Xanthine oxidase** is a key enzyme in **purine metabolism**, responsible for the oxidation of **hypoxanthine to xanthine** and further to **uric acid**. - **Molybdenum** is an essential trace element that serves as a **cofactor** for several enzymes, including xanthine oxidase, where it helps facilitate electron transfer reactions. *Selenium* - **Selenium** is a cofactor for **glutathione peroxidase**, an enzyme involved in antioxidant defense. - It is not directly involved in the function of **xanthine oxidase**. *Zinc* - **Zinc** is a cofactor for a wide range of enzymes, including **carbonic anhydrase** and **alcohol dehydrogenase**. - It does not serve as a cofactor for **xanthine oxidase**. *Magnesium* - **Magnesium** is a critical cofactor for many enzymes, particularly those involved in **ATP hydrolysis and synthesis** and **DNA/RNA synthesis**. - It is not a cofactor for **xanthine oxidase**.
Question 36: NADH via glycerophosphate shunt makes how many ATP?
- A. 1
- B. 4
- C. 2 (Correct Answer)
- D. 3
Explanation: ***2*** - The **glycerol phosphate shuttle** transfers electrons from **cytosolic NADH** to **FAD** in the mitochondrial electron transport chain. - Each **FADH2** molecule produced then enters the electron transport chain at **Complex II**, ultimately leading to the generation of approximately **2 ATP** molecules. *1* - This option would be correct if the electrons were transferred to a molecule that yields only **one ATP** equivalent, which is not the case for **FADH2**. - No direct mechanism in a shunt generates exactly one ATP per NADH equivalent. *3* - This value represents the ATP yield from **NADH** when it directly enters the electron transport chain via the **malate-aspartate shuttle**, not the **glycerophosphate shuttle**. - The **glycerophosphate shuttle** is less efficient than the **malate-aspartate shuttle**. *4* - This number is not a standard ATP yield for either **NADH** or **FADH2** in the electron transport chain. - The maximum yield for NADH is typically considered to be 2.5 or 3 ATP, and for FADH2 is 1.5 or 2 ATP, depending on the shuttle and precise calculations.
Question 37: Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?
- A. Pyruvate kinase
- B. Succinyl CoA thiokinase
- C. NADH dehydrogenase (Correct Answer)
- D. ATP synthase
Explanation: ***Correct NADH dehydrogenase*** - **NADH dehydrogenase**, also known as Complex I, is the enzyme that accepts electrons from **NADH** during oxidative phosphorylation, initiating the electron transport chain. - This enzyme **oxidizes NADH** to NAD+ and pumps protons from the mitochondrial matrix to the intermembrane space, contributing to the **proton gradient**. *Incorrect Pyruvate kinase* - **Pyruvate kinase** is an enzyme involved in **glycolysis**, catalyzing the final step of converting phosphoenolpyruvate to pyruvate. - It functions in the **cytoplasm** and is not directly involved in the electron transport chain or oxidative phosphorylation. *Incorrect Succinyl CoA thiokinase* - **Succinyl CoA thiokinase** (also known as succinate thiokinase or succinyl-CoA synthetase) is an enzyme in the **Krebs cycle** (citric acid cycle). - It catalyzes the reversible reaction of converting succinyl-CoA to succinate and is not directly part of the electron transport chain. *Incorrect ATP synthase* - **ATP synthase** (Complex V) is the enzyme responsible for synthesizing ATP using the **proton gradient** established by the electron transport chain. - While crucial for oxidative phosphorylation, it acts at the end of the process, utilizing the energy generated, rather than initiating electron transport.
Question 38: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
- A. 4 ATP, 2 NADH
- B. 4 ATP, 4 NADH
- C. 2 ATP, 4 NADH
- D. 2 ATP, 2 NADH (Correct Answer)
Explanation: **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.
Question 39: 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 40: 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.