Carbohydrate Metabolism Practice Questions

Q: NADPH+ and H+ is generated in the reaction catalyzed by which enzyme?

Glucose-6-phosphate dehydrogenase (G-6-PD). **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G-6-PD)**. This enzyme catalyzes the first and rate-limiting step of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). In this reaction, Glucose-6-phosphate is oxidized to 6-phosphogluconolactone, and **NADP+ is reduced to NADPH + H+**. This pathway is the primary source of NADPH in the body, which is essential for reductive biosynthesis (e.g., fatty acids, steroids) and maintaining reduced glutathione to protect cells against oxidative stress. **Analysis of Incorrect Options:** * **Lactate dehydrogenase (LDH):** Involved in anaerobic glycolysis; it interconverts pyruvate and lactate using **NADH/NAD+**, not NADPH. * **Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD):** A key enzyme in glycolysis that converts Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, producing **NADH**. * **Alcohol dehydrogenase:** Catalyzes the oxidation of ethanol to acetaldehyde in the cytosol, utilizing **NAD+** as a coenzyme to produce **NADH**. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Since RBCs lack mitochondria, they rely solely on the HMP shunt for NADPH. Deficiency leads to inadequate reduced glutathione, resulting in **hemolysis** triggered by oxidative stress (e.g., Fava beans, Primaquine, or infections). * **Heinz Bodies & Bite Cells:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs.

Q: The pentose phosphate pathway produces which of the following?

NADPH. **Explanation:** The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not energy production but the generation of biosynthetic precursors. **Why NADPH is the correct answer:** The oxidative phase of the PPP is the body's primary source of **NADPH** (Reduced Nicotinamide Adenine Dinucleotide Phosphate). This occurs via the rate-limiting enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids, cholesterol, and steroid hormones. 2. **Antioxidant Defense:** Maintaining reduced glutathione to protect cells (especially RBCs) against reactive oxygen species (ROS). **Why other options are incorrect:** * **ATP & ADP:** The PPP is unique because it **neither consumes nor produces ATP**. It is an energy-neutral pathway focused on carbon shuffling and redox potential. * **NADH:** While NADH is structurally similar to NADPH, it is primarily used in the **Electron Transport Chain (ETC)** for ATP generation. The PPP specifically produces NADPH, which has a phosphate group that directs it toward anabolic (building) reactions rather than catabolic (breaking down) energy production. **NEET-PG Clinical Pearls:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Non-oxidative phase:** Produces **Ribose-5-Phosphate**, essential for nucleotide (DNA/RNA) synthesis. * **Clinical Correlation:** G6PD deficiency leads to **hemolytic anemia** because RBCs cannot generate enough NADPH to combat oxidative stress (e.g., after eating fava beans or taking Primaquine), leading to the formation of **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** Highly active in the liver, adrenal cortex, lactating mammary glands, and RBCs.

Q: Which metabolic pathway occurs partly in the mitochondria and partly in the cytoplasm?

Gluconeogenesis. **Explanation:** Metabolic pathways are often compartmentalized to ensure efficient regulation. While many pathways occur entirely in one compartment, a few "dual-compartment" pathways are high-yield for NEET-PG. **1. Why Gluconeogenesis is Correct:** Gluconeogenesis begins in the **mitochondria** with the conversion of Pyruvate to Oxaloacetate (OAA) by *Pyruvate Carboxylase*. Since OAA cannot cross the mitochondrial membrane, it is converted to Malate (or Aspartate), shuttled into the **cytoplasm**, and converted back to OAA. The remaining steps, starting from the conversion of OAA to Phosphoenolpyruvate (PEP) by *PEPCK*, occur in the cytoplasm. **2. Analysis of Incorrect Options:** * **A. Electron Transport Chain:** Occurs exclusively in the **Inner Mitochondrial Membrane**. * **B. Link Reaction:** The conversion of Pyruvate to Acetyl-CoA by the PDH complex occurs entirely within the **Mitochondrial Matrix**. * **C. Fatty Acid Synthesis:** Occurs primarily in the **Cytoplasm** (the "Extramitochondrial" pathway). Note: Fatty acid *oxidation* occurs in the mitochondria. **3. Clinical Pearls & High-Yield Facts:** * **Mnemonic for Dual-Compartment Pathways:** Remember **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. These three pathways require enzymes from both the mitochondria and cytoplasm. * **Key Enzyme:** *Pyruvate Carboxylase* (the first step of gluconeogenesis) is a mitochondrial enzyme and requires **Biotin** as a cofactor. * **The "Shuttle":** The Malate-Aspartate shuttle is crucial for transporting carbon skeletons out of the mitochondria during gluconeogenesis.

Q: In glycogen metabolism, which enzyme acts on the 1-6 glycosidic bond of glycogen to produce glucose?

Glucose. **Explanation:** In glycogenolysis, the breakdown of glycogen involves two primary enzymes: **Glycogen Phosphorylase** and the **Debranching Enzyme**. The **Debranching Enzyme** is a bifunctional protein. Its second activity, **$\alpha$-1,6-glucosidase**, specifically hydrolyzes the $\alpha$-1,6-glycosidic bond at the branch point. Unlike the action of phosphorylase (which uses inorganic phosphate to release Glucose-1-Phosphate), the $\alpha$-1,6-glucosidase activity uses water (hydrolysis) to release a **free Glucose** molecule directly. For every branch point, one molecule of free glucose is produced, accounting for approximately 8-10% of the total glucose released from glycogen. **Analysis of Options:** * **Option A (Glucose-1-phosphate):** This is the product of Glycogen Phosphorylase acting on $\alpha$-1,4-linkages. It is the major product of glycogenolysis but is not released from the 1-6 bond. * **Option B (Glucose-6-phosphate):** This is formed in the liver by the isomerization of Glucose-1-phosphate via *Phosphoglucomutase*. It is not a direct product of the debranching enzyme. * **Option C (Maltose):** This is a disaccharide produced during the digestion of starch by amylase, not during intracellular glycogen metabolism. **NEET-PG High-Yield Pearls:** * **Debranching Enzyme Deficiency:** Leads to **Cori’s Disease (GSD Type III)**, characterized by the accumulation of "limit dextrins" (abnormal glycogen with short outer branches). * **Ratio:** Glycogenolysis yields Glucose-1-Phosphate and free Glucose in a ratio of approximately **10:1**. * **Key Difference:** Phosphorylase performs *phosphorolysis* (saves ATP), while debranching enzyme performs *hydrolysis*.

Q: Which of the following statements about GLUT is false?

GLUT-2 is needed in the brain. The correct answer is **A. GLUT-2 is needed in the brain**. This statement is false because the brain primarily relies on **GLUT-1** (for crossing the blood-brain barrier) and **GLUT-3** (for neuronal uptake). GLUT-2 is a high-capacity, low-affinity transporter found in the liver, pancreatic beta cells, small intestine, and renal tubules, where it acts as a "glucose sensor." ### Explanation of Options: * **Option A (False/Correct):** GLUT-2 is not the primary transporter for the brain. The brain requires a constant glucose supply regardless of blood sugar levels; therefore, it utilizes GLUT-3, which has a very low Km (high affinity). * **Option B (True):** GLUT-3 is indeed present in the placenta to ensure a steady transfer of glucose to the fetus. * **Option C (True):** GLUT-4 is the only **insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin. It is primarily found in skeletal muscle, cardiac muscle, and **adipose tissue**. * **Option D (True):** GLUT-3 has a wide distribution, including the brain (neurons), placenta, liver, kidneys, and specifically the **testis and intestine**. ### High-Yield Clinical Pearls for NEET-PG: * **GLUT-1 Deficiency Syndrome:** Presents with infantile seizures and developmental delay due to impaired glucose transport across the blood-brain barrier. * **SGLT vs. GLUT:** Remember that SGLT (1 & 2) are active transporters (secondary active) used in the intestine and kidneys, whereas all GLUTs (1–5) facilitate **passive diffusion**. * **GLUT-5:** Unique because it is primarily a **fructose** transporter, located in the small intestine and spermatozoa. * **Bidirectional Transport:** GLUT-2 is unique for its ability to transport glucose in both directions (e.g., during gluconeogenesis in the liver).

Q: Which of the following enzymes are NOT involved in glycogen metabolism?

Glycogen synthase A. ### Explanation In glycogen metabolism, enzymes exist in two interconvertible forms: an active form and an inactive form, regulated primarily by covalent modification (phosphorylation/dephosphorylation). **Why Option D is the Correct Answer:** The term **"Glycogen synthase A"** is a misnomer in standard biochemical nomenclature. While "Glycogen Synthase **a**" (lowercase) refers to the active, dephosphorylated form of the enzyme, "Glycogen Synthase **A**" (uppercase) is not a recognized clinical or biochemical designation for the enzyme. In the context of this specific competitive exam question, it serves as the "distractor" that does not exist in the metabolic pathway. **Analysis of Other Options:** * **Glycogen phosphorylase b (Option A):** This is the **inactive**, dephosphorylated form of the enzyme responsible for 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 that functions independently of the glucose-6-phosphate concentration. * **Glycogen synthase D (Option C):** The "D" stands for **Dependent**. This is the inactive, phosphorylated form of the enzyme which requires high concentrations of the allosteric activator, glucose-6-phosphate, to function. **High-Yield Clinical Pearls for NEET-PG:** * **Reciprocal Regulation:** Glucagon and Epinephrine trigger phosphorylation, which **activates** Glycogen Phosphorylase (promoting breakdown) but **inactivates** Glycogen Synthase (inhibiting synthesis). * **Insulin Effect:** Insulin triggers dephosphorylation via Protein Phosphatase-1, activating Glycogen Synthase (I-form). * **Rate-Limiting Enzymes:** Glycogen Synthase is the rate-limiting enzyme for glycogenesis; Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis.

Q: A 3-day-old child vomits everything he feeds, has a distended abdomen & diarrhea. The urine is positive for Benedict's test for reducing substances. What is the substance in the urine?

Galactose. ### Explanation The clinical presentation of a 3-day-old neonate with vomiting, abdominal distension, and diarrhea shortly after starting milk feeds, coupled with a positive Benedict’s test for reducing substances in the urine, is a classic description of **Classic Galactosemia** (deficiency of Galactose-1-phosphate uridyltransferase or GALT). **Why Galactose is the correct answer:** In Galactosemia, the body cannot metabolize galactose (a component of lactose found in breast milk and cow's milk). Galactose and its metabolites accumulate in the blood and are excreted in the urine. Since galactose is a **reducing sugar**, it reacts positively with Benedict’s reagent. The onset is typically within the first few days of life as soon as milk feeds begin. **Analysis of Incorrect Options:** * **A. Sucrose:** It is a **non-reducing sugar** and would yield a negative Benedict’s test. Furthermore, sucrose is not a primary component of early neonatal milk feeds. * **B. Glucose:** While glucose is a reducing sugar, isolated glucosuria in a 3-day-old with these systemic symptoms is less likely than Galactosemia. Glucosuria is usually associated with Diabetes Mellitus or renal tubular defects. * **D. Fructose:** Fructose is a reducing sugar (Hereditary Fructose Intolerance), but symptoms only appear when fructose or sucrose is introduced into the diet (e.g., fruit juices or honey), which typically occurs much later than 3 days of age. **NEET-PG High-Yield Pearls:** * **Screening:** Benedict’s test is a non-specific screening tool for reducing substances (Galactose, Glucose, Fructose, Lactose). A **Glucose Oxidase dipstick** test will be **negative** in Galactosemia because it only detects glucose. * **Complications:** If untreated, look for **Oil-drop cataracts**, hepatosplenomegaly, jaundice, and a high risk of **E. coli sepsis**. * **Management:** Immediate withdrawal of milk and initiation of a **soy-based (lactose-free)** formula.

Q: A patient has blood group antigen NAG transferase present but galactosyltransferase absent. Which blood group does this patient have?

Group A. ### Explanation The determination of ABO blood groups depends on the presence of specific enzymes (glycosyltransferases) that add terminal sugar molecules to a precursor substance known as the **H substance**. **1. Why Option A is Correct:** * **N-acetylgalactosaminyltransferase (NAG transferase):** This enzyme is coded by the **A gene**. It attaches N-acetylgalactosamine to the H substance, converting it into the **A antigen**. * **Galactosyltransferase:** This enzyme is coded by the **B gene**. It attaches D-galactose to the H substance to form the **B antigen**. * In this patient, NAG transferase is present (forming A antigen) but galactosyltransferase is absent (no B antigen formed). Therefore, the patient’s red blood cells express only the A antigen, making them **Blood Group A**. **2. Why the Other Options are Incorrect:** * **Group B:** Requires the presence of galactosyltransferase and the absence of NAG transferase. * **Group AB:** Requires both enzymes to be present, resulting in the expression of both A and B antigens. * **Group O:** Characterized by the absence of both functional enzymes. The H substance remains unconverted (H antigen only). **3. High-Yield Clinical Pearls for NEET-PG:** * **Immunogenetics:** The ABO gene is located on **Chromosome 9**. * **H Substance:** It is the precursor for both A and B antigens. It is formed by the action of fucosyltransferase (H enzyme) on a precursor chain. * **Bombay Phenotype (Oh):** Occurs when the **H gene is absent**. Even if the patient has A or B transferases, they cannot attach sugars because there is no H substance. These patients test as Group O but have potent anti-H antibodies. * **Universal Donor/Recipient:** Group O negative is the universal donor (no antigens); Group AB positive is the universal recipient (no antibodies).

Question 1

NADPH+ and H+ is generated in the reaction catalyzed by which enzyme?

Accepted Answer

Glucose-6-phosphate dehydrogenase (G-6-PD)

Answer

Lactate dehydrogenase (LDH)

Answer

Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD)

Answer

Alcohol dehydrogenase

Question 2

The pentose phosphate pathway produces which of the following?

Accepted Answer

NADPH

Answer

ATP

Answer

ADP

Answer

NADH

Question 3

Which metabolic pathway occurs partly in the mitochondria and partly in the cytoplasm?

Accepted Answer

Gluconeogenesis

Answer

Electron transport chain

Answer

Link reaction

Answer

Fatty acid synthesis

Question 4

A male patient presents with pain in his calf muscles upon exercise. Muscle biopsy reveals the presence of glycogen. Which enzyme deficiency is most likely responsible for this condition?

Accepted Answer

Phosphofructokinase I

Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Glucose 6 phosphatase

Question 5

In glycogen metabolism, which enzyme acts on the 1-6 glycosidic bond of glycogen to produce glucose?

Accepted Answer

Glucose

Answer

Glucose-1-phosphate

Answer

Glucose-6-phosphate

Answer

Maltose

Question 6

Which of the following statements about GLUT is false?

Accepted Answer

GLUT-2 is needed in the brain

Answer

GLUT-3 is present in the placenta

Answer

GLUT-4 is present in adipose tissue

Answer

GLUT-3 is present in both intestine and testis

Question 7

A 5-month-old child presents with hepatomegaly, ketosis, hyperuricemia, and malaise. What is the most likely diagnosis?

Accepted Answer

Mucopolysaccharidosis

Answer

Diabetes mellitus

Answer

Urea cycle defect

Answer

Glycogen storage disease

Question 8

Which of the following enzymes are NOT involved in glycogen metabolism?

Accepted Answer

Glycogen synthase A

Answer

Glycogen phosphorylase b

Answer

Glycogen synthase I

Answer

Glycogen synthase C

Question 9

A 3-day-old child vomits everything he feeds, has a distended abdomen & diarrhea. The urine is positive for Benedict's test for reducing substances. What is the substance in the urine?

Accepted Answer

Galactose

Answer

Sucrose

Answer

Glucose

Answer

Fructose

Question 10

A patient has blood group antigen NAG transferase present but galactosyltransferase absent. Which blood group does this patient have?

Accepted Answer

Group A

Answer

Group B

Answer

Group AB

Answer

Group O

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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