Carbohydrate Metabolism Practice Questions

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).

Q: In what form does the product of glycolysis enter the TCA cycle?

Acetyl-CoA. **Explanation:** The end product of aerobic glycolysis is **Pyruvate**. However, the TCA cycle (Krebs cycle) occurs in the mitochondrial matrix and cannot directly utilize pyruvate. **1. Why Acetyl-CoA is correct:** Before entering the TCA cycle, pyruvate must undergo **oxidative decarboxylation** to form **Acetyl-CoA**. This reaction is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme cluster located in the inner mitochondrial membrane. Acetyl-CoA then condenses with Oxaloacetate (OAA) to form Citrate, marking the official start of the TCA cycle. Therefore, Acetyl-CoA is the "connecting link" or the actual substrate that enters the cycle. **2. Why other options are incorrect:** * **Pyruvate:** While it is the product of glycolysis, it is a 3-carbon molecule that must be converted to the 2-carbon Acetyl-CoA before it can participate in the TCA cycle. * **NADH:** This is a coenzyme produced during glycolysis and the TCA cycle. It enters the Electron Transport Chain (ETC) to generate ATP but does not "enter" the TCA cycle as a substrate. * **Glucose:** This is the starting substrate for glycolysis, not the product. **3. High-Yield Clinical Pearls for NEET-PG:** * **PDH Complex Requirements:** It requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**eeds **N**ever **P**lay **L**oose). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to lipoic acid, leading to lactic acidosis and neurological symptoms. * **Regulation:** PDH is inhibited by its products (Acetyl-CoA and NADH) and activated by ADP and $Ca^{2+}$.

Q: Phosphofructokinase is the key enzyme of which metabolic pathway?

Glycolysis. **Explanation:** **Phosphofructokinase-1 (PFK-1)** is the most important regulatory and rate-limiting enzyme of **Glycolysis** (the Embden-Meyerhof pathway). It catalyzes the irreversible conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate using one molecule of ATP. This step is known as the "committed step" because once phosphorylated by PFK-1, the glucose metabolite is destined to complete the glycolytic pathway. **Analysis of Incorrect Options:** * **Glycogenolysis:** The rate-limiting enzyme is **Glycogen Phosphorylase**, which breaks down glycogen into glucose-1-phosphate. * **Glycogenesis:** The rate-limiting enzyme is **Glycogen Synthase**, responsible for forming $\alpha$-1,4-glycosidic bonds to synthesize glycogen. * **TCA Cycle:** The key regulatory enzyme is **Isocitrate Dehydrogenase**, which catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate. **High-Yield Clinical Pearls for NEET-PG:** * **Allosteric Regulation:** PFK-1 is allosterically **inhibited by ATP and Citrate** (signaling high energy status) and **activated by AMP and Fructose-2,6-bisphosphate**. * **Potent Activator:** Fructose-2,6-bisphosphate is the most potent physiological activator of PFK-1, produced by the bifunctional enzyme PFK-2. * **Clinical Correlation:** Deficiency of the M-isoform of PFK-1 in muscles leads to **Tarui Disease (Glycogen Storage Disease Type VII)**, characterized by exercise intolerance and muscle cramping. * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (promoting glycolysis), while glucagon decreases it.

Q: Which test is used to differentiate monosaccharides from disaccharides?

Barfoed's test. **Explanation:** The correct answer is **Barfoed’s test**. This test is specifically designed to distinguish between reducing monosaccharides and reducing disaccharides based on the speed of the reaction. **1. Why Barfoed’s test is correct:** Barfoed’s reagent consists of cupric acetate in dilute acetic acid (an acidic medium). Both monosaccharides and disaccharides can reduce cupric ions to cuprous oxide, forming a red precipitate. However, because the medium is acidic, monosaccharides (being stronger reducing agents) react much faster, typically within **2–3 minutes** of boiling. Reducing disaccharides (like lactose or maltose) require prolonged boiling (7–10 minutes) to show a reaction. **2. Analysis of Incorrect Options:** * **Benedict’s test:** Used to detect **reducing sugars** in general (e.g., glucose, fructose, lactose). It cannot differentiate between a monosaccharide and a disaccharide. * **Seliwanoff’s test:** Used to differentiate **keto-hexoses** (like fructose) from aldo-hexoses. It produces a cherry-red complex with ketoses. * **Rapid Furfural test:** Used to distinguish between **fructose (a keto-hexose)** and sucrose. It is a faster version of the Molisch test specifically for ketoses. **High-Yield Clinical Pearls for NEET-PG:** * **Barfoed’s Reagent:** Cupric acetate + Glacial acetic acid. * **Key distinction:** Monosaccharides = Scanty red precipitate in 7 mins. * **Osazone Test:** Another high-yield test where glucose and fructose form needle-shaped crystals, while lactose forms "powder-puff" or "hedgehog" shaped crystals. * **Bial’s Test:** Used specifically to detect **Pentoses** (e.g., ribose).

Q: All steps of N-glycosylation occur in the ER except?

Final trimming of the oligosaccharide. **Explanation:** N-glycosylation is a complex process where a pre-formed oligosaccharide is attached to the nitrogen atom of an **Asparagine (Asn)** residue. This process is spatially divided between the **Endoplasmic Reticulum (ER)** and the **Golgi apparatus**. **Why "Final trimming" is the correct answer:** The initial assembly of the core oligosaccharide and the initial trimming (removal of glucose and some mannose residues) occur in the **ER**. However, the **final trimming** and subsequent complex modifications (addition of galactose, sialic acid, or fucose) occur exclusively in the **Golgi apparatus**. Therefore, final trimming is not an ER-resident step. **Analysis of Incorrect Options:** * **A. Dolichol synthesis:** Dolichol phosphate is the essential lipid carrier located in the **ER membrane** upon which the oligosaccharide chain is built. * **B. Glycosyl transferase activity:** These enzymes are responsible for the sequential addition of sugars (N-acetylglucosamine and mannose) to the dolichol carrier within the **ER**. * **C. Protein-oligosaccharide transferase:** Also known as Oligosaccharyltransferase (OST), this enzyme complex resides in the **ER lumen** and catalyzes the transfer of the 14-sugar precursor from dolichol to the nascent protein. **High-Yield Clinical Pearls for NEET-PG:** * **Site of N-glycosylation:** Starts in ER, finishes in Golgi. (Contrast: **O-glycosylation** occurs exclusively in the Golgi). * **Sequence Motif:** N-glycosylation occurs at the **Asn-X-Ser/Thr** motif (where X is any amino acid except proline). * **Tunicamycin:** An antibiotic that inhibits the first step of N-glycosylation (blocks the formation of Dolichol-P-P-GlcNAC). * **I-Cell Disease:** Caused by a deficiency in phosphotransferase in the Golgi, leading to failure of mannose-6-phosphate tagging, causing lysosomal enzymes to be secreted extracellularly rather than reaching lysosomes.

Q: Which of the following reactions produces CO2?

HMP shunt pathway. **Explanation:** The correct answer is **B. HMP shunt pathway**. **1. Why HMP Shunt is correct:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway (PPP), consists of an oxidative and a non-oxidative phase. In the **oxidative phase**, the enzyme **6-phosphogluconate dehydrogenase** catalyzes the conversion of 6-phosphogluconate to Ribulose-5-phosphate. This reaction involves **oxidative decarboxylation**, where a carbon atom is released as **CO₂** while simultaneously generating NADPH. This is the primary source of CO₂ in this pathway. **2. Why other options are incorrect:** * **A. Amino acid synthesis:** While some specific amino acid degradative pathways (like the Glucose-Alanine cycle or Urea cycle) involve CO₂ fixation or release, general synthesis pathways are typically anabolic and do not characteristically produce CO₂ as a primary byproduct in the same way respiratory pathways do. * **C. Glycolysis:** This is a high-yield distinction. Glycolysis is the anaerobic breakdown of glucose to pyruvate (in the cytoplasm). It involves 10 enzymatic steps, **none of which release CO₂**. CO₂ is only released once pyruvate enters the mitochondria and undergoes the Link Reaction (Pyruvate Dehydrogenase complex) or the TCA cycle. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of HMP Shunt:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Key Products:** NADPH (for fatty acid synthesis and keeping glutathione reduced) and Ribose-5-phosphate (for nucleotide synthesis). * **Site:** Occurs entirely in the **cytosol**. It is highly active in tissues requiring NADPH, such as the adrenal cortex, liver, and RBCs. * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to neutralize free radicals in RBCs (low NADPH).

Question 1

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 2

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 3

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

Question 4

In what form does the product of glycolysis enter the TCA cycle?

Accepted Answer

Acetyl-CoA

Answer

Pyruvate

Answer

NADH

Answer

Glucose

Question 5

Phosphofructokinase is the key enzyme of which metabolic pathway?

Accepted Answer

Glycolysis

Answer

Glycogenolysis

Answer

Glycogenesis

Answer

TCA cycle

Question 6

Which test is used to differentiate monosaccharides from disaccharides?

Accepted Answer

Barfoed's test

Answer

Benedict's test

Answer

Seliwanoff's test

Answer

Rapid furfural test

Question 7

Glyconeogenic capability of a cell is determined by the presence of which enzyme?

Accepted Answer

Fructose 1,6-bisphosphatase

Answer

Pyruvate dehydrogenase

Answer

Glucose 6 phosphatase

Answer

Pyruvate carboxylase

Question 8

All steps of N-glycosylation occur in the ER except?

Accepted Answer

Final trimming of the oligosaccharide

Answer

Dolichol synthesis

Answer

Glycosyl transferase activity

Answer

Protein-oligosaccharide transferase activity

Question 9

Caffeine, a methyl xanthine, has been added to a variety of cell types. Which one of the following would be expected in various cell types treated with caffeine and epinephrine?

Accepted Answer

Decreased activity of liver glycogen synthase

Answer

Decreased activity of liver PKA

Answer

Decreased activity of muscle PKA

Answer

Increased activity of liver pyruvate kinase

Question 10

Which of the following reactions produces CO2?

Accepted Answer

HMP shunt pathway

Answer

Amino acid synthesis

Answer

Glycolysis

Answer

None of the above

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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