NEET-PG 2012 - Biochemistry PYQs Practice Questions

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]

Question 1

In apoptosis, cytochrome C acts through:

Accepted Answer

Apaf1

Answer

FADD

Answer

TNF

Answer

Bcl-2

Question 2

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

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Succinate thiokinase

Answer

Phosphoglycerate kinase

Question 3

The rate-limiting step in glycolysis is catalyzed by?

Accepted Answer

Phosphofructokinase

Answer

Enolase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 4

Which reaction requires Vitamin B1?

Accepted Answer

Oxidative decarboxylation

Answer

None of the options

Answer

Carboxylation

Answer

Transamination

Question 5

Cell-matrix adhesions are mediated by?

Accepted Answer

Integrins

Answer

Selectins

Answer

Calmodulin

Answer

Cadherins

Question 6

What are isoenzymes?

Accepted Answer

Physically distinct forms of the same enzyme

Answer

Physically same forms of different enzymes

Answer

Forms of same enzyme that catalyze different reactions

Answer

Forms of different enzyme that catalyze same reactions

Question 7

At which positions does pancreatic lipase hydrolyze the ester linkages of triacylglycerides?

Accepted Answer

1 and 3

Answer

1 and 2

Answer

2 and 3

Answer

Only 3

Question 8

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

Accepted Answer

Transferases

Answer

Ligases

Answer

Dipeptidases

Answer

Oxido-reductases

Question 9

Apoenzyme is ?

Accepted Answer

Protein moiety

Answer

Organic cofactor

Answer

Inactive enzyme component

Answer

Non-protein component required for enzyme activity

Question 10

Enzyme causing covalent bond cleavage without hydrolysis ?

Accepted Answer

Lyase

Answer

Ligase

Answer

Hydrolase

Answer

Transferase

NEET-PG 2012 — Biochemistry