Carbohydrate Metabolism Practice Questions

Q: The molecule marked by a blue question mark can be all of the following EXCEPT?

Glycerol. ***Glycerol*** - Glycerol enters gluconeogenesis at the **dihydroxyacetone phosphate (DHAP)** level and cannot be directly converted to or from pyruvate. - It is metabolically distinct from the **3-carbon pyruvate family** and follows a different biochemical pathway in glucose metabolism. *Pyruvate* - The central **3-carbon molecule** in carbohydrate metabolism that can be interconverted with lactate and alanine. - Serves as a key metabolic hub connecting **glycolysis**, **gluconeogenesis**, and **amino acid metabolism**. *Lactate* - Directly interconvertible with pyruvate via **lactate dehydrogenase** in anaerobic glycolysis. - Part of the **Cori cycle** where lactate is converted back to pyruvate in the liver for glucose production. *Alanine* - Interconvertible with pyruvate through **alanine aminotransferase (ALT)** in amino acid metabolism. - Part of the **glucose-alanine cycle** where muscle-derived alanine is converted to pyruvate in the liver.

Q: A 3-month-old infant presents with hepatosplenomegaly and failure to thrive. A liver biopsy reveals glycogen with an abnormal, amylopectin-like structure with long outer chains and missing branches. Which of the following enzymes would most likely be deficient?

Branching enzyme. ### Explanation The clinical presentation and biopsy findings point towards **Glycogen Storage Disease Type IV (Andersen Disease)**. **1. Why the Correct Answer is Right:** The **Branching Enzyme** (α-1,4 → α-1,6 transglucosidase) is responsible for creating α-1,6-glycosidic bonds by transferring a string of glucose residues to a side chain. A deficiency in this enzyme leads to the accumulation of an abnormal form of glycogen called **polyglucosan** (or amylopectin-like glycogen). Because the enzyme cannot create branches, the glycogen molecules have **fewer branch points and very long outer chains**. These insoluble, straight-chain structures trigger an immune response, leading to progressive liver cirrhosis, hepatosplenomegaly, and failure to thrive. **2. Why Incorrect Options are Wrong:** * **Alpha-amylase:** This is a digestive enzyme found in saliva and pancreatic secretions that breaks down dietary starch into maltose; it is not involved in intracellular glycogen synthesis. * **Debranching enzyme (GSD Type III / Cori Disease):** Deficiency leads to the accumulation of **Limit Dextrins** (glycogen with very short outer branches). Patients typically present with hypoglycemia, which is not the primary feature here. * **Glucose-6-phosphatase (GSD Type I / Von Gierke Disease):** This results in the accumulation of **normal-structured glycogen** in the liver and kidneys, presenting with severe fasting hypoglycemia, lactic acidosis, and hyperuricemia. **3. NEET-PG High-Yield Pearls:** * **Andersen Disease (Type IV):** Think "Long chains, No branches." It is the most "fatal" early-onset GSD due to liver cirrhosis. * **Cori Disease (Type III):** Think "Short branches" (Limit Dextrins). * **Mnemonic for GSDs:** **V**on Gierke (I), **P**ompe (II), **C**ori (III), **A**ndersen (IV), **M**cArdle (V), **H**ers (VI) → "**V**ery **P**oor **C**arbohydrate **A**nalysis **M**akes **H**umans" sick.

Q: Phosphorylase b is maintained in an inactivated state by?

Insulin. **Explanation:** The regulation of glycogenolysis centers on the enzyme **Glycogen Phosphorylase**, which exists in two forms: **Phosphorylase *a*** (phosphorylated, active) and **Phosphorylase *b*** (dephosphorylated, inactive). **Why Insulin is correct:** Insulin is an anabolic hormone that promotes glycogen synthesis and inhibits glycogen breakdown. It triggers **Protein Phosphatase-1 (PP1)**, which removes the phosphate group from Phosphorylase *a*, converting it back into the inactive **Phosphorylase *b***. Furthermore, insulin activates phosphodiesterase to lower cAMP levels, effectively maintaining the enzyme in its inactive state to prevent glucose release. **Analysis of Incorrect Options:** * **ATP:** In the muscle, ATP acts as an allosteric inhibitor of Phosphorylase *b*, but it does not "maintain" the state via hormonal signaling like insulin. High ATP signals high energy, reducing the *activity* of the enzyme rather than its phosphorylation state. * **cAMP:** This is a second messenger for Glucagon and Epinephrine. Increased cAMP activates Protein Kinase A (PKA), which leads to the activation of Phosphorylase Kinase, ultimately **activating** Phosphorylase *b* into *a*. * **Calcium:** In contracting muscles, Calcium binds to the calmodulin subunit of Phosphorylase Kinase, **activating** it. This ensures glycogen is broken down to provide energy for contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting step of glycogenolysis. * **Covalent Modification:** The conversion between *a* and *b* forms is the classic example of regulation via phosphorylation/dephosphorylation. * **Muscle vs. Liver:** Muscle phosphorylase is sensitive to **AMP** (allosteric activator), whereas liver phosphorylase is not; liver phosphorylase is primarily regulated by **Glucose**.

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).

Question 1

The molecule marked by a blue question mark can be all of the following EXCEPT?

Accepted Answer

Glycerol

Answer

Pyruvate

Answer

Lactate

Answer

Alanine

Question 2

A 3-month-old infant presents with hepatosplenomegaly and failure to thrive. A liver biopsy reveals glycogen with an abnormal, amylopectin-like structure with long outer chains and missing branches. Which of the following enzymes would most likely be deficient?

Accepted Answer

Branching enzyme

Answer

Alpha amylase

Answer

Debranching enzyme

Answer

Glucose-6-phosphatase

Question 3

Phosphorylase b is maintained in an inactivated state by?

Accepted Answer

Insulin

Answer

ATP

Answer

cAMP

Answer

Calcium

Question 4

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 5

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 6

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

Accepted Answer

Pentose phosphate pathway

Answer

Glycolysis

Answer

TCA cycle

Answer

Gluconeogenesis

Question 7

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 8

Glucose can be synthesized from which of the following substrates?

Accepted Answer

Glycerol

Answer

Adenine

Answer

Guanine

Answer

Palmitic acid

Question 9

The branching enzyme is involved in which process?

Accepted Answer

Glycogenesis

Answer

Glucogenesis

Answer

Glycogenolysis

Answer

Glycolysis

Question 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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