Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 601: Where is GLUT 4 primarily found?

A. Endothelium
B. Liver
C. Cardiac muscle (Correct Answer)
D. Lens

Explanation: **Explanation:** The correct answer is **Cardiac muscle**. GLUT 4 is the only glucose transporter that is **insulin-dependent**. It is primarily sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin or during exercise. It is found in tissues that require rapid glucose uptake following a meal: **Skeletal muscle, Cardiac muscle, and Adipose tissue.** **Analysis of Options:** * **A. Endothelium:** The blood-brain barrier and vascular endothelium primarily utilize **GLUT 1**, which provides basal glucose uptake independent of insulin. * **B. Liver:** The liver (along with pancreatic beta cells and the kidney) utilizes **GLUT 2**. This is a high-capacity, low-affinity transporter that functions as a "glucose sensor." * **D. Lens:** The lens and cornea primarily utilize **GLUT 1** for insulin-independent glucose uptake. In states of hyperglycemia (Diabetes), excess glucose in the lens is converted to sorbitol via the polyol pathway, leading to cataracts. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 1:** Found in RBCs, Blood-Brain Barrier, and Retina. (Mnemonic: **1** is for **B**lood). * **GLUT 2:** Bidirectional transporter. Found in **L**iver, **I**slets (Beta cells), **K**idney, and **S**mall Intestine (basolateral membrane). (Mnemonic: **LIKS**). * **GLUT 3:** Found in **Neurons** (highest affinity for glucose to protect the brain during hypoglycemia). * **GLUT 4:** The only **Insulin-responsive** transporter. (Mnemonic: Muscle and Fat). * **GLUT 5:** Specifically a **Fructose** transporter found in the small intestine and spermatozoa. * **SGLT 1/2:** Sodium-dependent active transporters found in the small intestine and renal tubules respectively.

Question 602: Essential pentosuria is due to deficiency of which enzyme?

A. Gulonolactone oxidase
B. Phosphoglucomutase
C. Xylulose reductase (Correct Answer)
D. Fructokinase

Explanation: **Explanation:** **Essential Pentosuria** is a rare, benign autosomal recessive metabolic disorder involving the **Uronic Acid Pathway**. 1. **Correct Answer: C. Xylulose reductase** In the uronic acid pathway, glucuronic acid is converted to L-xylulose. Under normal conditions, the enzyme **L-xylulose reductase** (also known as NADP-linked xylulose reductase) reduces L-xylulose to xylitol. A deficiency of this enzyme leads to the accumulation of L-xylulose in the blood and its subsequent excretion in the urine. Since L-xylulose is a reducing sugar, it gives a positive Benedict’s test, often leading to a misdiagnosis of diabetes mellitus. 2. **Why other options are incorrect:** * **A. Gulonolactone oxidase:** This enzyme converts L-gulonolactone to Vitamin C (Ascorbic acid). Humans lack this enzyme, making Vitamin C an essential dietary requirement (Evolutionary deficiency). * **B. Phosphoglucomutase:** This enzyme interconverts Glucose-1-Phosphate and Glucose-6-Phosphate. It is critical for glycogenesis and glycogenolysis, not the uronic acid pathway. * **D. Fructokinase:** Deficiency of this enzyme causes **Essential Fructosuria**, characterized by fructose in the urine. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Asymptomatic; usually an incidental finding of a reducing sugar in urine. * **Biochemical Marker:** High levels of **L-xylulose** in urine. * **Drug Interaction:** Administration of drugs like **Aminopyrine** or **Barbital** can increase the excretion of L-xylulose in patients with pentosuria by inducing the uronic acid pathway. * **Distinction:** Unlike Diabetes, blood glucose levels and Glucose Tolerance Tests (GTT) are normal in these patients.

Question 603: Good diabetic control is said to be present when glycosylated hemoglobin is:

A. 7-9% (Correct Answer)
B. >13%
C. 10-12%
D. 3-4%

Explanation: **Explanation:** Glycosylated hemoglobin (HbA1c) is formed by the non-enzymatic, irreversible covalent bonding of glucose to the N-terminal valine of the beta chain of hemoglobin. Since the average lifespan of a red blood cell is 120 days, HbA1c reflects the average blood glucose levels over the preceding **2–3 months**. **Why 7-9% is the correct answer:** In clinical practice, while the target for most non-pregnant adults with diabetes is <7%, a range of **7-9%** is traditionally considered "good to fair" control in a clinical examination context. It indicates that the patient’s mean plasma glucose is being managed effectively enough to significantly reduce the risk of microvascular complications (like retinopathy and nephropathy) without excessive risk of hypoglycemia. **Analysis of Incorrect Options:** * **B (>13%) & C (10-12%):** These values represent **poor glycemic control**. High HbA1c levels are associated with a high risk of chronic diabetic complications and indicate persistent hyperglycemia. * **D (3-4%):** This is below the **normal reference range (4-5.6%)**. Such low levels are typically seen in non-diabetic individuals or may indicate chronic hypoglycemia/hemolytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** HbA1c **≥ 6.5%** is diagnostic for Diabetes Mellitus. * **Prediabetes:** HbA1c range is **5.7% to 6.4%**. * **Formula:** Estimated Average Glucose (eAG) in mg/dL = $(28.7 \times HbA1c) - 46.7$. * **False Lows:** Conditions that decrease RBC lifespan (e.g., Hemolytic anemia, recent blood loss). * **False Highs:** Conditions that increase RBC lifespan (e.g., Splenectomy, Iron deficiency anemia).

Question 604: Which enzyme is NOT used in glycogen metabolism?

A. Glycogen phosphorylase B
B. Glycogen synthase I
C. Glycogen synthase C (Correct Answer)
D. Glycogen synthase D

Explanation: **Explanation:** The correct answer is **C. Glycogen synthase C**. This enzyme form does not exist in the nomenclature of glycogen metabolism. **1. Why Glycogen Synthase C is the correct answer:** In biochemistry, glycogen synthase exists in two interconvertible forms: **'a'** (active/independent) and **'b'** (inactive/dependent). There is no "C" form. The question asks for the enzyme NOT used, making this the correct choice. **2. Analysis of Incorrect Options:** * **Glycogen Phosphorylase B (Option A):** This is the **inactive**, dephosphorylated form of the rate-limiting enzyme in glycogenolysis. It is converted to the active 'a' form by phosphorylase kinase. * **Glycogen Synthase I (Option B):** The "I" stands for **Independent**. This is the active, dephosphorylated form of glycogen synthase (also known as Glycogen Synthase 'a'). It is called "independent" because its activity does not depend on the presence of Glucose-6-Phosphate. * **Glycogen Synthase D (Option D):** The "D" stands for **Dependent**. This is the inactive, phosphorylated form of glycogen synthase (also known as Glycogen Synthase 'b'). It is called "dependent" because it requires high concentrations of the allosteric activator **Glucose-6-Phosphate** to function. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzymes:** Glycogen Synthase (Glycogenesis) and Glycogen Phosphorylase (Glycogenolysis). * **Reciprocal Regulation:** Phosphorylation **activates** Glycogen Phosphorylase but **inactivates** Glycogen Synthase. * **Hormonal Control:** Insulin promotes the dephosphorylated state (activating synthesis), while Glucagon/Epinephrine promote the phosphorylated state (activating breakdown). * **Glycogenin:** A protein primer required to initiate de novo glycogen synthesis.

Question 605: The monosaccharide glucose is best described by which one of the following statements?

A. It usually exists in the furanose form
B. It is a ketose
C. It possesses an anomeric C-2 carbon atom
D. It forms part of the disaccharide sucrose (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. It forms part of the disaccharide sucrose** **1. Why the Correct Answer is Right:** Glucose is a hexose sugar that serves as a fundamental building block for many carbohydrates. **Sucrose** (common table sugar) is a disaccharide composed of one molecule of **α-D-glucose** and one molecule of **β-D-fructose** linked by an **α(1→2) glycosidic bond**. Since both anomeric carbons are involved in the bond, sucrose is a non-reducing sugar. **2. Analysis of Incorrect Options:** * **A. It usually exists in the furanose form:** Incorrect. In solution, glucose predominantly exists in the **pyranose** form (a six-membered ring). Furanose (five-membered ring) is more characteristic of ketohexoses like fructose or pentoses like ribose. * **B. It is a ketose:** Incorrect. Glucose is an **aldose** because it contains an aldehyde group at the C-1 position. Fructose is the most common example of a ketose. * **C. It possesses an anomeric C-2 carbon atom:** Incorrect. In aldoses like glucose, the **C-1 carbon** is the anomeric carbon (the carbonyl carbon that becomes chiral upon cyclization). In ketoses like fructose, the C-2 carbon is the anomeric carbon. **3. NEET-PG High-Yield Clinical Pearls:** * **Reducing vs. Non-reducing:** All monosaccharides are reducing sugars. Among disaccharides, **Sucrose** is the most notable **non-reducing sugar** (negative Benedict’s test) because its reducing groups are locked in the glycosidic bond. * **Epimers:** Glucose and Galactose are **C-4 epimers**; Glucose and Mannose are **C-2 epimers**. * **GLUT Transporters:** Glucose uptake in the brain is mediated by GLUT-1/3 (insulin-independent), while uptake in muscle and adipose tissue is mediated by **GLUT-4 (insulin-dependent)**. * **Sorbitol Pathway:** In states of hyperglycemia (Diabetes), glucose is reduced to sorbitol by *aldose reductase*, contributing to cataracts and neuropathy.

Question 606: Acetyl CoA is produced from various reactions in the body. Acetyl CoA can be directly converted to all EXCEPT:

A. Glucose (Correct Answer)
B. Fatty acids
C. Cholesterol
D. Ketone bodies

Explanation: **Explanation:** The correct answer is **Glucose**. In humans, Acetyl CoA cannot be used for the net synthesis of glucose because the **Pyruvate Dehydrogenase (PDH) complex reaction is irreversible**. 1. **Why Glucose is the correct answer:** Pyruvate is converted to Acetyl CoA by the PDH complex. However, there is no enzyme in the human body that can convert Acetyl CoA back into Pyruvate or Oxaloacetate (in a net-gain fashion). While Acetyl CoA enters the TCA cycle and condenses with Oxaloacetate, two carbons are lost as $CO_2$ during the cycle. Consequently, there is **no net synthesis of glucose** from Acetyl CoA. This is why fatty acids (which break down into Acetyl CoA) cannot be used for gluconeogenesis. 2. **Analysis of Incorrect Options:** * **Fatty acids:** Acetyl CoA is the primary building block for fatty acid synthesis (Lipogenesis) via its conversion to Malonyl CoA in the cytosol. * **Cholesterol:** Acetyl CoA is the precursor for HMG-CoA, which is the starting point of the mevalonate pathway for cholesterol synthesis. * **Ketone bodies:** During starvation or uncontrolled diabetes, excess Acetyl CoA is diverted to Ketogenesis (forming acetoacetate and $\beta$-hydroxybutyrate) in the liver mitochondria. **NEET-PG High-Yield Pearls:** * **The Exception:** Plants and some microorganisms can convert Acetyl CoA to glucose via the **Glyoxylate Cycle**, which bypasses the decarboxylation steps of the TCA cycle. * **Odd-chain Fatty Acids:** While even-chain fatty acids cannot form glucose, **Propionyl CoA** (from odd-chain fats) is glucogenic because it enters the TCA cycle as Succinyl CoA. * **PDH Complex:** It is a multi-enzyme complex requiring five cofactors: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid.

Question 607: Which enzyme is required for the hexose monophosphate shunt pathway?

A. Glucose-6-phosphatase
B. Phosphorylase
C. Aldolase
D. Glucose-6-phosphate dehydrogenase (Correct Answer)

Explanation: **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and is essential for generating **NADPH** and **Ribose-5-phosphate**. **Why Option D is Correct:** **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting and key regulatory enzyme** of the HMP shunt. It catalyzes the first step of the oxidative phase: the conversion of Glucose-6-phosphate to 6-phosphogluconolactone, reducing $NADP^+$ to $NADPH$ in the process. This pathway is unique because it does not consume or produce ATP directly. **Why Other Options are Incorrect:** * **A. Glucose-6-phosphatase:** This enzyme is involved in **Gluconeogenesis** and **Glycogenolysis** (specifically in the liver and kidneys), converting Glucose-6-phosphate back into free glucose to maintain blood sugar levels. * **B. Phosphorylase:** Specifically Glycogen Phosphorylase, this is the rate-limiting enzyme for **Glycogenolysis**, breaking down glycogen into Glucose-1-phosphate. * **C. Aldolase:** This is an enzyme of **Glycolysis** (Aldolase A) and Fructose metabolism (Aldolase B), responsible for cleaving Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., Fava beans, Primaquine, Infections) because RBCs cannot generate enough NADPH to maintain reduced glutathione, which protects against reactive oxygen species (ROS). * **Bite Cells & Heinz Bodies:** Classic peripheral smear findings in G6PD deficiency. * **Tissues involved:** The HMP shunt is highly active in the **Adrenal cortex, Liver, and Lactating mammary glands** (for fatty acid/steroid synthesis) and **RBCs** (for antioxidant defense).

Question 608: Fructose 2,6-bisphosphate regulates glycolysis at the level of which enzyme?

A. Glucose-6-phosphate dehydrogenase
B. Phosphofructokinase-1 (Correct Answer)
C. Glyceraldehyde-3-phosphate dehydrogenase
D. Pyruvate kinase

Explanation: **Explanation:** **Fructose 2,6-bisphosphate (F2,6-BP)** is the most potent allosteric activator of **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. It increases the affinity of PFK-1 for its substrate (Fructose-6-phosphate) and diminishes the inhibitory effect of ATP. Its levels are regulated by the bifunctional enzyme PFK-2/FBPase-2; insulin increases F2,6-BP levels to stimulate glycolysis, while glucagon decreases them to favor gluconeogenesis. **Analysis of Incorrect Options:** * **A. Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the **Pentose Phosphate Pathway (PPP)**, not glycolysis. It is regulated by the NADP+/NADPH ratio. * **C. Glyceraldehyde-3-phosphate dehydrogenase:** This is a reversible enzyme in glycolysis that catalyzes the conversion of G3P to 1,3-bisphosphoglycerate. It is not a major regulatory site. * **D. Pyruvate kinase:** While this is a regulatory enzyme of glycolysis (Step 10), it is primarily regulated by **Fructose 1,6-bisphosphate** (via feed-forward activation) and covalent modification (phosphorylation), not by F2,6-BP. **High-Yield Clinical Pearls for NEET-PG:** * **Reciprocal Regulation:** F2,6-BP simultaneously activates PFK-1 (glycolysis) and inhibits Fructose 1,6-bisphosphatase (gluconeogenesis), preventing a "futile cycle." * **The "Well-Fed" State:** High insulin → Dephosphorylation of the bifunctional enzyme → Active PFK-2 → High F2,6-BP → **Stimulated Glycolysis.** * **The "Starving" State:** High Glucagon → cAMP-mediated phosphorylation → Active FBPase-2 → Low F2,6-BP → **Stimulated Gluconeogenesis.**

Question 609: What is the enzyme defect in galactosemia?

A. Uridyl transferase
B. Galactokinase
C. Epimerase
D. All of the above (Correct Answer)

Explanation: **Explanation:** Galactosemia is a group of inherited metabolic disorders characterized by the body's inability to metabolize galactose into glucose. The conversion of galactose occurs via the **Leloir pathway**, which involves three primary enzymes. A deficiency in **any** of these enzymes results in a form of galactosemia, making "All of the above" the correct answer. 1. **Galactose-1-Phosphate Uridyltransferase (GALT):** Deficiency causes **Classic Galactosemia (Type I)**. This is the most common and severe form, presenting with liver failure, cataracts, and intellectual disability. 2. **Galactokinase (GALK):** Deficiency causes **Galactokinase Deficiency (Type II)**. It is characterized primarily by early-onset cataracts due to galactitol accumulation in the lens, but lacks the severe systemic involvement seen in Type I. 3. **UDP-Galactose-4-Epimerase (GALE):** Deficiency causes **Epimerase Deficiency (Type III)**. It can present in a benign peripheral form or a severe systemic form similar to the classic type. **High-Yield Clinical Pearls for NEET-PG:** * **Accumulated Metabolite:** In GALT deficiency, Galactose-1-Phosphate accumulates, which is toxic to the liver, kidneys, and brain. * **Cataract Mechanism:** Excess galactose is diverted to the polyol pathway, where **Aldose Reductase** converts it into **Galactitol**. Galactitol is osmotically active, causing water to enter the lens. * **Diagnostic Clue:** Presence of **reducing sugars** (Clinitest positive) in urine, but a **negative glucose oxidase test** (Dipstick). * **Management:** Immediate withdrawal of lactose/galactose from the diet (e.g., stop breastfeeding, switch to soy milk).

Question 610: Which of the following is NOT a substrate for gluconeogenesis?

A. Alanine
B. Fatty acid (Correct Answer)
C. Pyruvate
D. Lactate

Explanation: **Explanation:** The core concept of gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. **Fatty acids (Option B)** are not substrates for gluconeogenesis because their oxidation yields **Acetyl-CoA**. In humans, the Pyruvate Dehydrogenase (PDH) reaction—which converts pyruvate to Acetyl-CoA—is **irreversible**. Furthermore, for every two carbons of Acetyl-CoA that enter the TCA cycle, two carbons are lost as $CO_2$. Consequently, there is no net gain of carbon to form oxaloacetate for the gluconeogenic pathway. (Note: Odd-chain fatty acids are an exception as they yield Propionyl-CoA, which is glucogenic). **Analysis of Incorrect Options:** * **Alanine (Option A):** This is the primary glucogenic amino acid. Through the **Cahill Cycle**, alanine is deaminated in the liver to form pyruvate, a direct precursor for glucose. * **Pyruvate (Option C):** Pyruvate is the starting point of the gluconeogenic pathway in the mitochondria, where it is carboxylated to oxaloacetate by Pyruvate Carboxylase. * **Lactate (Option D):** Produced by anaerobic glycolysis in muscles and RBCs, lactate is transported to the liver and converted back to pyruvate by Lactate Dehydrogenase (the **Cori Cycle**). **High-Yield Clinical Pearls for NEET-PG:** * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase is the rate-limiting enzyme of gluconeogenesis. * **Energy Requirement:** Gluconeogenesis is energy-expensive, requiring 6 ATP/GTP per molecule of glucose synthesized. This energy is provided by **$\beta$-oxidation of fatty acids**. * **Glycerol Exception:** While fatty acids aren't glucogenic, the **glycerol backbone** of triacylglycerols can enter gluconeogenesis via Dihydroxyacetone phosphate (DHAP).

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

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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