Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 301: Which enzyme of glycolysis is also used in gluconeogenesis?

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

Explanation: **Explanation:** The core concept in understanding the relationship between glycolysis and gluconeogenesis is identifying **reversible** versus **irreversible** steps. While glycolysis and gluconeogenesis share many enzymes, they differ at three critical regulatory checkpoints. **Why Phosphohexose Isomerase is Correct:** Phosphohexose isomerase (also known as Phosphoglucose isomerase) catalyzes the reversible conversion of **Glucose-6-Phosphate to Fructose-6-Phosphate**. Because the Gibbs free energy change ($\Delta G$) for this reaction is near zero, the enzyme functions in both directions depending on substrate concentration. Therefore, it is used in both glycolysis (breakdown) and gluconeogenesis (synthesis). **Why the Other Options are Incorrect:** Options A, B, and C represent the **three irreversible "bottleneck" steps** of glycolysis. These steps have a large negative $\Delta G$ and must be bypassed in gluconeogenesis by specific, different enzymes: * **Glucokinase/Hexokinase (Step 1):** Bypassed by *Glucose-6-phosphatase* in gluconeogenesis. * **Phosphofructokinase-1 (Step 3):** The rate-limiting step of glycolysis; bypassed by *Fructose-1,6-bisphosphatase*. * **Pyruvate Kinase (Step 10):** Bypassed by a two-step process involving *Pyruvate carboxylase* and *PEP carboxykinase (PEPCK)*. **High-Yield NEET-PG Pearls:** * **Reversible Enzymes:** All enzymes of glycolysis are shared with gluconeogenesis **EXCEPT** Hexokinase, PFK-1, and Pyruvate Kinase. * **Location:** Gluconeogenesis occurs primarily in the liver and kidney. * **Mnemonic for Irreversible Steps:** "**H**ighly **P**roud **P**yruvate" (Hexokinase, PFK-1, Pyruvate Kinase). * **Clinical Link:** Deficiencies in gluconeogenic enzymes (like Glucose-6-phosphatase) lead to **Von Gierke Disease**, characterized by severe fasting hypoglycemia.

Question 302: Which of the following is most effective for gluconeogenesis?

A. Fructose 2,6 bisphosphate inhibits fructose 1,6 bisphosphatase
B. Acetyl CoA activates pyruvate carboxylase (Correct Answer)
C. Acetyl CoA inhibits pyruvate carboxylase
D. Citrate activates acetyl CoA carboxylase

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. The regulation of this pathway is crucial for maintaining blood glucose levels during fasting. **Why Option B is Correct:** The conversion of pyruvate to oxaloacetate by **pyruvate carboxylase** is the first committed step of gluconeogenesis. This enzyme is an obligate allosteric enzyme that requires **Acetyl CoA** for its activation. When Acetyl CoA levels rise (typically from fatty acid oxidation during fasting), it signals that the TCA cycle is saturated and diverts pyruvate toward gluconeogenesis instead of oxidative decarboxylation. **Analysis of Incorrect Options:** * **Option A:** While it is true that Fructose 2,6-bisphosphate inhibits Fructose 1,6-bisphosphatase, this action **inhibits** gluconeogenesis. The question asks for what is effective *for* (promotes) the process. * **Option C:** This is the opposite of the physiological reality; Acetyl CoA is a potent activator, not an inhibitor, of pyruvate carboxylase. * **Option D:** Citrate does activate Acetyl CoA Carboxylase, but this is the rate-limiting step for **Fatty Acid Synthesis (Lipogenesis)**, not gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Biotin (Vitamin B7):** Pyruvate carboxylase requires Biotin as a cofactor. Deficiency (often due to raw egg white consumption/avidin) impairs gluconeogenesis. * **Reciprocal Regulation:** Acetyl CoA simultaneously inhibits the Pyruvate Dehydrogenase (PDH) complex while activating Pyruvate Carboxylase, ensuring pyruvate is not wasted in the TCA cycle when glucose is needed. * **Location:** Pyruvate carboxylase is a **mitochondrial** enzyme, whereas the subsequent steps of gluconeogenesis occur in the cytosol.

Question 303: Conversion of lactate to glucose requires all except?

A. Pyruvate carboxylase
B. Phosphofructokinase (Correct Answer)
C. PEP carboxykinase
D. Glucose-6-phosphatase

Explanation: ### Explanation The conversion of lactate to glucose occurs via **Gluconeogenesis**, primarily in the liver (Cori Cycle). Gluconeogenesis is not a simple reversal of glycolysis because three steps in glycolysis are irreversible. These "bottlenecks" must be bypassed by specific gluconeogenic enzymes. **Why Phosphofructokinase (PFK) is the correct answer:** PFK is a key regulatory enzyme of **Glycolysis**. It converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. In gluconeogenesis, this step must be bypassed. The enzyme used for the reverse reaction is **Fructose-1,6-bisphosphatase**. Therefore, PFK is not required for synthesizing glucose; in fact, it is inhibited during gluconeogenesis to prevent a futile cycle. **Analysis of Incorrect Options:** * **Pyruvate Carboxylase (A):** Required to bypass Pyruvate Kinase. It converts pyruvate (derived from lactate) into oxaloacetate. It requires **Biotin** as a cofactor. * **PEP Carboxykinase (C):** Required to convert oxaloacetate into Phosphoenolpyruvate (PEP). * **Glucose-6-phosphatase (D):** The final bypass enzyme found in the endoplasmic reticulum of the liver and kidney. It converts Glucose-6-Phosphate to free Glucose, allowing it to enter the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **The Cori Cycle:** Lactate produced by anaerobic glycolysis in muscles/RBCs travels to the liver to be converted back to glucose. * **Key Bypass Enzymes:** Remember the "Big 4" of Gluconeogenesis: Pyruvate Carboxylase, PEP Carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase. * **Energy Requirement:** Synthesis of 1 mole of glucose from 2 moles of lactate requires **6 ATP**. * **Deficiency:** Deficiency of Glucose-6-phosphatase leads to **Von Gierke’s Disease (GSD Type I)**, characterized by severe fasting hypoglycemia and lactic acidosis.

Question 304: Which of the following types of reaction does NOT occur in glycolysis?

A. Hydration (Correct Answer)
B. Isomerization
C. Phosphoryl transfer
D. Aldol cleavage

Explanation: **Explanation:** In glycolysis, glucose is converted into pyruvate through a series of ten enzymatic steps. To identify the correct answer, we must analyze the chemical transformations occurring in this pathway. **Why "Hydration" is the correct answer:** Hydration involves the **addition of water** across a double bond. In glycolysis, the reaction catalyzed by **Enolase** (Step 9) converts 2-phosphoglycerate to phosphoenolpyruvate (PEP). This is a **Dehydration** reaction (removal of water), not hydration. While hydration occurs in the TCA cycle (e.g., Fumarase reaction), it does not occur in glycolysis. **Analysis of Incorrect Options:** * **Isomerization:** Occurs multiple times. Examples include Glucose-6-P to Fructose-6-P (Phosphohexose isomerase) and Dihydroxyacetone phosphate to Glyceraldehyde-3-P (Triose phosphate isomerase). * **Phosphoryl transfer:** This is the hallmark of the "investment" and "payoff" phases. Kinases (Hexokinase, PFK-1, Phosphoglycerate kinase, and Pyruvate kinase) transfer phosphate groups between ATP/ADP and substrate molecules. * **Aldol cleavage:** Catalyzed by **Aldolase** (Step 4), which cleaves the 6-carbon Fructose-1,6-bisphosphate into two 3-carbon molecules (DHAP and Glyceraldehyde-3-P). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Enolase Inhibition:** Fluoride inhibits Enolase by competing with Magnesium. This is why fluoride is added to blood collection tubes (grey top) to prevent glycolysis during glucose estimation. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the Glyceraldehyde-3-P dehydrogenase reaction, leading to ATP bypass (zero net ATP production). * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG, which shifts the oxygen dissociation curve to the right.

Question 305: What is the first product of glycogenolysis?

A. Glucose-6-phosphate
B. Glucose-1,6-diphosphate
C. Glucose-1-phosphate (Correct Answer)
D. Fructose-1-phosphate

Explanation: ### Explanation **Correct Option: C. Glucose-1-phosphate** Glycogenolysis is the biochemical breakdown of glycogen into glucose. The process begins with the enzyme **Glycogen Phosphorylase**, which is the rate-limiting enzyme of this pathway. It acts on the $\alpha(1\to4)$ glycosidic bonds at the non-reducing ends of the glycogen chain. Instead of using water (hydrolysis), it uses inorganic phosphate ($P_i$) to cleave the bond—a process called **phosphorolysis**. This reaction releases **Glucose-1-phosphate (G1P)** as the primary initial product. **Analysis of Incorrect Options:** * **A. Glucose-6-phosphate:** While G1P is eventually converted to G6P by the enzyme *Phosphoglucomutase*, G6P is the *second* intermediate, not the first product. * **B. Glucose-1,6-diphosphate:** This is a transient intermediate/cofactor involved in the Phosphoglucomutase reaction, but it is not a primary product of glycogen breakdown. * **D. Fructose-1-phosphate:** This is an intermediate of **fructose metabolism** (fructolysis), produced by the action of Fructokinase in the liver. It has no role in glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **The 90:10 Rule:** Approximately 90% of glycogen breakdown yields **G1P** (via phosphorylase). The remaining 10% (at the $\alpha(1\to6)$ branch points) is released as **free glucose** by the Debranching enzyme ($\alpha$-1,6-glucosidase activity). * **Key Enzyme:** Glycogen Phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. * **Tissue Specificity:** In the **liver**, G6P is converted to free glucose (via Glucose-6-phosphatase) to maintain blood sugar. In **muscle**, Glucose-6-phosphatase is absent; therefore, G6P enters glycolysis to provide ATP for contraction. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase, leading to severe fasting hypoglycemia and hepatomegaly.

Question 306: Which metabolic process is responsible for the conversion of fat to glucose?

A. Glycolysis
B. Kreb's cycle
C. Gluconeogenesis (Correct Answer)
D. Saponification

Explanation: **Explanation:** **1. Why Gluconeogenesis is Correct:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. In the context of "fat to glucose," it is important to note that while humans cannot convert even-chain fatty acids into glucose, the **glycerol backbone** of triglycerides is a major substrate for gluconeogenesis. Glycerol is phosphorylated to glycerol-3-phosphate and then converted to dihydroxyacetone phosphate (DHAP), which enters the gluconeogenic pathway to form glucose. Additionally, odd-chain fatty acids yield propionyl-CoA, which enters the Kreb’s cycle as succinyl-CoA and can eventually be converted to glucose. **2. Why Other Options are Incorrect:** * **Glycolysis:** This is the catabolic process of breaking down glucose into pyruvate to produce ATP. It is the functional opposite of gluconeogenesis. * **Kreb’s Cycle (TCA Cycle):** While this cycle is a common oxidative pathway for carbohydrates, fats, and proteins, it primarily serves to generate reducing equivalents (NADH, FADH2) for ATP production. It does not directly synthesize glucose. * **Saponification:** This is a chemical process (hydrolysis of triglycerides with an alkali) used in soap making; it is not a metabolic pathway in the human body. **3. NEET-PG High-Yield Pearls:** * **Key Substrates:** The four primary substrates for gluconeogenesis are **Lactate** (Cori Cycle), **Glycerol** (from fat), **Glucogenic Amino Acids** (mainly Alanine), and **Propionyl-CoA** (from odd-chain fatty acids). * **Rate-Limiting Enzyme:** Fructose-1,6-bisphosphatase. * **Location:** Occurs primarily in the **Liver** (90%) and Kidney (10%). * **The "Fat" Rule:** Acetyl-CoA (from even-chain fatty acids) **cannot** be converted to glucose because the Pyruvate Dehydrogenase reaction is irreversible. Only the glycerol portion of fat is gluconeogenic in humans.

Question 307: Which of the following is an aldose sugar?

A. Fructose
B. Erythrulose
C. Glucose (Correct Answer)
D. None of the above

Explanation: **Explanation:** Monosaccharides are classified based on the functional group they contain: **Aldoses** contain an aldehyde group (-CHO) at the C1 position, while **Ketoses** contain a keto group (>C=O), usually at the C2 position. **Why Glucose is Correct:** Glucose is a 6-carbon monosaccharide (hexose) that contains an aldehyde group at its first carbon. Therefore, it is classified as an **aldohexose**. It is the primary source of energy for the body and the most abundant monosaccharide. **Analysis of Incorrect Options:** * **Fructose:** This is a 6-carbon sugar (hexose) but contains a keto group at the C2 position. It is classified as a **ketohexose**. It is the sweetest natural sugar and is metabolized primarily in the liver. * **Erythrulose:** This is a 4-carbon sugar (tetrose) containing a keto group. It is classified as a **ketotetrose**. Its aldose counterpart is Erythrose. **High-Yield NEET-PG Clinical Pearls:** 1. **Reducing Sugars:** All monosaccharides (both aldoses and ketoses) are reducing sugars because they have a free reactive carbonyl group. 2. **Isomerism:** Glucose and Fructose are **functional isomers** (same molecular formula $C_6H_{12}O_6$, different functional groups). 3. **Epimers:** Glucose and Galactose are C-4 epimers; Glucose and Mannose are C-2 epimers. 4. **Seliwanoff’s Test:** This biochemical test is used to distinguish between aldoses and ketoses (ketoses like fructose give a cherry-red color more rapidly). 5. **Essential Pentoses:** Ribose is an aldopentose (found in RNA), while Ribulose is a ketopentose (involved in the HMP shunt).

Question 308: During the conversion of glycerol to pyruvic acid, what is the first glycolytic intermediate to form?

A. 2-phosphoglyceric acid
B. 3-phosphoglyceric acid
C. 3-phosphoglyceraldehyde
D. Dihydroxyacetone phosphate (Correct Answer)

Explanation: ### Explanation The conversion of glycerol into the glycolytic pathway occurs primarily in the liver. This process involves two key enzymatic steps: 1. **Glycerol Kinase:** Glycerol is first phosphorylated to **Glycerol-3-phosphate** (using ATP). 2. **Glycerol-3-phosphate Dehydrogenase:** Glycerol-3-phosphate is then oxidized to **Dihydroxyacetone phosphate (DHAP)**, reducing $NAD^+$ to $NADH$. **DHAP** is the first true glycolytic intermediate formed. It can then be converted into Glyceraldehyde-3-phosphate by *Triose phosphate isomerase* to proceed through the remainder of glycolysis to form pyruvic acid. #### Analysis of Options: * **D (Correct):** DHAP is the entry point into glycolysis for glycerol. * **C (Incorrect):** 3-phosphoglyceraldehyde (Glyceraldehyde-3-phosphate) is formed *after* DHAP via isomerization; it is not the first intermediate. * **A & B (Incorrect):** 2-phosphoglyceric acid and 3-phosphoglyceric acid are downstream intermediates in the payoff phase of glycolysis, occurring much later in the sequence toward pyruvate. #### NEET-PG High-Yield Pearls: * **Tissue Specificity:** Glycerol kinase is present in the **liver and kidneys** but is **absent in adipose tissue**. Therefore, adipocytes cannot reuse glycerol released from lipolysis; it must be transported to the liver. * **Gluconeogenesis:** This pathway also serves as the mechanism by which glycerol acts as a substrate for gluconeogenesis during fasting. * **Shuttle System:** The conversion of Glycerol-3-P to DHAP is also a component of the **Glycerol-3-phosphate shuttle**, which transports reducing equivalents from the cytosol to the mitochondria for the Electron Transport Chain.

Question 309: Arrange the steps of glycogenolysis in sequence.

A. Formation of limit dextrins
B. Transfer of glucose residues from branched chain to neighboring straight chain (Glucan Transferase)
C. Breakdown of alpha(1-4) bond from non-reducing end
D. Breakdown of alpha(1-6) bond (Correct Answer)

Explanation: **Explanation:** Glycogenolysis is the biochemical breakdown of glycogen into glucose-1-phosphate and glucose. The process follows a specific sequential order to navigate the branched structure of glycogen. **Sequential Steps of Glycogenolysis:** 1. **Step 1 (Option C):** **Glycogen Phosphorylase** cleaves $\alpha(1\to4)$ glycosidic bonds from the non-reducing ends. It stops when four glucose residues remain before a branch point. 2. **Step 2 (Option A):** The resulting structure, with short four-residue branches, is called a **Limit Dextrin**. 3. **Step 3 (Option B):** The **Debranching enzyme** (specifically the **4:4 transferase** activity) moves the outer three of the four glucose residues from the branch to a nearby straight chain. 4. **Step 4 (Option D):** Finally, the **$\alpha(1\to6)$ glucosidase** activity of the debranching enzyme hydrolyzes the single remaining glucose residue at the branch point. **Why Option D is the "Final" Step:** In a sequential arrangement, the breakdown of the $\alpha(1\to6)$ bond is the terminal step of the debranching process. While Phosphorylase initiates the process (Option C), it cannot complete it without the debranching enzyme first creating limit dextrins (Option A) and transferring residues (Option B). Thus, the $\alpha(1\to6)$ cleavage represents the completion of a "cycle" of debranching. **Clinical Pearls for NEET-PG:** * **Von Gierke Disease (Type I GSD):** Deficiency of Glucose-6-Phosphatase; presents with severe hypoglycemia and hepatomegaly. * **Cori Disease (Type III GSD):** Deficiency of **Debranching Enzyme**; results in the accumulation of **Limit Dextrins**. * **McArdle Disease (Type V GSD):** Deficiency of **Muscle Glycogen Phosphorylase**; presents with exercise-induced cramps and myoglobinuria. * **Rate-limiting enzyme:** Glycogen Phosphorylase (activated by cAMP-mediated phosphorylation).

Question 310: In glycolysis, which enzyme catalyzes the first committed step?

A. 2, 3-diphosphoglycerate
B. Glucokinase
C. Hexokinase
D. Phosphofructokinase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Phosphofructokinase-1 (PFK-1)**. In biochemistry, a "committed step" is an irreversible reaction that is unique to a specific pathway and effectively "commits" the substrate to that pathway's completion. 1. **Why PFK-1 is correct:** While Hexokinase catalyzes the first step of glycolysis, its product (Glucose-6-Phosphate) can enter other pathways like the Pentose Phosphate Pathway (PPP) or Glycogenesis. PFK-1 catalyzes the conversion of Fructose-6-Phosphate to **Fructose-1,6-bisphosphate**. This is the first irreversible reaction unique to glycolysis, making it the primary rate-limiting and committed step. 2. **Why other options are incorrect:** * **Hexokinase/Glucokinase:** These catalyze the first step of glycolysis, but not the *committed* step, as G6P has multiple metabolic fates. * **2,3-diphosphoglycerate (2,3-DPG):** This is a bypass product of glycolysis (Rapoport-Luebering shunt) found in RBCs; it is not an enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Regulation:** PFK-1 is allosterically inhibited by **ATP and Citrate** (energy-rich state) and activated by **AMP and Fructose-2,6-bisphosphate**. * **Fructose-2,6-bisphosphate:** This is the most potent allosteric activator of PFK-1, produced by the bifunctional enzyme PFK-2. * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (promoting glycolysis), while Glucagon decreases it. * **Rate-limiting enzymes:** Always distinguish between the "first step" (Hexokinase) and the "rate-limiting/committed step" (PFK-1).

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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