Carbohydrate Metabolism Practice Questions

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).

Q: A child presents with hepatomegaly and hypoglycemia. There is no improvement in blood sugar even after administration of epinephrine. What is the likely diagnosis?

Von Gierke's disease. ### Explanation **1. Why Von Gierke’s Disease (Type I GSD) is the Correct Answer:** The clinical triad of **hepatomegaly**, **fasting hypoglycemia**, and **failure to respond to epinephrine** is classic for Von Gierke’s disease. This condition is caused by a deficiency of **Glucose-6-Phosphatase**, the final enzyme required for both glycogenolysis and gluconeogenesis. * **Mechanism:** Epinephrine normally stimulates glycogen breakdown into Glucose-1-Phosphate, which is then converted to Glucose-6-Phosphate (G6P). However, because the G6Pase enzyme is missing, G6P cannot be converted into free glucose to enter the bloodstream. Consequently, blood sugar levels remain low despite hormonal stimulation. **2. Why the Other Options are Incorrect:** * **Anderson’s Disease (Type IV):** Caused by a branching enzyme deficiency. It presents with cirrhosis and "limit dextrin" accumulation, but hypoglycemia is not the primary presenting feature. * **Pompe’s Disease (Type II):** Caused by lysosomal acid maltase deficiency. It primarily affects the heart (massive cardiomegaly) and muscles. Blood sugar levels are typically **normal** because cytoplasmic glycogenolysis is intact. * **McArdle’s Disease (Type V):** Caused by muscle phosphorylase deficiency. It presents with muscle cramps and myoglobinuria after exercise. It does **not** cause hypoglycemia or hepatomegaly as the liver enzyme is unaffected. **3. NEET-PG High-Yield Pearls:** * **Biochemical Hallmarks:** Look for "Hyper-LACH": **H**yperuricemia (Gout), **L**actic **A**cidosis, **C**holesterolemia (Hyperlipidemia), and **H**ypoglycemia. * **Diagnostic Clue:** If the question mentions "doll-like facies" or "protuberant abdomen," think Von Gierke’s. * **Glucagon Test:** Similar to epinephrine, administration of glucagon will fail to raise blood glucose in these patients but will significantly increase lactate levels.

Q: Which GLUT receptor is primarily found on the pancreas?

GLUT 2. **Explanation:** **GLUT 2** is the correct answer because it serves as the primary glucose sensor for the body. It is a high-capacity, low-affinity (high $K_m$) bidirectional transporter. In the **pancreatic beta cells**, GLUT 2 allows glucose entry proportional to blood glucose levels. Once inside, glucose is metabolized to ATP, leading to the closure of ATP-sensitive $K^+$ channels, depolarization, and subsequent **insulin secretion**. This high $K_m$ ensures that insulin is released only when blood glucose levels are elevated. **Analysis of Incorrect Options:** * **GLUT 1:** Found in the **Blood-Brain Barrier (BBB)** and **RBCs**. It provides basal glucose uptake required for cellular respiration. * **GLUT 4:** The only **insulin-dependent** transporter. It is primarily located in **skeletal muscle and adipose tissue**. In the presence of insulin, GLUT 4 translocates from intracellular vesicles to the cell membrane. * **GLUT 5:** A specialized transporter primarily responsible for **fructose** absorption in the small intestine and spermatozoa. **High-Yield Clinical Pearls for NEET-PG:** * **Locations of GLUT 2:** Remember the mnemonic **"KLiP"** — **K**idney (PCT), **L**iver, **i**ntestine (basolateral side), and **P**ancreas. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in the **GLUT 2** transporter. * **SGLT vs. GLUT:** SGLT (1 & 2) are active transporters (secondary active) used for glucose absorption against a gradient, whereas GLUTs are passive transporters (facilitated diffusion).

Question 1

What is the enzyme involved in the following conversion?

Accepted Answer

Aldose reductase

Answer

Sorbitol dehydrogenase

Answer

Glucose reductase

Answer

Glucose oxidase

Question 2

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 3

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 4

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 5

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 6

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 7

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

Question 8

A child presents with hepatomegaly and hypoglycemia. There is no improvement in blood sugar even after administration of epinephrine. What is the likely diagnosis?

Accepted Answer

Von Gierke's disease

Answer

Anderson's disease

Answer

Pompe's disease

Answer

McArdle's disease

Question 9

The binding of epinephrine or glucagon to the corresponding membrane receptor has which of the following effects on glycogen metabolism?

Accepted Answer

Glycogen phosphorylase is activated, while glycogen synthase is inactivated.

Answer

The net synthesis of glycogen is increased.

Answer

Glycogen phosphorylase is inactivated, while glycogen synthase is activated.

Answer

Both glycogen synthase and phosphorylase are activated.

Question 10

Which GLUT receptor is primarily found on the pancreas?

Accepted Answer

GLUT 2

Answer

GLUT 1

Answer

GLUT 4

Answer

GLUT 5

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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