Carbohydrate Metabolism Practice Questions

Q: What are the steps involved in the production of fructose in seminal fluid?

Glucose-6-phosphate glucose sorbitol fructose. **Explanation:** The production of fructose in the seminal vesicles occurs via the **Polyol Pathway** (also known as the Sorbitol Pathway). This pathway is essential because fructose is the primary energy source for sperm motility. **Why Option C is Correct:** The synthesis follows a specific sequence starting from blood glucose: 1. **Glucose-6-phosphate** is converted back to **Glucose** by Glucose-6-phosphatase. 2. **Glucose** is reduced to **Sorbitol** (a polyol) by the enzyme **Aldose Reductase**, using NADPH as a cofactor. 3. **Sorbitol** is then oxidized to **Fructose** by the enzyme **Sorbitol Dehydrogenase**, using NAD+ as a cofactor. This bypasses the glycolytic hexokinase step, allowing for efficient fructose production in the seminal vesicles. **Why Other Options are Incorrect:** * **Option A:** Describes a simple isomerization (like in glycolysis), but the seminal vesicles utilize the polyol pathway to ensure high concentrations of fructose specifically. * **Option B:** "Sorbitol phosphate" is not a physiological intermediate in this pathway; the pathway involves free sugars, not phosphorylated intermediates. * **Option D:** Fructose-1-phosphate is an intermediate of fructose metabolism in the liver (fructolysis), not its synthesis in seminal fluid. **Clinical Pearls for NEET-PG:** * **Enzymes:** Aldose Reductase (Glucose → Sorbitol) and Sorbitol Dehydrogenase (Sorbitol → Fructose). * **Tissue Distribution:** The seminal vesicles and lens have both enzymes. However, tissues like the **retina, kidneys, and nerves** lack Sorbitol Dehydrogenase. * **Pathology:** In **Diabetes Mellitus**, hyperglycemia leads to excessive sorbitol accumulation in tissues lacking Sorbitol Dehydrogenase. Since sorbitol is osmotically active, it causes water influx, leading to **cataracts, retinopathy, and neuropathy**.

Q: What is the enzyme involved in the following conversion?

Aldose reductase. ***Aldose reductase*** - **Aldose reductase** catalyzes the first step of the **polyol pathway**, converting glucose to sorbitol using **NADPH** as a cofactor. - This enzyme is clinically significant in **diabetic complications** like neuropathy, retinopathy, nephropathy, and cataracts due to sorbitol accumulation. *Sorbitol dehydrogenase* - This enzyme catalyzes the **second step** of the polyol pathway, converting sorbitol to fructose using **NAD+**. - It does **not convert glucose** directly but acts on the product of aldose reductase. *Glucose reductase* - This is **not a recognized enzyme** in standard biochemical pathways involved in glucose metabolism. - The term is often confused with aldose reductase, but glucose reductase does not exist as a distinct enzyme. *Glucose oxidase* - This enzyme **oxidizes glucose** to gluconic acid and hydrogen peroxide, primarily used in **laboratory glucose assays**. - It does **not reduce glucose** to sorbitol and is not part of the polyol pathway.

Q: Under anaerobic conditions, the glycolysis of one mole of glucose yields how many moles of ATP?

Two. **Explanation:** In anaerobic glycolysis (occurring in the absence of oxygen or in cells lacking mitochondria like RBCs), the metabolic pathway converts one mole of glucose into two moles of **lactate**. 1. **Why Option B is Correct:** The process involves two phases: * **Investment Phase:** 2 ATP molecules are consumed (at the Hexokinase and Phosphofructokinase-1 steps). * **Payoff Phase:** 4 ATP molecules are generated via substrate-level phosphorylation (2 ATP from 1,3-bisphosphoglycerate and 2 ATP from Phosphoenolpyruvate). * **Net Yield:** 4 (Produced) – 2 (Invested) = **2 ATP**. Crucially, the NADH produced during the glyceraldehyde-3-phosphate dehydrogenase step is re-oxidized to NAD+ by converting pyruvate to lactate (catalyzed by Lactate Dehydrogenase). This ensures the cycle continues but prevents the NADH from entering the electron transport chain, resulting in no additional ATP. 2. **Why Other Options are Incorrect:** * **Option A:** Incorrect; the payoff phase generates more than is invested. * **Option C:** 8 ATP is the net yield of **aerobic glycolysis** in certain tissues (using the Malate-Aspartate shuttle) where NADH is oxidized in the mitochondria. * **Option D:** 30 (or 32) ATP is the total yield from the **complete oxidation** of glucose through glycolysis, the TCA cycle, and oxidative phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Mature RBCs** rely exclusively on anaerobic glycolysis for energy because they lack mitochondria. * **Lactic Acidosis:** Occurs when there is excessive anaerobic glycolysis (e.g., during shock or severe hypoxia), leading to lactate accumulation. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces 2,3-BPG, which decreases hemoglobin's affinity for oxygen, shifting the dissociation curve to the right. This process yields **zero net ATP**.

Q: Which of the following is true for phosphofructokinase?

It catalyzes an irreversible reaction in glycolysis. **Phosphofructokinase-1 (PFK-1)** is the most important regulatory enzyme and the **rate-limiting step** of glycolysis. ### **Explanation of the Correct Answer** **Option C** is correct because PFK-1 catalyzes the phosphorylation of Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This reaction is highly exergonic ($\Delta G$ is strongly negative), making it **physiologically irreversible**. In the cell, this step serves as the "committed step"; once glucose is converted to Fructose-1,6-bisphosphate, it is destined to complete glycolysis. ### **Analysis of Incorrect Options** * **Option A:** PFK-1 uses **Fructose-6-Phosphate** and ATP as substrates. Fructose-1,6-bisphosphate is the *product* of the reaction. * **Option B:** PFK-1 **consumes ATP** rather than generating it. It transfers a phosphate group from ATP to the substrate. ATP generation in glycolysis occurs later via Phosphoglycerate Kinase and Pyruvate Kinase (substrate-level phosphorylation). * **Option D:** PFK-1 is **inhibited** by high levels of ATP and Citrate (signals of high energy status). It is **activated** by AMP and **Fructose-2,6-bisphosphate** (the most potent allosteric activator). ### **High-Yield Clinical Pearls for NEET-PG** * **Rate-Limiting Step:** PFK-1 is the "Pacemaker" of glycolysis. * **Hormonal Regulation:** Insulin increases PFK-1 activity (via Fructose-2,6-BP), while Glucagon decreases it. * **Tissues:** In the liver, citrate inhibition of PFK-1 helps prioritize glucose for glycogen synthesis when energy is abundant. * **Comparison:** Do not confuse PFK-1 with **PFK-2**, which synthesizes Fructose-2,6-bisphosphate, the regulator that turns on PFK-1.

Q: High-energy phosphate is not produced in which of the following metabolic pathways?

Hexose monophosphate pathway. The correct answer is **B. Hexose monophosphate (HMP) pathway**, also known as the Pentose Phosphate Pathway (PPP). ### **Why HMP Pathway is Correct** The HMP pathway is a unique metabolic shunt that does **not** involve the production or consumption of ATP (high-energy phosphate). Instead, its primary objectives are: 1. **Production of NADPH:** Used for reductive biosynthesis (fatty acids, steroids) and maintaining reduced glutathione to prevent oxidative stress. 2. **Production of Ribose-5-phosphate:** Essential for nucleotide and nucleic acid synthesis. Because it bypasses the ATP-generating steps of glycolysis, it is considered a non-energetic pathway. ### **Why Other Options are Incorrect** * **TCA Cycle:** Produces high-energy phosphate directly via **Substrate Level Phosphorylation** (Succinyl CoA to Succinate produces **GTP**, which is energetically equivalent to ATP). * **Glycolysis:** Produces ATP via Substrate Level Phosphorylation at two steps: Phosphoglycerate kinase and Pyruvate kinase. * **Beta Oxidation:** While the pathway itself produces FADH₂ and NADH, these enter the Electron Transport Chain to generate large amounts of ATP via oxidative phosphorylation. ### **NEET-PG High-Yield Pearls** * **Rate-limiting enzyme of HMP:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to produce NADPH, which is required to keep glutathione reduced in RBCs. * **Tissue Distribution:** HMP pathway is most active in tissues requiring NADPH (Adrenal cortex, Liver, Lactating mammary glands, and RBCs). * **Thiamine (B1) Connection:** Transketolase, an enzyme in the non-oxidative phase of HMP, requires Thiamine pyrophosphate as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.

Q: Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin are glucose molecules attached?

Tyrosine. **Explanation:** **Glycogenin** is a specialized protein that acts as a primer for glycogen synthesis. Since **Glycogen Synthase** can only add glucose units to an existing chain of at least four glucose residues, glycogenin initiates the process through **autocatalysis**. 1. **Why Tyrosine is Correct:** Glycogenin possesses glucosyltransferase activity. It attaches the first glucose molecule from UDP-glucose to the **hydroxyl (-OH) group** of a specific **Tyrosine residue (Tyr-194)** within its own structure. This serves as the foundation upon which a short chain of about 8 glucose units is built, providing the necessary primer for Glycogen Synthase to take over. 2. **Why Other Options are Incorrect:** * **Serine & Threonine:** While these amino acids also have hydroxyl groups and are common sites for O-linked glycosylation in many glycoproteins (and phosphorylation sites for enzyme regulation), they are not the attachment sites for the glycogen primer. * **Hydroxylysine:** This is a modified amino acid primarily found in collagen, where it serves as a site for glycosylation (attachment of glucose and galactose), but it plays no role in glycogen metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Glycogenin** remains buried at the core of every mature glycogen granule. * **UDP-Glucose** is the active donor of glucose units for both glycogenin and glycogen synthase. * **Branching Enzyme (4:6 transferase)** is required to create $\alpha$1-6 linkages, while **Glycogen Synthase** only creates $\alpha$1-4 linkages. * In the absence of glycogenin, glycogen synthesis cannot be initiated, a concept relevant to understanding certain rare Glycogen Storage Diseases (GSD Type 0).

Question 1

Which of the following statements is false regarding glycolysis?

Accepted Answer

The net production of ATP from anaerobic glycolysis is 2 ATP.

Answer

Glycolysis occurs in the cytosol of all cells.

Answer

The net production of ATP from aerobic glycolysis is approximately 7 ATP.

Answer

None of the above statements are false.

Question 2

What are the steps involved in the production of fructose in seminal fluid?

Accepted Answer

Glucose-6-phosphate 	 glucose 	 sorbitol 	 fructose

Answer

Glucose-6-phosphate 	 fructose-6-phosphate 	 fructose

Answer

Glucose-6-phosphate 	 sorbitol phosphate 	 fructose phosphate 	 fructose

Answer

Glucose-6-phosphate 	 fructose-1-phosphate 	 fructose

Question 3

What is the effect of oral administration of 50 gm of glucose on the body?

Accepted Answer

Decreased gluconeogenesis

Answer

Decreased ketone body production

Answer

Increased lactate production upon exercise

Answer

Increased gluconeogenesis

Question 4

What is the enzyme involved in the following conversion?

Accepted Answer

Aldose reductase

Answer

Sorbitol dehydrogenase

Answer

Glucose reductase

Answer

Glucose oxidase

Question 5

Why is fructose not used in intravenous infusions?

Accepted Answer

None of the above

Answer

It can cause irritability.

Answer

It can cause mental retardation.

Answer

It can increase erythrocyte protoporphyrin.

Question 6

Inulin, like other fructans, is used as a prebiotic because it is non-digestible. What causes its resistance to digestion in the upper gastrointestinal tract?

Accepted Answer

Beta configuration of the anomeric carbon (C2)

Answer

Absence of digestive enzymes in the upper gastrointestinal tract

Answer

Low pH of the stomach

Answer

Presence of alpha-glycosidic linkages

Question 7

Under anaerobic conditions, the glycolysis of one mole of glucose yields how many moles of ATP?

Accepted Answer

Two

Answer

One

Answer

Eight

Answer

Thirty

Question 8

Which of the following is true for phosphofructokinase?

Accepted Answer

It catalyzes an irreversible reaction in glycolysis

Answer

It uses Fructose 1,6 bisphosphate as substrate.

Answer

It generates ATP

Answer

It is activated by increased levels of ATP and citrate.

Question 9

High-energy phosphate is not produced in which of the following metabolic pathways?

Accepted Answer

Hexose monophosphate pathway

Answer

TCA cycle

Answer

Glycolysis

Answer

Beta oxidation of fatty acid

Question 10

Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin are glucose molecules attached?

Accepted Answer

Tyrosine

Answer

Serine

Answer

Threonine

Answer

Hydroxylysine

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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