NEET-PG 2012 — Biochemistry

Q: In apoptosis, cytochrome C acts through:

Apaf1. ***Apaf1*** - Cytochrome C released from the mitochondria binds to **Apaf1**, which leads to the formation of the **apoptosome** [1][2]. - This complex activates **caspase-9**, initiating the caspase cascade that leads to apoptosis [2]. *TNF* - Tumor Necrosis Factor (TNF) is involved in **necrosis** and **inflammatory processes**, not directly in the intrinsic pathway of apoptosis. - It activates **caspase-8**, which is part of the **extrinsic pathway**, differing from the role of cytochrome C [1]. *FADD* - FADD (Fas-associated protein with death domain) is part of the **death receptor pathway**, linking to caspase-8, not associated with cytochrome C [1]. - It does not play a role in the assembly of the apoptosome like Apaf1 does. *Bcl_2* - Bcl-2 is an **anti-apoptotic protein** that inhibits apoptosis rather than inducing it or acting through cytochrome C [1]. - It functions by preventing the release of cytochrome C from mitochondria, thereby opposing the apoptotic process [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, p. 310. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 64-67. [Practice more Biochemistry PYQs on OnCourse]

Q: Which of the following enzymes does not catalyze a reaction that directly produces ATP via substrate-level phosphorylation?

Hexokinase. ***Correct: Hexokinase*** **Hexokinase** catalyzes the transfer of a phosphate group from **ATP to glucose**, producing **glucose-6-phosphate** and ADP. This step **consumes ATP** rather than producing it via substrate-level phosphorylation. **Substrate-level phosphorylation** directly synthesizes ATP from ADP by transferring a high-energy phosphate group from a phosphorylated substrate; hexokinase performs the **opposite reaction** (ATP consumption). *Incorrect: Pyruvate kinase* **Pyruvate kinase** catalyzes the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, forming **pyruvate** and ATP. This is a classic example of **substrate-level phosphorylation** in glycolysis, directly generating ATP. *Incorrect: Succinate thiokinase* **Succinate thiokinase** (also known as succinyl-CoA synthetase) catalyzes the conversion of **succinyl-CoA to succinate**, simultaneously forming **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate. The GTP produced can be converted to ATP through nucleoside diphosphate kinase, representing substrate-level phosphorylation in the TCA cycle. *Incorrect: Phosphoglycerate kinase* **Phosphoglycerate kinase** catalyzes the transfer of a phosphate group from **1,3-bisphosphoglycerate** to ADP, yielding **3-phosphoglycerate** and ATP. This is a key enzymatic step in glycolysis that directly produces ATP through **substrate-level phosphorylation**. [Practice more Biochemistry PYQs on OnCourse]

Q: The rate-limiting step in glycolysis is catalyzed by?

Phosphofructokinase. ***Phosphofructokinase*** - **Phosphofructokinase-1 (PFK-1)** is the primary regulatory enzyme and **rate-limiting step** in glycolysis. - It catalyzes the irreversible phosphorylation of **fructose-6-phosphate to fructose-1,6-bisphosphate**, a crucial commitment step. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate to phosphoenolpyruvate** in glycolysis. - While essential for glycolysis, it is not the rate-limiting step. *Glucokinase* - **Glucokinase** catalyzes the phosphorylation of glucose to **glucose-6-phosphate** in the liver and pancreatic beta cells. - This is the first step in glycolysis but is not the rate-limiting step for the entire pathway once glucose has entered the cell. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, converting **phosphoenolpyruvate to pyruvate**. - Although it is a regulated enzyme, it is not the primary rate-limiting step that controls the overall flux through the glycolytic pathway. [Practice more Biochemistry PYQs on OnCourse]

Q: Which reaction requires Vitamin B1?

Oxidative decarboxylation. ***Oxidative decarboxylation*** - Vitamin B1, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for enzymes catalyzing **oxidative decarboxylation** reactions. - Key examples include the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**, essential for cellular respiration and the citric acid cycle. *Transamination* - This type of reaction, involving the transfer of an **amino group**, primarily requires **pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**. - It is vital for amino acid metabolism but does not utilize Vitamin B1. *Carboxylation* - **Carboxylation** reactions, which add a carboxyl group to a substrate, typically require **biotin** (Vitamin B7) as a coenzyme. - Examples include pyruvate carboxylase and acetyl-CoA carboxylase, which are not dependent on Vitamin B1. *None of the options* - As **oxidative decarboxylation** specifically requires Vitamin B1, this option is incorrect. - The other listed reactions depend on different vitamins as coenzymes. [Practice more Biochemistry PYQs on OnCourse]

Q: Cell-matrix adhesions are mediated by?

Integrins. ***Integrins*** - **Integrins** are transmembrane receptors that mediate cell adhesion to the **extracellular matrix (ECM)**, linking it to the cell's cytoskeleton. - They bind to various ECM components like **fibronectin**, **collagen**, and **laminin**. *Cadherins* - **Cadherins** are primarily involved in **cell-to-cell adhesion**, forming junctions like **adherens junctions** and **desmosomes**. - They are **calcium-dependent adhesion molecules** that do not directly bind to the extracellular matrix. *Selectins* - **Selectins** are cell adhesion molecules involved in **leukocyte rolling** and **adhesion to endothelial cells** during inflammation. - They mediate **transient cell-to-cell interactions**, not cell-matrix adhesion. *Calmodulin* - **Calmodulin** is a **calcium-binding protein** that acts as a signal transducer, regulating various intracellular processes. - It is involved in **calcium-dependent signaling pathways** and enzyme activation, not cell adhesion. [Practice more Biochemistry PYQs on OnCourse]

184 Previous Year Questions with Answers & Explanations

184

Questions

In apoptosis, cytochrome C acts through:

Which of the following enzymes does not catalyze a reaction that directly produces ATP via substrate-level phosphorylation?

The rate-limiting step in glycolysis is catalyzed by?

Which reaction requires Vitamin B1?

Cell-matrix adhesions are mediated by?

Enzymes that move a molecular group from one molecule to another are known as -

Km value is defined as:

In the electron transport chain (ETC), which enzyme does cyanide inhibit?

Apoenzyme is ?

Q10

Enzyme causing covalent bond cleavage without hydrolysis ?

NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs

Question 1: In apoptosis, cytochrome C acts through:

A. FADD
B. TNF
C. Apaf1 (Correct Answer)
D. Bcl-2

Explanation: ***Apaf1*** - Cytochrome C released from the mitochondria binds to **Apaf1**, which leads to the formation of the **apoptosome** [1][2]. - This complex activates **caspase-9**, initiating the caspase cascade that leads to apoptosis [2]. *TNF* - Tumor Necrosis Factor (TNF) is involved in **necrosis** and **inflammatory processes**, not directly in the intrinsic pathway of apoptosis. - It activates **caspase-8**, which is part of the **extrinsic pathway**, differing from the role of cytochrome C [1]. *FADD* - FADD (Fas-associated protein with death domain) is part of the **death receptor pathway**, linking to caspase-8, not associated with cytochrome C [1]. - It does not play a role in the assembly of the apoptosome like Apaf1 does. *Bcl_2* - Bcl-2 is an **anti-apoptotic protein** that inhibits apoptosis rather than inducing it or acting through cytochrome C [1]. - It functions by preventing the release of cytochrome C from mitochondria, thereby opposing the apoptotic process [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, p. 310. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 64-67.

Question 2: Which of the following enzymes does not catalyze a reaction that directly produces ATP via substrate-level phosphorylation?

A. Pyruvate kinase
B. Hexokinase (Correct Answer)
C. Succinate thiokinase
D. Phosphoglycerate kinase

Explanation: ***Correct: Hexokinase*** **Hexokinase** catalyzes the transfer of a phosphate group from **ATP to glucose**, producing **glucose-6-phosphate** and ADP. This step **consumes ATP** rather than producing it via substrate-level phosphorylation. **Substrate-level phosphorylation** directly synthesizes ATP from ADP by transferring a high-energy phosphate group from a phosphorylated substrate; hexokinase performs the **opposite reaction** (ATP consumption). *Incorrect: Pyruvate kinase* **Pyruvate kinase** catalyzes the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, forming **pyruvate** and ATP. This is a classic example of **substrate-level phosphorylation** in glycolysis, directly generating ATP. *Incorrect: Succinate thiokinase* **Succinate thiokinase** (also known as succinyl-CoA synthetase) catalyzes the conversion of **succinyl-CoA to succinate**, simultaneously forming **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate. The GTP produced can be converted to ATP through nucleoside diphosphate kinase, representing substrate-level phosphorylation in the TCA cycle. *Incorrect: Phosphoglycerate kinase* **Phosphoglycerate kinase** catalyzes the transfer of a phosphate group from **1,3-bisphosphoglycerate** to ADP, yielding **3-phosphoglycerate** and ATP. This is a key enzymatic step in glycolysis that directly produces ATP through **substrate-level phosphorylation**.

Question 3: The rate-limiting step in glycolysis is catalyzed by?

A. Phosphofructokinase (Correct Answer)
B. Enolase
C. Glucokinase
D. Pyruvate kinase

Explanation: ***Phosphofructokinase*** - **Phosphofructokinase-1 (PFK-1)** is the primary regulatory enzyme and **rate-limiting step** in glycolysis. - It catalyzes the irreversible phosphorylation of **fructose-6-phosphate to fructose-1,6-bisphosphate**, a crucial commitment step. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate to phosphoenolpyruvate** in glycolysis. - While essential for glycolysis, it is not the rate-limiting step. *Glucokinase* - **Glucokinase** catalyzes the phosphorylation of glucose to **glucose-6-phosphate** in the liver and pancreatic beta cells. - This is the first step in glycolysis but is not the rate-limiting step for the entire pathway once glucose has entered the cell. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, converting **phosphoenolpyruvate to pyruvate**. - Although it is a regulated enzyme, it is not the primary rate-limiting step that controls the overall flux through the glycolytic pathway.

Question 4: Which reaction requires Vitamin B1?

A. None of the options
B. Oxidative decarboxylation (Correct Answer)
C. Carboxylation
D. Transamination

Explanation: ***Oxidative decarboxylation*** - Vitamin B1, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for enzymes catalyzing **oxidative decarboxylation** reactions. - Key examples include the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**, essential for cellular respiration and the citric acid cycle. *Transamination* - This type of reaction, involving the transfer of an **amino group**, primarily requires **pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**. - It is vital for amino acid metabolism but does not utilize Vitamin B1. *Carboxylation* - **Carboxylation** reactions, which add a carboxyl group to a substrate, typically require **biotin** (Vitamin B7) as a coenzyme. - Examples include pyruvate carboxylase and acetyl-CoA carboxylase, which are not dependent on Vitamin B1. *None of the options* - As **oxidative decarboxylation** specifically requires Vitamin B1, this option is incorrect. - The other listed reactions depend on different vitamins as coenzymes.

Question 5: Cell-matrix adhesions are mediated by?

A. Integrins (Correct Answer)
B. Selectins
C. Calmodulin
D. Cadherins

Explanation: ***Integrins*** - **Integrins** are transmembrane receptors that mediate cell adhesion to the **extracellular matrix (ECM)**, linking it to the cell's cytoskeleton. - They bind to various ECM components like **fibronectin**, **collagen**, and **laminin**. *Cadherins* - **Cadherins** are primarily involved in **cell-to-cell adhesion**, forming junctions like **adherens junctions** and **desmosomes**. - They are **calcium-dependent adhesion molecules** that do not directly bind to the extracellular matrix. *Selectins* - **Selectins** are cell adhesion molecules involved in **leukocyte rolling** and **adhesion to endothelial cells** during inflammation. - They mediate **transient cell-to-cell interactions**, not cell-matrix adhesion. *Calmodulin* - **Calmodulin** is a **calcium-binding protein** that acts as a signal transducer, regulating various intracellular processes. - It is involved in **calcium-dependent signaling pathways** and enzyme activation, not cell adhesion.

Question 6: Enzymes that move a molecular group from one molecule to another are known as -

A. Transferases (Correct Answer)
B. Ligases
C. Dipeptidases
D. Oxido-reductases

Explanation: ***Transferases*** - **Transferases** are a class of enzymes that catalyze the transfer of a specific functional group (e.g., methyl, acetyl, phosphate) from one molecule (the donor) to another (the acceptor). - This broad category includes enzymes vital for many metabolic pathways, such as **kinases** (transferring phosphate groups) and **transaminases** (transferring amino groups). *Ligases* - **Ligases** are enzymes responsible for joining two large molecules together, typically by forming a new chemical bond. - This process usually involves the concomitant hydrolysis of a small, energy-rich molecule such as **ATP**, to provide the necessary energy for bond formation. *Dipeptidases* - **Dipeptidases** are a type of hydrolase enzyme that specifically cleaves the peptide bond within a **dipeptide**, releasing two free amino acids. - They are crucial for the final stages of protein digestion, breaking down small peptides into absorbable **amino acid units**. *Oxido-reductases* - **Oxido-reductases** are enzymes that catalyze **oxidation-reduction reactions** (redox reactions), where electrons are transferred from one molecule to another. - This class includes enzymes like **dehydrogenases** and **oxidases**, which play critical roles in cellular respiration and energy production.

Question 7: Km value is defined as:

A. Substrate concentration at Vmax/2 (Correct Answer)
B. Substrate concentration at which reaction rate is maximum
C. Substrate concentration at Vmax
D. Substrate concentration at which enzyme activity is optimal

Explanation: ***Substrate concentration at Vmax/2*** - The **Michaelis constant (Km)** is defined as the **substrate concentration** at which the reaction velocity is **half of the maximum velocity (Vmax/2)**. - It reflects the **affinity of an enzyme for its substrate**; a lower Km indicates higher affinity. *Substrate concentration at which reaction rate is maximum* - The **maximum reaction rate (Vmax)** is achieved when the enzyme is **saturated with substrate**, meaning all active sites are occupied. - Km specifically refers to the substrate concentration needed to reach **half of this maximum rate**, not the maximum rate itself. *Substrate concentration at Vmax* - At **Vmax**, the enzyme is fully saturated with substrate, and the reaction rate cannot increase further by adding more substrate. - The **Km value** is a measure related to the **efficiency of substrate binding** at conditions below saturation, specifically at half Vmax. *Substrate concentration at which enzyme activity is optimal* - **Optimal enzyme activity** is generally influenced by factors such as **pH and temperature**, which affect the enzyme's structure and catalytic efficiency. - Km is specifically related to the **substrate concentration** required to achieve a specific reaction rate, not the overall optimal environmental conditions for the enzyme.

Question 8: In the electron transport chain (ETC), which enzyme does cyanide inhibit?

A. Complex II (Succinate dehydrogenase)
B. Cytochrome c oxidase (Complex IV) (Correct Answer)
C. Complex I (NADH dehydrogenase)
D. Complex III (Cytochrome bc1 complex)

Explanation: ***Cytochrome c oxidase (Complex IV)*** - Cyanide binds to the **ferric iron (Fe3+)** in the heme a3 component of cytochrome c oxidase, blocking the final transfer of electrons to oxygen. - This inhibition effectively halts the entire **electron transport chain** and **oxidative phosphorylation**, leading to rapid cellular energy depletion. *Complex I (NADH dehydrogenase)* - While other toxins can inhibit Complex I (e.g., rotenone, amytal), **cyanide specifically targets Complex IV**. - Inhibition here prevents the entry of electrons from **NADH** into the ETC, but it's not cyanide's primary site of action. *Complex III (Cytochrome bc1 complex)* - Complex III is involved in transferring electrons from **ubiquinol** to cytochrome c, but it is not directly inhibited by cyanide. - Antimycin A is a well-known inhibitor of Complex III. *Complex II (Succinate dehydrogenase)* - Complex II directly receives electrons from **succinate** in the citric acid cycle and passes them to ubiquinone, bypassing Complex I. - Cyanide does not inhibit Complex II; inhibitors of this complex include malonate.

Question 9: Apoenzyme is ?

A. Protein moiety (Correct Answer)
B. Organic cofactor
C. Inactive enzyme component
D. Non-protein component required for enzyme activity

Explanation: ***Protein moiety*** - An **apoenzyme** is the **protein component of an enzyme** that is catalytically inactive by itself. - It requires a **non-protein cofactor** (either an inorganic ion or an organic molecule) to become active. *Organic cofactor* - An **organic cofactor** is also known as a **coenzyme**, which binds to the apoenzyme to form a functional holoenzyme. - While essential for enzyme activity, the apoenzyme itself is the protein part, not the organic cofactor. *Inactive enzyme component* - While an apoenzyme is **inactive on its own**, this description is too broad and doesn't specify its chemical nature. - It is specifically the **protein component** that is inactive until bound to its cofactor. *Non-protein component required for enzyme activity* - This describes a **cofactor** (either inorganic or organic), not the apoenzyme itself. - The apoenzyme is the **protein portion**, which *requires* the non-protein component for activity.

Question 10: Enzyme causing covalent bond cleavage without hydrolysis ?

A. Lyase (Correct Answer)
B. Ligase
C. Hydrolase
D. Transferase

Explanation: ***Lyase*** - **Lyases** are enzymes that catalyze the cleavage of **covalent bonds** (C-C, C-O, C-N, and others) by means other than hydrolysis or oxidation, often creating a new double bond or a ring structure. - They remove groups from substrates to form double bonds, or conversely, add groups to double bonds. - **Examples:** Aldolase (cleaves C-C bonds in glycolysis), carbonic anhydrase (reversible cleavage of C-O bond), fumarase (C-C bond cleavage in TCA cycle). *Ligase* - **Ligases** are enzymes that join two large molecules by forming a new chemical bond, usually accompanied by the **hydrolysis of ATP**. - They are involved in synthesis reactions, not the cleavage of bonds. *Hydrolase* - **Hydrolases** specifically catalyze the hydrolysis of a chemical bond, involving the **addition of water** across the bond. - They break down large molecules into smaller ones using water - this is the key difference from lyases. *Transferase* - **Transferases** catalyze the transfer of a **functional group** from one molecule (the donor) to another (the acceptor). - They do not cause covalent bond cleavage without hydrolysis but rather move existing groups between molecules.