ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?
Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
Carboxypeptidase contains which mineral?
Which element is required by phosphofructokinase?
Which kinetic parameter is primarily associated with enzyme specificity?
Kcat/Km is a measure of which of the following?
What is the typical Q10 value for enzymatic reactions?
What type of enzyme is hexokinase?
According to IUB system, hydrolases belong to which class?
Which of the following is a natural uncoupler found in brown adipose tissue?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 31: ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?
- A. Cl- ATPase
- B. ADP Kinase
- C. FoF1 ATPase (Correct Answer)
- D. Na+/K+ ATPase
Explanation: ***FoF1 ATPase*** - The **FoF1 ATPase**, also known as **ATP synthase**, is the complex enzyme responsible for synthesizing ATP using the **proton gradient** generated by the electron transport chain. - The **Fo subunit** forms a channel that allows protons to flow back into the mitochondrial matrix, driving the rotation of the **F1 subunit** which catalyzes ATP synthesis from ADP and inorganic phosphate. *Na+/K+ ATPase* - This enzyme is a **pump** that actively transports **three sodium ions out** of the cell and **two potassium ions into** the cell, maintaining membrane potential. - It uses **ATP hydrolysis** as its energy source, meaning it **consumes ATP** rather than producing it directly in the ETC. *Cl- ATPase* - **Cl- ATPase** refers to a family of pumps that transport **chloride ions**, typically using ATP hydrolysis as an energy source. - These enzymes are involved in ion homeostasis and fluid balance, but they do **not generate ATP** in the electron transport chain. *ADP Kinase* - **ADP Kinase** is a general term for enzymes that catalyze the phosphorylation of ADP to ATP, often by transferring a phosphate group from another high-energy molecule. - While it produces ATP, it is not the specific enzyme that directly harnesses the **proton gradient** in the electron transport chain for oxidative phosphorylation.
Question 32: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
- A. 2,3-Bisphosphoglycerate (2,3-BPG)
- B. Fructose 2,6-bisphosphate (Correct Answer)
- C. Glucokinase
- D. Phosphoenolpyruvate (PEP)
Explanation: ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.
Question 33: Carboxypeptidase contains which mineral?
- A. Copper
- B. Zinc (Correct Answer)
- C. Iron
- D. None of the options
Explanation: ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.
Question 34: Which element is required by phosphofructokinase?
- A. Magnesium (Correct Answer)
- B. Inorganic phosphate
- C. Manganese
- D. Copper
Explanation: **Magnesium** - **Phosphofructokinase** (PFK) is an enzyme in **glycolysis** that catalyzes the phosphorylation of fructose-6-phosphate. - This reaction requires **ATP**, and like many enzymes that utilize ATP, PFK requires **magnesium ions (Mg²⁺)** as a cofactor, typically forming a complex with ATP (MgATP²⁻). *Inorganic phosphate* - **Inorganic phosphate** is a substrate for some kinase reactions, but not a direct cofactor requirement for the *activation* of phosphofructokinase itself. - While phosphate is incorporated into molecules during phosphorylation, it does not act as a metal ion cofactor to facilitate the enzyme's activity. *Manganese* - While **manganese (Mn²⁺)** can sometimes substitute for magnesium in certain enzyme reactions, it is not the primary or required cofactor for phosphofructokinase under normal physiological conditions. - Many enzymes have a preference for specific metal ions based on their active site structure and coordination chemistry. *Copper* - **Copper (Cu²⁺)** is a cofactor for a variety of enzymes, particularly those involved in **redox reactions** (e.g., cytochrome c oxidase, superoxide dismutase). - However, copper is not a required metallic cofactor for the activity of **phosphofructokinase** in glycolysis.
Question 35: Which kinetic parameter is primarily associated with enzyme specificity?
- A. Both
- B. Km
- C. Vmax
- D. None of the options (Correct Answer)
Explanation: ***None of the options*** - **Enzyme specificity** is primarily determined by the unique three-dimensional **active site structure** of the enzyme, which allows it to bind only to specific substrates through complementary shape and chemical interactions. - This structural complementarity involves steric fit and specific non-covalent interactions (hydrogen bonds, van der Waals forces, electrostatic interactions) between the enzyme and its substrate. - **Neither Km nor Vmax are determinants of enzyme specificity**—they are kinetic parameters that describe enzyme behavior, not structural selectivity. *Km (Michaelis constant)* - Represents the substrate concentration at which the reaction rate is half of Vmax. - Indicates the **affinity** of an enzyme for its substrate (lower Km = higher affinity). - While enzymes may show different Km values for different substrates, **Km reflects binding affinity, not the structural basis of specificity**. *Vmax (Maximum velocity)* - The maximum rate of reaction when the enzyme is saturated with substrate. - Reflects **catalytic efficiency** and the amount of active enzyme present. - Does not relate to the enzyme's ability to discriminate between different substrate molecules. *Both* - Incorrect because neither Km nor Vmax determines which substrates an enzyme can recognize and bind. - Enzyme specificity is a **structural property** of the active site, while Km and Vmax are **kinetic properties** that describe reaction rates.
Question 36: Kcat/Km is a measure of which of the following?
- A. Speed of enzymatic reaction
- B. Concentration of substrate
- C. Enzyme turnover
- D. Enzyme efficiency (Correct Answer)
Explanation: **Correct: Enzyme efficiency** - The ratio **kcat/Km** is the definitive measure of an enzyme's **catalytic efficiency** or **specificity constant** - It reflects how effectively an enzyme converts substrate to product at low substrate concentrations - A higher **kcat/Km** value indicates greater efficiency, combining high catalytic rate (kcat) with strong substrate affinity (low Km) - This is the most important parameter for comparing different enzymes or different substrates for the same enzyme *Incorrect: Speed of enzymatic reaction* - **kcat** (turnover number) alone measures the maximum speed when enzyme is saturated with substrate - **kcat/Km** is a more comprehensive measure that includes substrate binding affinity, not just reaction speed - Speed also depends on enzyme and substrate concentrations, which kcat/Km doesn't directly represent *Incorrect: Concentration of substrate* - **Km** (Michaelis constant) represents the substrate concentration at which reaction velocity is half of Vmax - **kcat/Km** is a ratio that describes enzyme performance across substrate concentrations, not the concentration itself - It's particularly useful for predicting enzyme behavior at physiological (low) substrate concentrations *Incorrect: Enzyme turnover* - **kcat** specifically measures enzyme turnover: the number of substrate molecules converted per enzyme molecule per unit time at saturation - **kcat/Km** incorporates both kcat and Km, providing overall efficiency rather than just turnover rate - Turnover is only one component of the efficiency measure
Question 37: What is the typical Q10 value for enzymatic reactions?
- A. 2 (Correct Answer)
- B. 3
- C. 4
- D. 5
Explanation: ***2*** - The **Q10 value** represents the factor by which the rate of a reaction increases for every 10°C rise in temperature. - For most enzymatic and biological reactions, the **Q10 value** is typically around **2 to 3**. *3* - While **3** is within the typical range for some biological reactions, **2** is often considered the most common or average value cited for enzymatic reactions. - A **Q10 of 3** means the reaction rate triples with a 10°C increase, which is observed in certain cases but is not the most general "typical" value. *4* - A **Q10 value of 4** indicates a significantly higher temperature sensitivity than what is commonly observed for most enzymatic reactions. - Such a high Q10 would imply that the reaction rate quadruples for every 10°C increase, which is less typical. *5* - A **Q10 value of 5** is exceptionally high and rarely observed for common enzymatic reactions under physiological conditions. - This would suggest an extreme sensitivity to temperature changes, which is not characteristic of most enzyme kinetics.
Question 38: What type of enzyme is hexokinase?
- A. Ligase
- B. Transferase (Correct Answer)
- C. Oxidoreductase
- D. Reductase
Explanation: ***Transferase*** - Hexokinase catalyzes the transfer of a **phosphate group** from **ATP** to glucose, forming glucose-6-phosphate. - Enzymes that catalyze the transfer of functional groups from one molecule to another are classified as **transferases**. *Ligase* - **Ligases** are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, usually accompanied by the hydrolysis of a small pendant chemical group on one of the larger molecules or the less-stable of the two products. - This activity usually involves reactions like **DNA ligation**, not phosphate group transfer to a sugar. *Oxidoreductase* - **Oxidoreductases** catalyze **oxidation-reduction reactions**, involving the transfer of electrons from one molecule to another. - Hexokinase does not perform redox reactions; it transfers a phosphate group. *Reductase* - **Reductases** are a specific type of **oxidoreductase** that catalyze reactions where a molecule is reduced (gains electrons). - This is a subset of oxidation-reduction chemistry and is not the function of hexokinase.
Question 39: According to IUB system, hydrolases belong to which class?
- A. EC-1
- B. EC-2
- C. EC-3 (Correct Answer)
- D. EC-4
Explanation: ***EC-3*** - **Hydrolases** catalyze the **hydrolysis** of chemical bonds, which involves the addition of water to break the bond. - This class includes enzymes like **esterases**, **peptidases**, and **glycosidases**, all of which use water to cleave molecules. *EC-1* - **EC-1** refers to **oxidoreductases**, which catalyze **oxidation-reduction reactions**. - These enzymes are involved in the transfer of electrons or hydrogen atoms, not the hydrolysis of bonds. *EC-2* - **EC-2** represents **transferases**, enzymes that catalyze the **transfer of a functional group** from one molecule to another. - Examples include **kinases** and **transaminases**, which are distinct from hydrolytic enzymes. *EC-4* - **EC-4** encompasses **lyases**, which catalyze the **cleavage of various bonds** by means other than hydrolysis or oxidation, often forming double bonds. - This class includes enzymes like **decarboxylases** and **aldolases**, which are not primarily involved in breaking bonds with water.
Question 40: Which of the following is a natural uncoupler found in brown adipose tissue?
- A. Thermogenin (Correct Answer)
- B. 2,4-Nitrophenol
- C. 2,4-Dinitrophenol
- D. Oligomycin
Explanation: ***Correct: Thermogenin*** - Also known as **uncoupling protein 1 (UCP1)**, it is a **mitochondrial inner membrane protein** naturally expressed in **brown adipose tissue** - Thermogenin creates a **proton leak** across the inner mitochondrial membrane, bypassing ATP synthase and dissipating the proton gradient as heat, thereby mediating **non-shivering thermogenesis** - This is the only natural uncoupler among the options listed *Incorrect: 2,4-Nitrophenol* - This compound is **not a naturally occurring uncoupler** in mammalian tissues - While it can act as a synthetic uncoupler in laboratory settings, it is not found in biological systems *Incorrect: 2,4-Dinitrophenol* - This is a well-known **synthetic chemical uncoupler** of oxidative phosphorylation, historically used as a weight-loss drug (now banned due to toxicity) - It works by carrying protons across the inner mitochondrial membrane, but it is **not a natural biological molecule** found in the body *Incorrect: Oligomycin* - Oligomycin is an **inhibitor of ATP synthase (Complex V)**, not an uncoupler - It binds to the F0 subunit of ATP synthase, blocking the flow of protons through the enzyme and thereby preventing ATP synthesis - This blocks both the proton gradient dissipation AND ATP production, which is mechanistically different from uncoupling