Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 611: Which of the following enzymes is responsible for the release of free glucose from glycogen during glycogenolysis in muscle?

A. Glycogen phosphorylase
B. Glucose-1-phosphatase
C. Glucose-6-phosphatase
D. Debranching enzyme (Correct Answer)

Explanation: ### Explanation The correct answer is **Debranching enzyme**. **1. Why Debranching Enzyme is Correct:** Glycogenolysis primarily produces Glucose-1-Phosphate via glycogen phosphorylase. However, the debranching enzyme is a bifunctional protein. While its first activity is transferase, its second activity is **$\alpha$-1,6-glucosidase**. This specific action hydrolytically cleaves the $\alpha$-1,6-glycosidic bond at the branch point, releasing a molecule of **free glucose** (approximately 8–10% of glycogen breakdown products). In the muscle, this is the only mechanism to produce free glucose because muscle lacks the enzyme to dephosphorylate glucose-6-phosphate. **2. Analysis of Incorrect Options:** * **A. Glycogen phosphorylase:** This is the rate-limiting enzyme of glycogenolysis. It breaks $\alpha$-1,4 bonds to release **Glucose-1-Phosphate**, not free glucose. * **B. Glucose-1-phosphatase:** This enzyme does not play a physiological role in human glycogen metabolism. * **C. Glucose-6-phosphatase:** This enzyme converts Glucose-6-Phosphate to free glucose. It is present in the **liver and kidney** but is notably **absent in muscle**. Therefore, muscle cannot contribute to blood glucose levels; it uses glycogen strictly for its own energy needs. **3. NEET-PG High-Yield Pearls:** * **Cori’s Disease (GSD Type III):** Caused by a deficiency of the Debranching enzyme. It presents with hepatomegaly and hypoglycemia, but symptoms are often milder than Von Gierke’s. * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. * **Product Ratio:** For every branch point, glycogenolysis yields roughly **8-10 molecules of G-1-P** (via phosphorylase) for every **1 molecule of free glucose** (via debranching enzyme). * **Muscle Metabolism:** Because muscle lacks Glucose-6-phosphatase, the glucose-6-phosphate produced enters the glycolytic pathway to generate ATP for contraction.

Question 612: The L or D configuration of a carbohydrate is determined by its stereochemical relationship to which of the following compounds?

A. Fructose
B. Glycogen
C. Glyceraldehyde (Correct Answer)
D. Glucose

Explanation: ### Explanation The D and L configuration of carbohydrates is based on the **Rosanoff convention**, which uses **glyceraldehyde** as the reference standard. Glyceraldehyde is the simplest aldose containing a chiral center (asymmetric carbon). **Why Glyceraldehyde is the Correct Answer:** The configuration is determined by the orientation of the hydroxyl (-OH) group on the **penultimate carbon** (the chiral carbon furthest from the carbonyl group). * If the -OH group is on the **right**, it is the **D-isomer** (Dextro). * If the -OH group is on the **left**, it is the **L-isomer** (Levo). Because all higher sugars can be chemically derived from or related back to glyceraldehyde, it serves as the structural template for this nomenclature. **Analysis of Incorrect Options:** * **Fructose & Glucose:** While these are common hexoses, they are classified as D or L based on their relationship to glyceraldehyde, not the other way around. Most naturally occurring sugars in humans are D-isomers. * **Glycogen:** This is a complex polysaccharide (homopolymer of glucose) used for storage; it is not a reference molecule for stereoisomerism. **NEET-PG High-Yield Pearls:** 1. **Biological Preference:** In the human body, almost all utilized sugars are **D-isomers** (e.g., D-glucose), whereas almost all amino acids are **L-isomers**. 2. **Enzymatic Specificity:** Enzymes in the human body are stereospecific; for example, salivary amylase acts on D-glucose polymers but cannot act on L-glucose. 3. **Optical Activity:** D and L designations refer to structural configuration and do not necessarily indicate the direction of optical rotation (+/- or d/l). A D-sugar can be levorotatory (e.g., D-fructose).

Question 613: Gluconeogenesis can occur from all except?

A. Lactic acid
B. Acetoacetate (Correct Answer)
C. Glycerol
D. Alanine

Explanation: **Explanation:** The core concept of gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. To be "glucogenic," a substance must be capable of being converted into **Pyruvate** or an intermediate of the **TCA cycle** (like Oxaloacetate). **Why Acetoacetate is the correct answer:** Acetoacetate is a **ketone body**. In the body, fatty acids and ketogenic substances are broken down into **Acetyl-CoA**. In humans, the conversion of Acetyl-CoA to Pyruvate is irreversible because the Pyruvate Dehydrogenase (PDH) complex reaction is unidirectional. Since Acetyl-CoA cannot be converted back to glucose, purely ketogenic substances like acetoacetate and even-chain fatty acids cannot serve as precursors for gluconeogenesis. **Analysis of Incorrect Options:** * **Lactic Acid:** Produced via anaerobic glycolysis, it enters the **Cori Cycle** where it is transported to the liver and converted into Pyruvate (by LDH), then to glucose. * **Glycerol:** Derived from triacylglycerol breakdown, it is phosphorylated to glycerol-3-phosphate and then converted to **Dihydroxyacetone phosphate (DHAP)**, an intermediate of glycolysis/gluconeogenesis. * **Alanine:** This is the primary glucogenic amino acid. Through the **Cahill Cycle** (Glucose-Alanine cycle), it undergoes transamination to form **Pyruvate**. **High-Yield Clinical Pearls for NEET-PG:** * **Leucine and Lysine** are the only two amino acids that are **purely ketogenic** (cannot form glucose). * While even-chain fatty acids are not glucogenic, **Odd-chain fatty acids** are glucogenic because their terminal metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * The major site of gluconeogenesis is the **Liver**, followed by the Kidney cortex.

Question 614: Dehydrogenase enzymes in the HMP shunt's oxidative phase generate which of the following?

A. NADP+
B. NADPH (Correct Answer)
C. FAD+
D. FADH

Explanation: **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and consists of two phases: oxidative and non-oxidative. **Why NADPH is the correct answer:** The **oxidative phase** is irreversible and involves two key dehydrogenase enzymes: 1. **Glucose-6-Phosphate Dehydrogenase (G6PD):** The rate-limiting enzyme that converts Glucose-6-P to 6-Phosphogluconolactone. 2. **6-Phosphogluconate Dehydrogenase:** Converts 6-Phosphogluconate to Ribulose-5-Phosphate. Both these enzymes utilize **NADP+ as a coenzyme** and reduce it to **NADPH**. Therefore, the primary products of the oxidative phase are NADPH (used for reductive biosynthesis) and Pentoses (used for nucleotide synthesis). **Why other options are incorrect:** * **NADP+:** This is the *oxidized* form used as a substrate/coenzyme; it is consumed, not generated, by the dehydrogenases. * **FAD+ / FADH:** These flavin nucleotides are typically involved in the TCA cycle (e.g., Succinate dehydrogenase) and the Mitochondrial Electron Transport Chain, not the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **Tissues involved:** The HMP shunt is highly active in tissues requiring NADPH for lipid/steroid synthesis (Adrenal cortex, Liver, Lactating mammary gland) or for maintaining reduced glutathione (Erythrocytes). * **G6PD Deficiency:** This is the most common enzymopathy. A lack of NADPH in RBCs leads to an inability to regenerate reduced glutathione, resulting in hemoglobin oxidation, **Heinz bodies**, and hemolytic anemia under oxidative stress (e.g., Fava beans, Primaquine). * **Regulation:** The pathway is regulated by the **NADPH/NADP+ ratio**; high levels of NADPH inhibit G6PD.

Question 615: In which metabolic pathway is NADPH primarily utilized?

A. Fatty acid synthesis (Correct Answer)
B. Gluconeogenesis
C. Beta oxidation
D. Glycogenolysis

Explanation: **Explanation:** The primary role of **NADPH** (Nicotinamide Adenine Dinucleotide Phosphate) is to serve as a reducing agent in **reductive biosynthesis** and to protect cells against oxidative stress. **1. Why Fatty Acid Synthesis is Correct:** Fatty acid synthesis (lipogenesis) occurs in the cytosol and requires a significant amount of reducing power to convert acetyl-CoA and malonyl-CoA into long-chain fatty acids. The enzyme **Fatty Acid Synthase (FAS)** complex utilizes NADPH at two specific steps: the reduction of the keto group and the reduction of the double bond. The major sources of this NADPH are the **Hexose Monophosphate (HMP) Shunt** (via G6PD) and the **Malic Enzyme** reaction. **2. Why the Other Options are Incorrect:** * **Gluconeogenesis:** This is an anabolic pathway but primarily utilizes **NADH** (at the glyceraldehyde-3-phosphate dehydrogenase step) rather than NADPH. * **Beta-oxidation:** This is a catabolic process (breakdown of fats) that **produces** energy in the form of **FADH₂ and NADH**, which then enter the electron transport chain. * **Glycogenolysis:** This involves the phosphorolysis of glycogen into glucose-1-phosphate. It is a simple degradative process that does not involve redox cofactors like NADPH. **High-Yield Clinical Pearls for NEET-PG:** * **NADPH Producers:** HMP Shunt (Main source), Malic Enzyme, and Isocitrate Dehydrogenase (cytosolic). * **NADPH Consumers:** Fatty acid synthesis, Cholesterol synthesis, Steroid hormone synthesis, and **Microsomal Cytochrome P450 system**. * **Erythrocyte Integrity:** NADPH is essential for the **Glutathione Reductase** enzyme, which keeps glutathione in its reduced state to neutralize H₂O₂. A deficiency in NADPH (as seen in **G6PD deficiency**) leads to hemolysis. * **Mnemonic:** NADPH is for **"P"** (Positive/Building/Production) — Anabolic pathways.

Question 616: Which enzyme is NOT involved in substrate-level phosphorylation?

A. Succinyl thiokinase
B. Phosphofructokinase (Correct Answer)
C. Pyruvate kinase
D. Phosphoglycerate kinase

Explanation: ### Explanation **Substrate-Level Phosphorylation (SLP)** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, without the requirement of oxygen or the electron transport chain. **Why Phosphofructokinase (PFK) is the correct answer:** Phosphofructokinase is the rate-limiting enzyme of glycolysis. It catalyzes the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. This reaction is a **phosphorylation** step that **consumes** one molecule of ATP rather than generating it. Therefore, it is not involved in substrate-level phosphorylation. **Analysis of Incorrect Options:** * **Succinyl thiokinase (Succinyl-CoA Synthetase):** Involved in the **TCA Cycle**. It converts Succinyl-CoA to Succinate, generating one molecule of **GTP** (equivalent to ATP) via SLP. This is the only SLP step in the mitochondria. * **Phosphoglycerate kinase:** Involved in **Glycolysis**. It converts 1,3-bisphosphoglycerate to 3-phosphoglycerate, yielding one ATP. * **Pyruvate kinase:** The final step of **Glycolysis**. It converts Phosphoenolpyruvate (PEP) to Pyruvate, yielding one ATP. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP yield:** In aerobic glycolysis, SLP produces 4 ATP (net 2); in the TCA cycle, SLP produces 1 GTP per turn. * **PFK-1 Regulation:** It is the "Pacemaker of Glycolysis," inhibited by ATP and Citrate, and activated by AMP and Fructose-2,6-bisphosphate. * **Arsenic Poisoning:** Arsenite inhibits the pyruvate dehydrogenase complex, while **Arsenate** bypasses the SLP step of Phosphoglycerate kinase, resulting in zero net ATP gain during glycolysis.

Question 617: What is the net ATP formed in glycolysis?

A. 5
B. 7 (Correct Answer)
C. 10
D. 15

Explanation: In aerobic glycolysis, the net ATP yield is calculated by subtracting the energy consumed from the energy produced. **1. Why Option B (7 ATP) is Correct:** The net yield is derived from two stages: * **ATP Consumption:** 2 ATP are used in the preparatory phase (Hexokinase and Phosphofructokinase-1 reactions). * **ATP Production:** * **Substrate-Level Phosphorylation:** 4 ATP are produced (2 from Phosphoglycerate kinase and 2 from Pyruvate kinase). * **Oxidative Phosphorylation:** 2 NADH are produced (Glyceraldehyde-3-phosphate dehydrogenase step). In the Malate-Aspartate shuttle (predominant in heart, liver, and kidney), each NADH yields 2.5 ATP. * **Calculation:** (4 ATP + 5 ATP from NADH) - 2 ATP = **7 ATP**. *(Note: If the Glycerol-3-phosphate shuttle is used, the yield is 1.5 ATP per NADH, totaling 3 ATP, resulting in a net of 5 ATP. However, 7 is the standard high-yield answer for aerobic glycolysis in most tissues).* **2. Why other options are incorrect:** * **Option A (5):** This is the net yield if the **Glycerol-3-phosphate shuttle** is used (common in skeletal muscle/brain). * **Option C (10):** This represents the total (gross) ATP produced before subtracting the 2 ATP consumed. * **Option D (15):** This does not correspond to any standard calculation for a single cycle of glycolysis. **NEET-PG Clinical Pearls:** * **Anaerobic Glycolysis:** The net yield is always **2 ATP** because NADH is consumed to reduce pyruvate to lactate. * **Rate-limiting enzyme:** Phosphofructokinase-1 (PFK-1). * **Arsenic Poisoning:** Inhibits ATP production at the Glyceraldehyde-3-phosphate dehydrogenase step by bypassing substrate-level phosphorylation, resulting in a net yield of **0 ATP**. * **Mature RBCs:** Always produce a net of 2 ATP as they lack mitochondria and rely solely on anaerobic glycolysis.

Question 618: What is the primary metabolic effect of insulin?

A. Inhibition of glycolysis
B. Stimulation of gluconeogenesis
C. Induction of lipogenesis (Correct Answer)
D. Increased glycogenolysis

Explanation: **Explanation:** Insulin is the body’s primary **anabolic hormone**, secreted by the pancreatic beta cells in response to high blood glucose levels (fed state). Its overarching goal is to lower blood glucose by promoting storage and utilization while inhibiting the production of new glucose. **Why Option C is Correct:** Insulin promotes **lipogenesis** (fat synthesis) through several mechanisms. It increases the activity of **Acetyl-CoA Carboxylase (ACC)**, the rate-limiting enzyme in fatty acid synthesis. Furthermore, insulin enhances glucose uptake into adipocytes via **GLUT-4** transporters, providing the glycerol-3-phosphate backbone required for triglyceride synthesis. It also inhibits Hormone-Sensitive Lipase (HSL), thereby preventing lipolysis. **Why Other Options are Incorrect:** * **A. Inhibition of glycolysis:** Incorrect. Insulin **stimulates** glycolysis by inducing key enzymes like Glucokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase to utilize glucose for energy. * **B. Stimulation of gluconeogenesis:** Incorrect. Insulin **represses** the genes for PEPCK and Glucose-6-Phosphatase, the key enzymes of gluconeogenesis, to prevent the liver from producing excess glucose. * **D. Increased glycogenolysis:** Incorrect. Insulin inhibits glycogen breakdown and instead **stimulates glycogenesis** (glycogen synthesis) by activating Glycogen Synthase. **NEET-PG High-Yield Pearls:** * **GLUT-4:** The only insulin-dependent glucose transporter, found primarily in skeletal muscle and adipose tissue. * **Key Enzyme Activation:** Insulin acts via a **tyrosine kinase receptor** pathway, leading to the dephosphorylation (activation) of Glycogen Synthase and the dephosphorylation (inactivation) of Glycogen Phosphorylase. * **Potassium Shift:** Insulin promotes the entry of $K^+$ into cells; hence, it is used clinically in the emergency management of hyperkalemia.

Question 619: Which of the following is NOT gluconeogenic?

A. Acetyl CoA (Correct Answer)
B. Lactate
C. Glycerol
D. Alanine

Explanation: **Explanation:** The core concept in gluconeogenesis is the ability of a molecule to be converted into **Oxaloacetate (OAA)** or other intermediates of the TCA cycle that can eventually form glucose. **1. Why Acetyl CoA is the Correct Answer:** Acetyl CoA cannot be used for the net synthesis of glucose in humans for two primary reasons: * **The PDH Reaction is Irreversible:** The conversion of Pyruvate to Acetyl CoA by the Pyruvate Dehydrogenase (PDH) complex is irreversible. There is no enzyme in humans to convert Acetyl CoA back into Pyruvate. * **Carbon Loss in TCA Cycle:** When Acetyl CoA enters the TCA cycle, its two carbons are lost as two molecules of $CO_2$ before it reaches Oxaloacetate. Therefore, there is **no net gain** of carbon atoms to contribute to glucose synthesis. **2. Why the other options are incorrect (They ARE Gluconeogenic):** * **Lactate:** Produced by anaerobic glycolysis, it is converted to Pyruvate by *Lactate Dehydrogenase* (Cori Cycle) and enters gluconeogenesis. * **Glycerol:** Derived from triglyceride breakdown, it is phosphorylated to Glycerol-3-Phosphate and then to Dihydroxyacetone phosphate (DHAP), a direct intermediate of glycolysis/gluconeogenesis. * **Alanine:** The primary glucogenic amino acid. It undergoes transamination to form Pyruvate (Cahill Cycle). **High-Yield Clinical Pearls for NEET-PG:** * **Odd-chain fatty acids** are gluconeogenic because their terminal product is **Propionyl CoA**, which enters the TCA cycle as Succinyl CoA. Even-chain fatty acids (producing Acetyl CoA) are NOT gluconeogenic. * **Leucine and Lysine** are the only purely ketogenic amino acids; they cannot form glucose. * **Pyruvate Carboxylase** is the key regulatory enzyme that bypasses the PDH block by converting Pyruvate directly to Oxaloacetate (requires Biotin).

Question 620: Which step in the HMP pathway requires Thiamine Pyrophosphate (TPP)?

A. Glucose-6-phosphate dehydrogenase (G6PD)
B. 6-Phosphogluconate dehydrogenase
C. Transketolase (Correct Answer)
D. Transaldolase

Explanation: ### Explanation **1. Why Transketolase is Correct:** The Hexose Monophosphate (HMP) Shunt consists of an irreversible oxidative phase and a reversible non-oxidative phase. **Transketolase** is a key enzyme in the non-oxidative phase that catalyzes the transfer of 2-carbon units between sugar phosphates. This enzyme requires **Thiamine Pyrophosphate (TPP)**, a derivative of Vitamin B1, as a mandatory cofactor. It serves as a bridge between the HMP shunt and glycolysis by recycling pentose phosphates back into glycolytic intermediates (Glyceraldehyde-3-P and Fructose-6-P). **2. Why Other Options are Incorrect:** * **Options A & B (G6PD and 6-Phosphogluconate dehydrogenase):** These are the regulatory enzymes of the *oxidative* phase. They require **NADP+** as a cofactor to produce NADPH; they do not require TPP. * **Option D (Transaldolase):** While this enzyme also functions in the non-oxidative phase (transferring 3-carbon units), it does **not** require any cofactor. It utilizes a Schiff base mechanism involving a lysine residue at its active site. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Utility:** Erythrocyte transketolase activity is the **most sensitive biochemical marker** to diagnose Thiamine (B1) deficiency. An increase in enzyme activity after adding TPP in vitro confirms the deficiency. * **Wernicke-Korsakoff Syndrome:** Some individuals have a genetic variation in transketolase that reduces its affinity for TPP, making them more susceptible to neuropsychiatric symptoms during thiamine deficiency (often seen in chronic alcoholism). * **Mnemonic:** TPP is required by "The **A**lpha-**K**eto **T**eam": **A**lpha-ketoglutarate dehydrogenase, **K**eto-acid dehydrogenase (branched-chain), **T**ransketolase, and Pyruvate dehydrogenase.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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