Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate — MCQs
The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
Which of the following is NOT true regarding the role of NAD+?
Which of the following statements about the electron transport chain is CORRECT?
What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
In the malate shuttle, how many ATPs are produced from one NADH?
Kreb's cycle and urea cycle are linked by-
Which of the following is active in dephosphorylated state?
During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
NADPH is required in which of the following cellular processes?
All are cofactors for Dehydrogenase except:
Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate Indian Medical PG Practice Questions and MCQs
Question 1: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
- A. Cyanide (Correct Answer)
- B. Carbon dioxide
- C. Oligomycin
- D. Ouabain
Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.
Question 2: Which of the following is NOT true regarding the role of NAD+?
- A. Acts as an electron carrier
- B. Functions as an antioxidant (Correct Answer)
- C. Participates in glycolysis
- D. Involved in TCA cycle
Explanation: ***Functions as an antioxidant*** - **NAD+** primarily functions as an **electron carrier** in redox reactions, not as an antioxidant that directly neutralizes reactive oxygen species. - While it plays a role in maintaining cellular redox balance, its direct function is not scavenging free radicals like **glutathione** or **vitamins C and E**. *Acts as an electron carrier* - **NAD+** is a crucial coenzyme that accepts electrons and protons during metabolic reactions, converting into **NADH**. - **NADH** then donates these electrons to the **electron transport chain** to generate **ATP**. *Participates in glycolysis* - In glycolysis, **NAD+** is reduced to **NADH** during the oxidation of **glyceraldehyde-3-phosphate** to **1,3-bisphosphoglycerate**. - This step is vital for producing **ATP** and regenerating **NAD+** for continued glycolytic flux. *Involved in TCA cycle* - **NAD+** is reduced to **NADH** at several steps in the **TCA cycle**, including the conversion of **isocitrate to α-ketoglutarate**, **α-ketoglutarate to succinyl CoA**, and **malate to oxaloacetate**. - These **NADH** molecules are then funneled into the **electron transport chain** for oxidative phosphorylation.
Question 3: Which of the following statements about the electron transport chain is CORRECT?
- A. NADH enters at Complex I
- B. FADH2 enters at Complex II
- C. FADH2 gives 1.5 ATP (Correct Answer)
- D. NADH gives 3 ATP
Explanation: ***FADH2 gives 1.5 ATP*** - Each **FADH2** molecule that enters the electron transport chain generates approximately **1.5 ATP molecules** via oxidative phosphorylation based on modern P/O ratio calculations. - FADH2 bypasses Complex I and enters at **Complex II (succinate dehydrogenase)**, thus contributing to fewer proton pumping sites (only Complexes III and IV) compared to NADH. - This is the modern, accurate value based on current understanding of mitochondrial bioenergetics. *NADH enters at Complex I* - While this statement is **factually true**, NADH entering at Complex I is well-established biochemistry. - However, when combined with the context of other options, **option C provides the most clinically relevant quantitative information** about ATP yield. - NADH oxidation at Complex I pumps protons at three sites (Complexes I, III, and IV). *FADH2 enters at Complex II* - This statement is also **factually correct** - FADH2 does enter the electron transport chain at Complex II. - However, without mentioning the ATP yield, this statement is less complete than option C which provides quantitative information. *NADH gives 3 ATP* - This is **INCORRECT** based on modern biochemistry. - The current accepted value is approximately **2.5 ATP** per NADH molecule. - The older estimate of 3 ATP was based on integer P/O ratios that didn't account for the energy cost of ATP/ADP translocase and phosphate transporter.
Question 4: What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
- A. NAD
- B. NADP (Correct Answer)
- C. FAD
- D. FMN
Explanation: ***NADP*** - **NADP+** (nicotinamide adenine dinucleotide phosphate) acts as the **electron acceptor** in the **glucose-6-phosphate dehydrogenase (G6PD)** reaction, becoming **NADPH**. - **NADPH** is crucial for maintaining the **redox balance** in cells, particularly in red blood cells, by reducing **oxidative stress**. *NAD* - **NAD+** (nicotinamide adenine dinucleotide) is a primary cofactor for many **dehydrogenase reactions** in catabolic pathways like **glycolysis** and the **Krebs cycle**. - It primarily functions as an electron acceptor in pathways that generate **ATP**, distinct from the role of **NADPH** in reductive biosynthesis and antioxidant defense. *FAD* - **FAD** (flavin adenine dinucleotide) is a coenzyme derived from **riboflavin (vitamin B2)** that is involved in various redox reactions, often in the form of **flavoproteins**. - Enzymes like **succinate dehydrogenase** in the electron transport chain utilize **FAD** as an electron acceptor, which is not the case for G6PD. *FMN* - **FMN** (flavin mononucleotide) is another coenzyme derived from **riboflavin**, structurally similar to FAD but lacking the additional adenosine monophosphate. - It participates in electron transfer reactions, particularly within **complex I** of the **electron transport chain**, but is not a cofactor for G6PD.
Question 5: In the malate shuttle, how many ATPs are produced from one NADH?
- A. 1 ATP
- B. 3 ATP
- C. 2 ATP
- D. 2.5 ATP (Correct Answer)
Explanation: ***2.5 ATP*** - In the **malate-aspartate shuttle**, mitochondrial **NADH** is regenerated from cytosolic NADH, and then enters the electron transport chain at **Complex I**. - **Complex I** entry means that NADH contributes to the pumping of enough protons to generate approximately **2.5 ATP** through oxidative phosphorylation. *1 ATP* - **1 ATP** is not the direct equivalent produced from the reoxidation of one NADH via the malate shuttle into the electron transport chain. - This value is typically associated with the direct hydrolysis of **ATP** or the energy equivalent of **GTP** produced in the citric acid cycle. *3 ATP* - Historically, **3 ATP** was the accepted stoichiometry for one NADH, but more accurate measurements of proton pumping and ATP synthase activity have revised this. - The value of 3 ATP per NADH does not reflect the most current understanding of mitochondrial bioenergetics. *2 ATP* - **2 ATP** is the approximate yield for **FADH2** entering the electron transport chain at **Complex II**, bypassing Complex I, and thus pumping fewer protons. - This value is not applicable to NADH transferred via the malate-aspartate shuttle, as NADH enters at Complex I.
Question 6: Kreb's cycle and urea cycle are linked by-
- A. Malate
- B. Succinate
- C. α-ketoglutarate
- D. Fumarate (Correct Answer)
Explanation: ***Fumarate*** - **Fumarate** is a key intermediate produced in the **urea cycle** during the conversion of argininosuccinate to arginine, which then enters the **Krebs cycle** to be converted into malate and then oxaloacetate. - This molecule acts as a direct link, allowing metabolic crosstalk between the two cycles. *Malate* - While **malate** is an intermediate in the Krebs cycle and is derived from fumarate, it is not the direct molecule that links the two cycles. - Malate is formed in the cytoplasm from fumarate but must be transported into the mitochondria to continue in the Krebs cycle. *α-ketoglutarate* - **α-ketoglutarate** is an important intermediate in the Krebs cycle involved in amino acid metabolism, but it does not directly link the urea cycle to the Krebs cycle. - It plays a role in nitrogen metabolism by accepting amino groups, but not in the *direct* transference of carbon skeletons between the cycles in the same way fumarate does. *Succinate* - **Succinate** is an intermediate of the Krebs cycle that is formed from succinyl CoA, but it does not directly participate in the urea cycle as a connecting molecule. - Its primary role is in **oxidative phosphorylation** as it is converted to fumarate by succinate dehydrogenase within the electron transport chain.
Question 7: Which of the following is active in dephosphorylated state?
- A. PEPCK
- B. Pyruvate Carboxylase
- C. Glycogen Synthase (Correct Answer)
- D. Glycogen Phosphorylase
Explanation: ***Glycogen Synthase*** - **Glycogen synthase** is primarily active in its **dephosphorylated state**, which is promoted by insulin and signals glycogen synthesis. - Dephosphorylation relieves the inhibitory effect of phosphorylation, allowing the enzyme to efficiently add glucose units to a **growing glycogen chain**. *PEPCK* - **Phosphoenolpyruvate carboxykinase (PEPCK)** activity is primarily regulated at the transcriptional level, not typically by phosphorylation state for activation. - Its expression is induced by **glucagon** and **cortisol** during gluconeogenesis. *Pyruvate Carboxylase* - **Pyruvate carboxylase** is allosterically activated by **acetyl-CoA** and its activity is not directly regulated by phosphorylation/dephosphorylation in the same manner as glycogen synthase. - This enzyme plays a key role in **gluconeogenesis** by converting pyruvate to oxaloacetate. *Glycogen Phosphorylase* - **Glycogen phosphorylase** is active in its **phosphorylated state**, particularly the 'a' form, which is promoted by glucagon and adrenaline for glycogen breakdown. - Phosphorylation activates the enzyme, leading to the **breakdown of glycogen** into glucose-1-phosphate.
Question 8: During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
- A. Phosphofructokinase
- B. Phosphocreatine (Correct Answer)
- C. Glucose 1 - phosphate
- D. Creatine phosphokinase
Explanation: ***Phosphocreatine*** - **Phosphocreatine (PCr)** is the primary energy source for a **100m sprint** (lasting 10-20 seconds). - The **ATP-PC (phosphagen) system** provides **immediate energy** by rapidly regenerating **ATP** from ADP through the transfer of a high-energy phosphate group. - This system is crucial for **short bursts of maximal intensity exercise** where energy demand exceeds the capacity of glycolysis and oxidative phosphorylation to respond quickly enough. - Phosphocreatine stores can fuel maximum effort for approximately **10-15 seconds**, making it ideal for sprint activities. *Phosphofructokinase* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, not an energy substrate. - While PFK-catalyzed glycolysis contributes ATP during intense exercise, it cannot provide energy as rapidly as the phosphocreatine system. - Glycolysis becomes more prominent after the first 10-15 seconds of maximal effort. *Glucose 1-phosphate* - **Glucose 1-phosphate** is an intermediate in **glycogenolysis** (breakdown of glycogen to glucose-6-phosphate). - It is part of the pathway leading to glucose availability for glycolysis, but is not a **direct, immediate energy source** for muscle contraction. - Unlike phosphocreatine, it cannot directly regenerate ATP. *Creatine phosphokinase* - **Creatine phosphokinase (CPK)**, also known as **creatine kinase (CK)**, is the **enzyme** that catalyzes the reversible transfer of phosphate from phosphocreatine to ADP. - It facilitates the energy transfer reaction but is **not an energy substrate** itself. - The enzyme enables the phosphocreatine system to function, but the actual energy comes from phosphocreatine.
Question 9: NADPH is required in which of the following cellular processes?
- A. HMP shunt
- B. Glycogenolysis
- C. Gluconeogenesis
- D. Lipogenesis (Correct Answer)
Explanation: ***Lipogenesis*** - **NADPH** is critically required for anabolic processes such as **fatty acid synthesis** (lipogenesis), where it acts as a **reducing agent**. - It supplies the electrons necessary for the sequential reduction steps in the conversion of acetyl-CoA to fatty acids in the cytoplasm. *HMP shunt* - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is the primary cellular source of **NADPH**. - Therefore, it produces NADPH rather than requiring it as a substrate for its main function. *Gluconeogenesis* - **Gluconeogenesis** is the metabolic pathway that produces **glucose** from non-carbohydrate precursors. - This process primarily uses **ATP** and **GTP** as energy sources, and NADH (not NADPH) is involved in some reduction reactions. *Glycogenolysis* - **Glycogenolysis** is the breakdown of **glycogen** into glucose-6-phosphate and then glucose. - This catabolic process does not directly require **NADPH**; instead, it releases glucose for energy or other metabolic uses.
Question 10: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
Explanation: ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.
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