Carbohydrate Metabolism Practice Questions

Q: A boy presents with vomiting, a bloated abdomen, and abdominal pain. He attended an ice-cream eating competition last night and has a past history of similar episodes following the ingestion of milk and milk products. What is the likely cause?

Lactase deficiency. **Explanation:** The clinical presentation of abdominal bloating, pain, and vomiting following the ingestion of milk products (ice cream) is a classic manifestation of **Lactose Intolerance**, caused by **Lactase deficiency**. **Why the correct answer is right:** Lactase is a brush-border enzyme in the small intestine that hydrolyzes lactose into glucose and galactose. In its absence, undigested lactose passes into the colon. Here, it acts osmotically, drawing water into the lumen (causing diarrhea and bloating), and is fermented by colonic bacteria into hydrogen gas, methane, and lactic acid. This fermentation leads to flatulence, abdominal distension, and pain. **Why incorrect options are wrong:** * **Pancreatic/Salivary Amylase deficiency:** Amylase breaks down complex starches (polysaccharides) into maltose. Deficiency would lead to malabsorption of starches, not specifically milk-based sugars. * **Food poisoning:** While it causes vomiting and pain, the recurrent history ("past history of similar episodes") specifically linked to milk products points toward a metabolic/enzymatic defect rather than an acute infection. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The **Hydrogen Breath Test** is the gold standard (increased H₂ due to bacterial fermentation). Stool analysis shows **low pH** (acidic) and the presence of **reducing sugars**. * **Types:** Primary (age-related decline), Secondary (due to mucosal damage like Celiac or Rotavirus), and Congenital (rare). * **Biochemical Note:** Lactose is a disaccharide with a **β-1,4 glycosidic linkage**.

Q: Which of the following statements is true regarding GLUT-5?

Sodium independent transport. **Explanation:** Glucose transporters (GLUTs) are a family of membrane proteins that facilitate the transport of glucose and other sugars across the cell membrane via **facilitated diffusion**. This process is **sodium-independent**, unlike the SGLT (Sodium-Glucose Linked Transporters) found in the kidneys and intestines, which require a sodium gradient. **Why Option D is correct:** GLUT-5 is unique among the GLUT family because its primary substrate is **fructose**, not glucose. Like all members of the GLUT family (GLUT 1-14), it operates via facilitated diffusion, meaning it moves solutes down their concentration gradient without requiring energy or sodium ions. **Analysis of Incorrect Options:** * **Option A:** GLUT-1 and GLUT-3 are the primary transporters in the **brain**, ensuring a constant glucose supply regardless of blood sugar levels. * **Option B:** GLUT-4 is the specific transporter found in **adipose tissue and skeletal muscle**. * **Option C:** Only **GLUT-4** is insulin-dependent. GLUT-5 is insulin-independent and is primarily expressed in the small intestine (apical membrane of enterocytes), spermatozoa, and kidneys. **High-Yield NEET-PG Pearls:** * **GLUT-1:** Blood-brain barrier, RBCs (Basal uptake). * **GLUT-2:** Bidirectional; found in Liver, Pancreas (B-cells), and Kidney. It has a high $K_m$ (low affinity). * **GLUT-3:** Neurons (Highest affinity). * **GLUT-4:** Insulin-responsive (Muscle/Fat). * **GLUT-5:** Fructose specific. * **SGLT-1/2:** Secondary active transport (Sodium-dependent); found in the small intestine and proximal convoluted tubule of the kidney.

Q: Which metabolic pathway provides the reducing agent primarily used in lipogenesis?

Pentose phosphate pathway. **Explanation:** The correct answer is **A. Pentose Phosphate Pathway (PPP)**. **1. Why it is correct:** Lipogenesis (fatty acid synthesis) occurs in the cytosol and requires **NADPH** as a reducing agent to reduce keto groups to hydroxyl groups and double bonds to single bonds. The primary source of this NADPH is the **Pentose Phosphate Pathway** (specifically the oxidative phase via Glucose-6-phosphate dehydrogenase). Another secondary source is the activity of the **Malic enzyme**, which converts malate to pyruvate. **2. Why other options are incorrect:** * **B. Glycolysis:** This pathway occurs in the cytosol but produces **NADH**, not NADPH. NADH is primarily used for ATP production in the electron transport chain, not for reductive biosynthesis. * **C. TCA Cycle:** This mitochondrial pathway produces **NADH and FADH₂**, which are used for oxidative phosphorylation. While it provides the substrate (Citrate) for lipogenesis, it does not provide the reducing power. * **D. Gluconeogenesis:** This is an anabolic pathway that consumes energy (ATP/GTP) and reducing equivalents (NADH) to synthesize glucose; it does not generate NADPH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of PPP:** Glucose-6-phosphate dehydrogenase (G6PD). * **Tissues with high PPP activity:** Adrenal cortex, liver, lactating mammary glands, and RBCs (all require NADPH for either steroid/fatty acid synthesis or maintaining reduced glutathione). * **The "Citrate Shuttle":** Since Acetyl-CoA cannot cross the mitochondrial membrane, it exits as **Citrate**. In the cytosol, Citrate is cleaved by *ATP Citrate Lyase* to provide Acetyl-CoA for lipogenesis. * **Key Regulatory Enzyme:** Acetyl-CoA Carboxylase (inhibited by Glucagon/Palmitoyl-CoA; activated by Insulin/Citrate).

Q: In a well-fed state, what is the major fate of glucose-6-phosphate in tissues?

Conversion to glycogen. **Explanation:** In a **well-fed state**, high blood glucose levels trigger the release of **insulin**. Insulin promotes anabolic pathways to store excess energy. Glucose entering the cell is immediately phosphorylated to **Glucose-6-Phosphate (G6P)** by hexokinase/glucokinase. In tissues like the liver and muscle, the primary fate of this G6P is storage. It is converted to Glucose-1-Phosphate and then incorporated into **glycogen** via glycogen synthase (the rate-limiting enzyme activated by insulin). This ensures a glucose reserve for future fasting states. **Analysis of Incorrect Options:** * **Option A (Hydrolysis to glucose):** This occurs primarily in the liver during the **fasting state** (via Glucose-6-Phosphatase) to maintain blood glucose levels. In a well-fed state, this reaction is inhibited. * **Option C (Isomerization to fructose-6-phosphate):** While this is the second step of glycolysis, in a well-fed state, the "major fate" for storage takes precedence once immediate energy needs are met. Glycolysis provides ATP, but glycogen synthesis is the hallmark of the "excess" glucose state. * **Option D (Conversion to ribulose-5-phosphate):** This is the Pentose Phosphate Pathway (PPP). While active in the well-fed state (to provide NADPH for fatty acid synthesis), it is a minor pathway compared to the bulk storage of glucose as glycogen. **NEET-PG High-Yield Pearls:** * **Glucokinase vs. Hexokinase:** Glucokinase (liver/pancreas) has a high $K_m$ and high $V_{max}$, allowing it to handle the large glucose load in the well-fed state. * **Rate-limiting enzyme:** Glycogen synthase is active when **dephosphorylated** (stimulated by insulin). * **Tissue Specificity:** Muscle glycogen is used for local energy; only liver glycogen can contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**.

Q: Glucose can be synthesized from which of the following substrates?

Glycerol. **Explanation:** The process of synthesizing glucose from non-carbohydrate precursors is known as **Gluconeogenesis**. This metabolic pathway primarily occurs in the liver and kidneys during periods of fasting or starvation. **Why Glycerol is Correct:** Glycerol is derived from the hydrolysis of triacylglycerols (TAGs) in adipose tissue. Once released, it is transported to the liver where it is phosphorylated by **glycerol kinase** to glycerol-3-phosphate and then oxidized to **Dihydroxyacetone phosphate (DHAP)**. DHAP is a direct intermediate of the glycolytic/gluconeogenic pathway, allowing glycerol to enter the pathway and be converted into glucose. **Why the other options are Incorrect:** * **Adenine & Guanine (Options B & C):** These are purine bases. The catabolism of purines in humans results in **uric acid**, which is excreted in the urine. Purines do not possess a carbon skeleton that can be converted into pyruvate or any TCA cycle intermediate; therefore, they cannot contribute to gluconeogenesis. * **Palmitic Acid (Option D):** This is a long-chain even-carbon fatty acid. Beta-oxidation of even-chain fatty acids yields **Acetyl-CoA**. In humans, Acetyl-CoA cannot be converted back to pyruvate because the **Pyruvate Dehydrogenase (PDH) reaction is irreversible**. Furthermore, the two carbons of Acetyl-CoA are lost as $CO_2$ in the TCA cycle, resulting in no net synthesis of glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Major Gluconeogenic Precursors:** Lactate (Cori Cycle), Glucogenic amino acids (primarily Alanine), and Glycerol. * **Odd-chain Fatty Acids:** Unlike Palmitic acid, odd-chain fatty acids *can* be gluconeogenic because their final breakdown product is **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Key Enzyme:** Glycerol kinase is absent in adipose tissue; therefore, glycerol must travel to the liver for gluconeogenesis.

Q: The branching enzyme is involved in which process?

Glycogenesis. **Explanation:** **1. Why Glycogenesis is Correct:** Glycogenesis is the process of glycogen synthesis. The **Branching Enzyme** (also known as **Amylo-(1,4→1,6)-transglucosidase**) is essential for creating the branched structure of glycogen. Once glycogen synthase extends a glucose chain via α-1,4-glycosidic bonds to about 11 residues, the branching enzyme removes a fragment (at least 6-7 residues) and reattaches it via an **α-1,6-glycosidic bond**. Branching increases the solubility of glycogen and creates multiple non-reducing ends, allowing for rapid mobilization of glucose during stress or fasting. **2. Why Other Options are Incorrect:** * **Glucogenesis:** This is a general term for glucose formation (often confused with Gluconeogenesis). It does not involve branching enzymes. * **Glycogenolysis:** This is the breakdown of glycogen. It requires the **Debranching Enzyme** (which has two activities: 4:4 transferase and α-1,6-glucosidase) to remove branches, not create them. * **Glycolysis:** This is the metabolic pathway that converts glucose into pyruvate. It involves enzymes like Hexokinase and Phosphofructokinase-1; it does not involve glycogen or branching. **3. High-Yield Clinical Pearls for NEET-PG:** * **Andersen Disease (GSD Type IV):** Caused by a deficiency of the **Branching Enzyme**. It results in the formation of abnormal glycogen with very long outer chains (resembling amylopectin), leading to liver cirrhosis and early death. * **Cori Disease (GSD Type III):** Caused by a deficiency of the **Debranching Enzyme**, leading to the accumulation of "limit dextrins." * **Key Regulatory Enzyme:** Remember that **Glycogen Synthase** is the rate-limiting enzyme for glycogenesis, but the Branching Enzyme is required for structural integrity.

Q: Which of the following enzymes does not participate in the TCA cycle?

Pyruvate dehydrogenase. **Explanation:** The **Tricarboxylic Acid (TCA) cycle**, also known as the Krebs cycle, occurs in the mitochondrial matrix. It begins with the condensation of Acetyl-CoA and Oxaloacetate. **Why Pyruvate Dehydrogenase (PDH) is the correct answer:** Pyruvate dehydrogenase is part of the **PDH Complex**, which serves as a "bridge" or "link reaction" between glycolysis and the TCA cycle. It converts Pyruvate (the end product of glycolysis) into Acetyl-CoA. While it provides the substrate necessary for the cycle to begin, it is **not** considered a member of the TCA cycle itself. **Analysis of Incorrect Options:** * **Citrate Synthase (Option A):** This is the first regulatory enzyme of the TCA cycle. It catalyzes the synthesis of Citrate from Acetyl-CoA and Oxaloacetate. * **Isocitrate Dehydrogenase (Option B):** This is the **rate-limiting enzyme** of the TCA cycle. It catalyzes the oxidative decarboxylation of Isocitrate to α-Ketoglutarate, producing the first molecule of NADH and $CO_2$. * **Malate Dehydrogenase (Option D):** This is the final enzyme of the cycle, which oxidizes Malate to Oxaloacetate, completing the loop and generating NADH. **High-Yield Clinical Pearls for NEET-PG:** * **PDH Deficiency:** The most common cause of congenital lactic acidosis. * **Cofactors:** Both PDH and α-Ketoglutarate dehydrogenase require five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**eves **N**ever **P**lay **L**ate). * **Inhibitors:** Fluoroacetate inhibits Aconitase; Arsenite inhibits α-Ketoglutarate dehydrogenase. * **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1).

Q: Keratin sulfate in urine is found in which of the following conditions?

Morquio's syndrome. **Explanation:** The question pertains to **Mucopolysaccharidoses (MPS)**, a group of metabolic disorders caused by the deficiency of lysosomal enzymes needed to break down Glycosaminoglycans (GAGs). **1. Why Morquio’s Syndrome is Correct:** Morquio’s syndrome (MPS IV) is uniquely characterized by the accumulation and urinary excretion of **Keratan Sulfate**. It occurs in two forms: Type A (Galactose-6-sulfatase deficiency) and Type B (β-galactosidase deficiency). Unlike other MPS types, Morquio’s presents with severe skeletal dysplasia (spondyloepiphyseal dysplasia) and ligamentous laxity, but notably **spares intelligence**. **2. Analysis of Incorrect Options:** * **Hurler’s Syndrome (MPS IH):** Characterized by the deficiency of α-L-iduronidase. The GAGs found in urine are **Dermatan sulfate** and **Heparan sulfate**. It is the most severe form, presenting with corneal clouding and mental retardation. * **Hunter’s Syndrome (MPS II):** Caused by Iduronate sulfatase deficiency. Like Hurler’s, it involves **Dermatan and Heparan sulfate**, but it is **X-linked recessive** and lacks corneal clouding. * **Sanfilippo’s Syndrome (MPS III):** Primarily involves the accumulation of **Heparan sulfate**. It is clinically distinct due to profound central nervous system involvement (aggressive behavior, dementia) with relatively mild physical symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Keratan Sulfate** is also found in the cornea and cartilage; hence, its accumulation leads to the skeletal features of Morquio’s. * **Mnemonic for GAGs:** "Mor-K" (Morquio = Keratan); "San-H" (Sanfilippo = Heparan). * All MPS are Autosomal Recessive **except Hunter’s**, which is X-linked Recessive (Mnemonic: *The Hunter needs X-ray vision to see the target*). * **Corneal clouding** is absent in Hunter’s and Sanfilippo’s.

Q: During caloric deprivation, how many hours are typically required for complete depletion of hepatic glycogen stores?

18 hours. **Explanation:** **1. Why 18 hours is correct:** Hepatic glycogen serves as the primary source of blood glucose during the early stages of fasting. In a healthy adult, the liver stores approximately 75–100 grams of glycogen. Following a meal, blood glucose levels are maintained by dietary intake for about 4 hours. As the body enters the post-absorptive state, **glycogenolysis** (breakdown of glycogen) becomes the dominant process. Under standard caloric deprivation, these stores are progressively depleted and typically run out within **12 to 18 hours**. By this point, the body must transition almost entirely to **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources like amino acids and glycerol) to maintain glycemia. **2. Analysis of Incorrect Options:** * **9 hours (Option A):** At this stage, hepatic glycogenolysis is still at its peak. The stores are significantly reduced but not yet exhausted. * **24 hours (Option C):** While some textbooks suggest a range of 12–24 hours, 18 hours is the physiological "tipping point" emphasized in standard medical biochemistry (like Harper’s). By 24 hours, gluconeogenesis is already the sole source of blood glucose. * **48 hours (Option D):** This represents prolonged fasting. By 48 hours, the body has shifted toward ketosis to spare muscle protein. **3. NEET-PG High-Yield Pearls:** * **Muscle Glycogen:** Unlike the liver, muscle glycogen **cannot** contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Rate-Limiting Enzyme:** The rate-limiting enzyme for glycogenolysis is **Glycogen Phosphorylase**. * **Gluconeogenesis Trigger:** As glycogen levels fall, the **Insulin:Glucagon ratio** decreases, activating PEPCK and Fructose-1,6-bisphosphatase. * **Clinical Correlation:** In Von Gierke’s Disease (G6Pase deficiency), severe hypoglycemia occurs much earlier because neither glycogenolysis nor gluconeogenesis can release glucose into the blood.

Q: Which of the following is the rate-limiting enzyme in glycogenesis?

Glycogen synthase. **Explanation:** **Glycogenesis** is the process of glycogen synthesis from glucose. The correct answer is **Glycogen synthase** because it catalyzes the formation of $\alpha$-1,4-glycosidic bonds, adding glucose units from UDP-glucose to a pre-existing glycogen primer. It is the primary site of regulation through both allosteric activation (by Glucose-6-Phosphate) and covalent modification (inhibition by phosphorylation via Protein Kinase A). **Analysis of Incorrect Options:** * **Glucokinase (Option A):** While it initiates glucose metabolism by phosphorylating glucose to Glucose-6-Phosphate in the liver, it is not specific to glycogenesis; it also feeds into glycolysis and the HMP shunt. * **Glycogen phosphorylase (Option B):** This is the rate-limiting enzyme for **Glycogenolysis** (glycogen breakdown), not synthesis. It cleaves $\alpha$-1,4-glycosidic bonds to release Glucose-1-Phosphate. * **Branching enzyme (Option D):** Also known as Amylo-$(1,4 \to 1,6)$-transglucosidase, it creates $\alpha$-1,6-glycosidic bonds. While essential for increasing glycogen solubility, it is not the rate-limiting step. **High-Yield Clinical Pearls for NEET-PG:** * **Hormonal Regulation:** Insulin stimulates glycogen synthase (via dephosphorylation), while Glucagon and Epinephrine inhibit it. * **Glycogenin:** This protein acts as the initial primer required for glycogen synthase to start adding glucose units. * **Clinical Correlation:** A deficiency in Glycogen Synthase leads to **Glycogen Storage Disease (GSD) Type 0**, characterized by fasting hypoglycemia and postprandial hyperglycemia. * **Branching Enzyme Deficiency:** Leads to **Andersen’s Disease (GSD Type IV)**, resulting in the accumulation of abnormal glycogen with long outer chains (resembling amylopectin), causing liver cirrhosis.

Question 1

A boy presents with vomiting, a bloated abdomen, and abdominal pain. He attended an ice-cream eating competition last night and has a past history of similar episodes following the ingestion of milk and milk products. What is the likely cause?

Accepted Answer

Lactase deficiency

Answer

Pancreatic amylase deficiency

Answer

Salivary amylase deficiency

Answer

Food poisoning

Question 2

Which of the following statements is true regarding GLUT-5?

Accepted Answer

Sodium independent transport

Answer

Present in the brain

Answer

Present in adipose tissue, skeletal muscle, and skin

Answer

Mediated by insulin transport

Question 3

Which metabolic pathway provides the reducing agent primarily used in lipogenesis?

Accepted Answer

Pentose phosphate pathway

Answer

Glycolysis

Answer

TCA cycle

Answer

Gluconeogenesis

Question 4

In a well-fed state, what is the major fate of glucose-6-phosphate in tissues?

Accepted Answer

Conversion to glycogen

Answer

Hydrolysis to glucose

Answer

Isomerization to fructose-6-phosphate

Answer

Conversion to ribulose-5-phosphate

Question 5

Glucose can be synthesized from which of the following substrates?

Accepted Answer

Glycerol

Answer

Adenine

Answer

Guanine

Answer

Palmitic acid

Question 6

The branching enzyme is involved in which process?

Accepted Answer

Glycogenesis

Answer

Glucogenesis

Answer

Glycogenolysis

Answer

Glycolysis

Question 7

Which of the following enzymes does not participate in the TCA cycle?

Accepted Answer

Pyruvate dehydrogenase

Answer

Citrate synthase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Question 8

Keratin sulfate in urine is found in which of the following conditions?

Accepted Answer

Morquio's syndrome

Answer

Hurler's syndrome

Answer

Hunter's syndrome

Answer

Sanfillipo's syndrome

Question 9

During caloric deprivation, how many hours are typically required for complete depletion of hepatic glycogen stores?

Accepted Answer

18 hours

Answer

9 hours

Answer

24 hours

Answer

48 hours

Question 10

Which of the following is the rate-limiting enzyme in glycogenesis?

Accepted Answer

Glycogen synthase

Answer

Glucokinase

Answer

Glycogen phosphorylase

Answer

Branching enzyme

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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