Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 11: Which mucopolysaccharide possesses anticoagulant activity?

A. Heparin (Correct Answer)
B. Phenindione
C. Rivaroxaban
D. Dabigatran

Explanation: **Explanation:** **Heparin** is the correct answer because it is a classic example of a **Glycosaminoglycan (GAG)**, also known as a **mucopolysaccharide**. It is a highly sulfated, linear polysaccharide consisting of repeating disaccharide units (D-glucosamine and uronic acid). * **Mechanism:** Heparin acts as an indirect anticoagulant by binding to **Antithrombin III**. This binding induces a conformational change in Antithrombin III, accelerating its ability to inactivate Thrombin (Factor IIa) and Factor Xa by 1,000-fold. * **Location:** It is naturally produced by mast cells and basophils. **Analysis of Incorrect Options:** * **Phenindione (B):** This is a synthetic vitamin K antagonist (indanedione derivative). While it is an anticoagulant, it is a small molecule drug, not a mucopolysaccharide. * **Rivaroxaban (C):** This is a Direct Factor Xa inhibitor (NOAC). It is a synthetic small molecule, not a carbohydrate-based molecule. * **Dabigatran (D):** This is a Direct Thrombin Inhibitor (DTI). Like the others, it is a synthetic drug and does not belong to the class of mucopolysaccharides. **High-Yield Clinical Pearls for NEET-PG:** * **Most Acidic GAG:** Heparin is the most acidic (most negatively charged) substance in the human body due to its high sulfate content. * **Antidote:** The negative charge of Heparin is neutralized by the positively charged **Protamine Sulfate**. * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein core. * **Chondroitin Sulfate:** The most abundant GAG in the body (found in cartilage and bone).

Question 12: Glycogenin primer is glucosylated by:

A. UDP Glucose (Correct Answer)
B. Glucose 1-phosphate
C. UDP Glucose 1-phosphate
D. UDP Glucose 6-phosphate

Explanation: ### Explanation **Concept Overview:** Glycogenesis (glycogen synthesis) cannot start *de novo*. It requires a primer called **Glycogenin**, a self-glucosylating enzyme. Glycogenin acts as a scaffold and an enzyme to initiate the synthesis of a short glycogen chain (about 8 glucose residues) before Glycogen Synthase takes over. **Why Option A is Correct:** The active donor of glucose units in glycogenesis is **UDP-Glucose**. Glycogenin possesses glucosyltransferase activity; it attaches a glucose molecule from UDP-Glucose to the hydroxyl group of a specific **Tyrosine residue (Tyr-194)** on itself. This process is called autoglucosylation. Once a short chain of glucose is formed, Glycogen Synthase extends the chain using more UDP-Glucose. **Why Other Options are Incorrect:** * **B. Glucose 1-phosphate:** This is an intermediate in the pathway, but it lacks the high-energy bond required for the transfer. It must be converted to UDP-Glucose by the enzyme *UDP-Glucose pyrophosphorylase*. * **C & D. UDP Glucose 1-phosphate / 6-phosphate:** These molecules do not exist as standard substrates in the glycogenesis pathway. Glucose must be phosphorylated at the 6th position (G6P), isomerized to the 1st position (G1P), and then activated by UTP to form UDP-Glucose. **High-Yield Clinical Pearls for NEET-PG:** * **The Primer:** Glycogenin is both an enzyme and a protein primer. * **The Linkage:** The first glucose is attached to the **Tyrosine** residue of Glycogenin via an **O-glycosidic bond**. * **Rate-Limiting Step:** While Glycogenin initiates the process, **Glycogen Synthase** is the rate-limiting enzyme of glycogenesis. * **Energy Requirement:** The formation of one UDP-Glucose molecule from Glucose-1-Phosphate consumes one **UTP** (equivalent to one ATP).

Question 13: Essential Pentosuria is due to a defect in which metabolic pathway?

A. Glycolysis
B. Hexose Monophosphate (HMP) Shunt
C. Tricarboxylic Acid (TCA) Cycle
D. Uronic acid pathway (Correct Answer)

Explanation: **Explanation:** **Essential Pentosuria** is a rare, benign autosomal recessive condition caused by a deficiency of the enzyme **L-xylulose reductase**. This enzyme is a key component of the **Uronic Acid Pathway** (also known as the Glucuronic Acid Pathway). 1. **Why the Uronic Acid Pathway is correct:** In this pathway, Glucuronic acid is converted into L-xylulose. Under normal conditions, L-xylulose reductase reduces L-xylulose to **xylitol**, which then enters the HMP shunt. In Essential Pentosuria, the deficiency of this enzyme leads to the accumulation of L-xylulose in the blood and its subsequent excretion in the urine. 2. **Why other options are incorrect:** * **Glycolysis:** This is the primary pathway for glucose breakdown into pyruvate/lactate to produce ATP. Defects here (e.g., Pyruvate Kinase deficiency) typically lead to hemolytic anemia, not pentosuria. * **HMP Shunt:** While the HMP shunt produces pentoses (like Ribose-5-phosphate), Essential Pentosuria specifically involves the inability to process L-xylulose *before* it can re-enter the HMP shunt. * **TCA Cycle:** This is the final common pathway for the oxidation of carbohydrates, fats, and proteins. Defects here are usually incompatible with life or result in severe multisystemic lactic acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Patients excrete large amounts of **L-xylulose** in the urine. * **Clinical Significance:** It is a **benign** condition (asymptomatic). Its primary clinical importance is that L-xylulose is a **reducing sugar**, which can lead to a false-positive diagnosis of Diabetes Mellitus during routine urine copper reduction tests (Benedict's test). * **Drug Interaction:** Administration of drugs like **Aminopyrine** or **Barbiturates** can increase the rate of the uronic acid pathway, thereby increasing the excretion of L-xylulose in these patients.

Question 14: The malate shuttle is important in which of the following processes?

A. Glycolysis only
B. Glycogenolysis
C. Glycolysis and gluconeogenesis (Correct Answer)
D. Glycogen synthesis

Explanation: **Explanation:** The **Malate-Aspartate Shuttle** is a crucial biochemical mechanism used to transport reducing equivalents across the inner mitochondrial membrane, which is impermeable to NADH. **Why Option C is Correct:** 1. **Glycolysis:** This process occurs in the cytosol and generates NADH. For ATP production via the Electron Transport Chain (ETC), these electrons must enter the mitochondria. The malate shuttle facilitates this by reducing oxaloacetate to malate (using NADH), which then crosses into the mitochondria to regenerate NADH for the ETC. 2. **Gluconeogenesis:** A key step involves converting pyruvate to phosphoenolpyruvate (PEP). Pyruvate enters the mitochondria and is converted to oxaloacetate (OAA). However, OAA cannot cross back into the cytosol. It is reduced to malate, transported out via the shuttle, and then re-oxidized to OAA in the cytosol to continue gluconeogenesis. **Why other options are incorrect:** * **Option A:** While essential for aerobic glycolysis, the shuttle is equally vital for gluconeogenesis (transporting OAA equivalents). * **Options B & D:** Glycogen synthesis and glycogenolysis are primarily cytosolic processes involving the phosphorylation and dephosphorylation of glucose molecules; they do not directly depend on the mitochondrial transport of NADH or OAA via the malate shuttle. **High-Yield Clinical Pearls for NEET-PG:** * **ATP Yield:** The Malate-Aspartate Shuttle is more efficient than the Glycerol-3-Phosphate shuttle, yielding **2.5 ATP** per NADH (vs. 1.5 ATP). * **Tissue Specificity:** It is predominantly found in the **heart, liver, and kidneys**. * **Key Enzymes:** It requires Malate Dehydrogenase and Aspartate Aminotransferase (which requires **Vitamin B6**).

Question 15: Keratan sulfate I and II is found in which of the following locations?

A. Cornea
B. Cartilage
C. Loose connective tissue
D. All of the above (Correct Answer)

Explanation: **Explanation:** Keratan sulfate (KS) is a unique glycosaminoglycan (GAG) because it contains galactose instead of the usual uronic acid. It exists in two primary forms, distinguished by their location and the type of glycosidic linkage to their core protein: * **Keratan Sulfate I (KS I):** This is found predominantly in the **cornea**. It is N-linked to asparagine residues. Its precise arrangement is critical for corneal transparency; a deficiency or structural abnormality leads to Macular Corneal Dystrophy. * **Keratan Sulfate II (KS II):** This is found in **cartilage**, bone, and **loose connective tissue**. It is O-linked to serine or threonine residues. It acts as a shock absorber in joints alongside chondroitin sulfate. Since KS I is specific to the cornea (Option A) and KS II is found in cartilage (Option B) and connective tissues (Option C), the correct answer is **All of the above**. **Why other options are included:** * **Cornea:** While KS I is specific here, selecting only this ignores the systemic distribution of KS II. * **Cartilage/Connective Tissue:** These are the primary sites for KS II, but excluding the cornea would overlook the most clinically significant site of KS I. **High-Yield Clinical Pearls for NEET-PG:** * **Morquio Syndrome (MPS IV):** Caused by a deficiency in enzymes required to degrade Keratan Sulfate. It presents with severe skeletal dysplasia and corneal clouding, but notably **normal intelligence**. * **Unique Structure:** KS is the only GAG that **lacks uronic acid** (it has galactose instead). * **Linkage:** Remember **I** is **N**-linked (Cornea) and **II** is **O**-linked (Skeletal).

Question 16: Which of the following is an epimer of glucose?

A. Mannose (Correct Answer)
B. Glyceraldehyde
C. Fructose
D. None of the above

Explanation: **Explanation:** **1. Why Mannose is the Correct Answer:** Epimers are stereoisomers that differ in configuration around only one specific carbon atom (other than the carbonyl carbon). Glucose and Mannose are **C-2 epimers**. They have the same molecular formula ($C_6H_{12}O_6$) and structure, except for the orientation of the hydroxyl (-OH) group at the second carbon atom. In glucose, the -OH at C-2 is on the right (Fischer projection), while in mannose, it is on the left. **2. Why Other Options are Incorrect:** * **Glyceraldehyde:** This is a triose (3-carbon sugar) and the simplest aldose. It is not an epimer of glucose because it lacks the same number of carbon atoms. * **Fructose:** Fructose is a **functional isomer** (keto-hexose) of glucose (aldo-hexose). While they share the same molecular formula, they differ in their functional groups (ketone vs. aldehyde), not just the configuration at a single chiral center. **3. High-Yield Clinical Pearls for NEET-PG:** * **C-4 Epimer:** Galactose is the C-4 epimer of glucose. This is a frequent exam favorite. * **Epimerization Enzyme:** The interconversion of epimers (e.g., UDP-glucose to UDP-galactose) is catalyzed by enzymes called **epimerases**. * **Essential Concept:** All epimers are isomers, but not all isomers are epimers. * **Mnemonics:** * **M**annose = **2** (M looks like a 2 upside down) $\rightarrow$ **C-2** epimer. * **G**alactose = **4** (G has 4 curves) $\rightarrow$ **C-4** epimer.

Question 17: Cellulose is a:

A. Fructose polymer
B. Non-starch polysaccharide (Correct Answer)
C. Starch polysaccharide
D. Glycosaminoglycan

Explanation: **Explanation:** **Cellulose** is a linear homopolysaccharide composed of **β-D-glucose** units linked by **β(1→4) glycosidic bonds**. 1. **Why B is correct:** In nutritional biochemistry, polysaccharides are broadly categorized into starch and **Non-Starch Polysaccharides (NSP)**. NSPs are the primary components of dietary fiber. Unlike starch (which has α-linkages), the β-linkages in cellulose cannot be hydrolyzed by human digestive enzymes (amylases). Therefore, cellulose remains undigested in the human gut, classifying it as a non-starch polysaccharide. 2. **Why other options are incorrect:** * **A (Fructose polymer):** These are called **Fructans** (e.g., Inulin). Cellulose is strictly a glucose polymer (Glucan). * **C (Starch polysaccharide):** Starch consists of Amylose and Amylopectin, which contain **α(1→4)** and **α(1→6)** linkages. These are easily digested by humans. * **D (Glycosaminoglycan):** GAGs (like Heparin or Hyaluronic acid) are heteropolysaccharides containing amino sugars and uronic acids. Cellulose is a simple homopolysaccharide. **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Fiber:** Cellulose provides "bulk" to the stool, promoting peristalsis and preventing constipation. * **Ruminants:** Unlike humans, ruminants can digest cellulose because their rumen contains bacteria that secrete the enzyme **cellulase**. * **Inulin vs. Insulin:** Do not confuse Inulin (a fructose polymer used to measure GFR) with Insulin (a peptide hormone). * **Bonding:** The β(1→4) linkage allows cellulose to form long, straight chains that pack into rigid fibrils, providing structural support to plant cell walls.

Question 18: Which of the following carbohydrates is most lipogenic?

A. Fructose (Correct Answer)
B. Glucose
C. Galactose
D. Ribose

Explanation: **Explanation:** **Fructose** is the most lipogenic carbohydrate because it bypasses the major rate-limiting step of glycolysis. In the liver, glucose metabolism is strictly regulated by the enzyme **Phosphofructokinase-1 (PFK-1)**. However, fructose enters the glycolytic pathway distal to this step via the action of fructokinase, which converts it to fructose-1-phosphate, eventually forming dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate. By bypassing PFK-1, fructose provides an unregulated flow of carbon precursors (Acetyl-CoA and Glycerol-3-phosphate) for **De Novo Lipogenesis (DNL)**. This leads to a rapid increase in fatty acid synthesis and VLDL secretion, often contributing to non-alcoholic fatty liver disease (NAFLD). **Analysis of Incorrect Options:** * **B. Glucose:** Its metabolism is tightly controlled by PFK-1 (inhibited by high ATP and Citrate). This "bottleneck" prevents the rapid flooding of the lipogenic pathway. * **C. Galactose:** It is primarily converted to Glucose-1-phosphate and enters the standard glycolytic pathway, thus remaining subject to the same regulatory constraints as glucose. * **D. Ribose:** This is a pentose sugar primarily utilized in the Pentose Phosphate Pathway (PPP) for nucleotide synthesis rather than being a primary substrate for energy or fat storage. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Fructokinase has a much higher $V_{max}$ than glucokinase, leading to rapid fructose metabolism. * **Clinical Link:** High dietary fructose is a major driver of hypertriglyceridemia and insulin resistance. * **Essential Fructosuria:** Caused by a deficiency of Fructokinase (asymptomatic). * **Hereditary Fructose Intolerance (HFI):** Caused by a deficiency of Aldolase B (severe; presents with hypoglycemia and jaundice after weaning).

Question 19: Insulin increases the following pathways in the liver EXCEPT:

A. Fatty acid synthesis
B. Glycogen synthesis
C. Protein synthesis
D. Glucose synthesis (Correct Answer)

Explanation: **Explanation:** Insulin is the body’s primary **anabolic hormone**, secreted by the pancreatic beta cells in response to high blood glucose levels (the fed state). Its primary goal is to lower blood glucose by promoting storage and utilization while inhibiting the production of new glucose. **Why "Glucose Synthesis" is the correct answer:** Glucose synthesis (Gluconeogenesis) is a **catabolic/fasting state pathway** primarily regulated by Glucagon and Cortisol. Insulin **inhibits** gluconeogenesis in the liver by downregulating key enzymes like *PEPCK* and *Fructose-1,6-bisphosphatase*. Therefore, insulin decreases, rather than increases, glucose synthesis. **Analysis of Incorrect Options:** * **Fatty acid synthesis:** Insulin promotes lipogenesis by activating *Acetyl-CoA Carboxylase*. It converts excess glucose into triglycerides for storage. * **Glycogen synthesis:** Insulin stimulates *Glycogen Synthase* (via dephosphorylation) to store glucose as glycogen in the liver and muscles. * **Protein synthesis:** Insulin is strongly anabolic for proteins; it increases amino acid uptake and stimulates ribosomal translation. **NEET-PG High-Yield Pearls:** * **Rate-Limiting Enzyme:** Insulin activates **Phosphofructokinase-1 (PFK-1)** indirectly by increasing Fructose-2,6-bisphosphate levels, thereby stimulating glycolysis. * **Mechanism of Action:** Insulin acts via a **Tyrosine Kinase receptor** (catalytic receptor). * **GLUT-4:** Remember that while insulin increases glucose uptake in muscles and adipose tissue via GLUT-4, glucose uptake in the **liver is insulin-independent (via GLUT-2)**. Insulin’s effect on the liver is strictly metabolic (regulating enzymes).

Question 20: Blood samples for glucose estimation are collected in fluoride bulbs/tubes as fluoride prevents glycolysis by inhibition of which enzyme?

A. Enolase (Correct Answer)
B. Aldolase
C. Glucokinase
D. Phosphofructokinase

Explanation: **Explanation:** The correct answer is **Enolase (Option A)**. **1. Why Enolase is the correct answer:** In clinical practice, blood glucose estimation requires the prevention of *in vitro* glycolysis by red blood cells (RBCs), which can otherwise decrease glucose levels by approximately 5–10 mg/dL per hour. Sodium fluoride (NaF) is added to the collection tubes (grey-top bulbs) to act as a glycolytic inhibitor. * **Mechanism:** Fluoride ions ($F^-$) bind with magnesium ($Mg^{2+}$) and phosphate to form a **magnesium-fluorophosphate complex**. This complex competitively inhibits **Enolase**, the enzyme responsible for the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). Since Enolase requires $Mg^{2+}$ as a cofactor, its removal halts the glycolytic pathway. **2. Why other options are incorrect:** * **Aldolase (B):** Cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. It is not inhibited by fluoride. * **Glucokinase (C):** The first step of glycolysis in the liver. While it regulates glucose entry, it is not the target of fluoride. * **Phosphofructokinase (D):** The rate-limiting enzyme of glycolysis, inhibited by high ATP and citrate, but not by fluoride. **3. Clinical Pearls for NEET-PG:** * **The "Grey Top" Tube:** Contains Sodium Fluoride (antiglycolytic agent) and Potassium Oxalate (anticoagulant). * **Delayed Effect:** Fluoride inhibition of Enolase takes about 1–2 hours to fully stabilize glucose levels; therefore, immediate processing is still ideal. * **Fluoride & Urease:** Fluoride also inhibits the enzyme **Urease**. Therefore, fluoride bulbs should **not** be used for blood urea estimation if the laboratory uses the urease method. * **Enolase Isoforms:** Neuron-specific enolase (NSE) is a high-yield clinical marker for small cell lung cancer and neuroblastoma.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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