Carbohydrate Metabolism Practice Questions

Q: Which enzyme is stimulated in gluconeogenesis during starvation?

Carboxylase. **Explanation:** In starvation, the body must maintain blood glucose levels through **gluconeogenesis**. The correct answer is **Pyruvate Carboxylase**, which is the first regulatory enzyme of this pathway. **1. Why Carboxylase is Correct:** During starvation, high levels of **Acetyl-CoA** (from fatty acid oxidation) act as an obligatory allosteric activator of **Pyruvate Carboxylase**. This enzyme converts pyruvate into oxaloacetate (OAA) in the mitochondria. This is a crucial "bypass" step to overcome the irreversible nature of the glycolytic enzyme pyruvate kinase, effectively shunting substrates toward glucose synthesis. **2. Why the Other Options are Incorrect:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts pyruvate to Acetyl-CoA for the TCA cycle. In starvation, PDH is **inhibited** by Acetyl-CoA and NADH to conserve three-carbon compounds for glucose production. * **Pyruvate Kinase:** This is a glycolytic enzyme. During starvation, it is **inhibited** by glucagon-mediated phosphorylation (via cAMP) to prevent the breakdown of phosphoenolpyruvate (PEP), ensuring PEP is used for gluconeogenesis instead. * **Glucokinase:** This enzyme functions in the liver to trap glucose during the well-fed state. In starvation, its activity is **decreased** (low insulin/high glucagon) to prevent the liver from consuming the glucose it is trying to produce. **High-Yield NEET-PG Pearls:** * **Biotin (Vitamin B7):** Pyruvate Carboxylase requires Biotin as a co-factor. Remember: "All carboxylases require Biotin." * **Subcellular Localization:** Pyruvate Carboxylase is located in the **mitochondria**, whereas the rest of gluconeogenesis occurs primarily in the cytosol. * **Key Activator:** Acetyl-CoA is the most important metabolic signal that switches the cell from glucose oxidation to glucose synthesis.

Q: Where is GLUT 4 primarily found?

Cardiac muscle. **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.

Q: Essential pentosuria is due to deficiency of which enzyme?

Xylulose reductase. **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.

Q: Which of the following is the FAD-linked dehydrogenase of the TCA cycle?

Succinate dehydrogenase. **Explanation:** The correct answer is **Succinate dehydrogenase (SDH)**. In the TCA cycle, most dehydrogenases utilize $NAD^+$ as an electron acceptor. However, Succinate dehydrogenase catalyzes the oxidation of **Succinate to Fumarate**, which involves the reduction of **FAD to $FADH_2$**. **Why Succinate Dehydrogenase?** The free energy change ($\Delta G$) of the succinate-to-fumarate reaction is insufficient to reduce $NAD^+$. Therefore, the enzyme utilizes the more powerful oxidant, **FAD**, which is covalently bound to the enzyme. Uniquely, SDH is the only enzyme of the TCA cycle that is **integral to the inner mitochondrial membrane**, doubling as **Complex II** of the Electron Transport Chain (ETC). **Analysis of Incorrect Options:** * **A. Isocitrate dehydrogenase:** Catalyzes the oxidative decarboxylation of Isocitrate to $\alpha$-ketoglutarate; it is $NAD^+$-linked and is the rate-limiting step of the TCA cycle. * **B. Malate dehydrogenase:** Catalyzes the oxidation of Malate to Oxaloacetate; it is $NAD^+$-linked. * **D. Alpha-ketoglutarate dehydrogenase:** A multi-enzyme complex that converts $\alpha$-ketoglutarate to Succinyl-CoA; it is $NAD^+$-linked (though it uses FAD as a prosthetic group internally, the final electron exit is via $NAD^+$). **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** Succinate dehydrogenase is competitively inhibited by **Malonate** (a classic exam favorite). * **Location:** While all other TCA enzymes are in the mitochondrial matrix, SDH is in the **inner mitochondrial membrane**. * **Marker Enzyme:** SDH is used as a marker enzyme for mitochondria. * **Vitamins:** FAD is derived from **Riboflavin (Vitamin $B_2$)**.

Q: Which enzyme is NOT used in glycogen metabolism?

Glycogen synthase C. **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.

Q: All are features of Pompe's disease except?

Hyperglycemia. **Explanation:** **Pompe’s Disease (Glycogen Storage Disease Type II)** is unique among GSDs because it is a **lysosomal storage disorder**, not a disorder of glycogenolysis or gluconeogenesis. It is caused by a deficiency of **Acid α-1,4-glucosidase (Acid Maltase)**, which breaks down glycogen within lysosomes. 1. **Why Hyperglycemia is the correct answer (the "Except"):** In Pompe’s disease, the metabolic pathways in the cytosol (glycogenolysis and gluconeogenesis) remain **intact**. Therefore, the liver can still maintain blood glucose levels normally. Patients do **not** present with hypoglycemia; conversely, they definitely do not present with **hyperglycemia**. Blood glucose levels are typically normal. 2. **Why the other options are incorrect (Features of Pompe’s):** * **Cardiomyopathy (Option B):** This is the hallmark of the infantile-onset form. Massive glycogen accumulation in cardiac muscle leads to hypertrophic cardiomyopathy and "globular heart" on X-ray. * **Skeletal myopathy (Option C):** Glycogen accumulates in skeletal muscle lysosomes, causing profound hypotonia (Floppy Baby Syndrome) and respiratory muscle weakness. * **Hepatomegaly (Option D):** While the liver is not the primary organ of clinical failure (unlike GSD Type I), mild to moderate hepatomegaly occurs due to lysosomal glycogen buildup in hepatocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart), **Liver**, and **Muscle**." * **Enzyme Deficiency:** Acid Maltase (Acid α-1,4-glucosidase). * **Diagnosis:** PAS-positive material in lysosomes; "EKG with massive QRS complexes." * **Treatment:** Enzyme Replacement Therapy (Alglucosidase alfa). * **Key Distinction:** Unlike von Gierke’s (Type I), Pompe’s has **no** lactic acidosis, **no** hyperuricemia, and **normal** blood glucose.

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

Phosphofructokinase-1. **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.**

Q: Which of the following enzymes is responsible for the release of free glucose from glycogen during glycogenolysis in muscle?

Debranching enzyme. ### Explanation The correct answer is **Debranching enzyme**. **1. Why Debranching Enzyme is Correct:** Glycogenolysis primarily produces Glucose-1-Phosphate via glycogen phosphorylase. However, the debranching enzyme is a bifunctional protein. While its first activity is transferase, its second activity is **$\alpha$-1,6-glucosidase**. This specific action hydrolytically cleaves the $\alpha$-1,6-glycosidic bond at the branch point, releasing a molecule of **free glucose** (approximately 8–10% of glycogen breakdown products). In the muscle, this is the only mechanism to produce free glucose because muscle lacks the enzyme to dephosphorylate glucose-6-phosphate. **2. Analysis of Incorrect Options:** * **A. Glycogen phosphorylase:** This is the rate-limiting enzyme of glycogenolysis. It breaks $\alpha$-1,4 bonds to release **Glucose-1-Phosphate**, not free glucose. * **B. Glucose-1-phosphatase:** This enzyme does not play a physiological role in human glycogen metabolism. * **C. Glucose-6-phosphatase:** This enzyme converts Glucose-6-Phosphate to free glucose. It is present in the **liver and kidney** but is notably **absent in muscle**. Therefore, muscle cannot contribute to blood glucose levels; it uses glycogen strictly for its own energy needs. **3. NEET-PG High-Yield Pearls:** * **Cori’s Disease (GSD Type III):** Caused by a deficiency of the Debranching enzyme. It presents with hepatomegaly and hypoglycemia, but symptoms are often milder than Von Gierke’s. * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. * **Product Ratio:** For every branch point, glycogenolysis yields roughly **8-10 molecules of G-1-P** (via phosphorylase) for every **1 molecule of free glucose** (via debranching enzyme). * **Muscle Metabolism:** Because muscle lacks Glucose-6-phosphatase, the glucose-6-phosphate produced enters the glycolytic pathway to generate ATP for contraction.

Q: Which of the following is a glycogen storage disease?

Von Gierke's disease. **Explanation:** **1. Why Von Gierke’s Disease is Correct:** Von Gierke’s disease, also known as **Glycogen Storage Disease (GSD) Type I**, is caused by a deficiency of the enzyme **Glucose-6-Phosphatase**. This enzyme is crucial for the final step of both glycogenolysis and gluconeogenesis. Its absence prevents the liver from releasing free glucose into the blood, leading to massive accumulation of glycogen in the liver and kidneys. Clinically, it presents with severe fasting hypoglycemia, hepatomegaly, lactic acidosis, hyperuricemia, and hyperlipidemia. **2. Why the Other Options are Incorrect:** * **Fabry’s Disease:** This is a **Lysosomal Storage Disorder (Sphingolipidosis)** caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. Key features include angiokeratomas and renal failure. * **Fragile X Syndrome:** This is a **Genetic Tri-nucleotide Repeat Disorder (CGG)** on the FMR1 gene. It is the most common cause of inherited intellectual disability and is not related to glycogen metabolism. * **Krabbe’s Disease:** This is also a **Lysosomal Storage Disorder (Sphingolipidosis)** caused by a deficiency of **galactocerebrosidase**, leading to the destruction of myelin (leukodystrophy) and the presence of characteristic globoid cells. **3. NEET-PG High-Yield Pearls:** * **GSD Type II (Pompe):** Deficiency of Acid Maltase ($\alpha$-1,4-glucosidase). It is unique because it is both a GSD and a Lysosomal Storage Disorder. "Pompe trashes the Pump (Heart)." * **GSD Type III (Cori):** Deficiency of Debranching enzyme. Presents with milder hypoglycemia than Type I. * **GSD Type V (McArdle):** Deficiency of Skeletal Muscle Glycogen Phosphorylase. Presents with exercise-induced cramps and myoglobinuria. * **Mnemonic for Type I:** "G" for **G**ierke, **G**lucose-6-Phosphatase, and **G**out (due to hyperuricemia).

Q: The L or D configuration of a carbohydrate is determined by its stereochemical relationship to which of the following compounds?

Glyceraldehyde. ### Explanation The D and L configuration of carbohydrates is based on the **Rosanoff convention**, which uses **glyceraldehyde** as the reference standard. Glyceraldehyde is the simplest aldose containing a chiral center (asymmetric carbon). **Why Glyceraldehyde is the Correct Answer:** The configuration is determined by the orientation of the hydroxyl (-OH) group on the **penultimate carbon** (the chiral carbon furthest from the carbonyl group). * If the -OH group is on the **right**, it is the **D-isomer** (Dextro). * If the -OH group is on the **left**, it is the **L-isomer** (Levo). Because all higher sugars can be chemically derived from or related back to glyceraldehyde, it serves as the structural template for this nomenclature. **Analysis of Incorrect Options:** * **Fructose & Glucose:** While these are common hexoses, they are classified as D or L based on their relationship to glyceraldehyde, not the other way around. Most naturally occurring sugars in humans are D-isomers. * **Glycogen:** This is a complex polysaccharide (homopolymer of glucose) used for storage; it is not a reference molecule for stereoisomerism. **NEET-PG High-Yield Pearls:** 1. **Biological Preference:** In the human body, almost all utilized sugars are **D-isomers** (e.g., D-glucose), whereas almost all amino acids are **L-isomers**. 2. **Enzymatic Specificity:** Enzymes in the human body are stereospecific; for example, salivary amylase acts on D-glucose polymers but cannot act on L-glucose. 3. **Optical Activity:** D and L designations refer to structural configuration and do not necessarily indicate the direction of optical rotation (+/- or d/l). A D-sugar can be levorotatory (e.g., D-fructose).

Question 1

Which enzyme is stimulated in gluconeogenesis during starvation?

Accepted Answer

Carboxylase

Answer

Pyruvate dehydrogenase

Answer

Pyruvate kinase

Answer

Glucokinase

Question 2

Where is GLUT 4 primarily found?

Accepted Answer

Cardiac muscle

Answer

Endothelium

Answer

Liver

Answer

Lens

Question 3

Essential pentosuria is due to deficiency of which enzyme?

Accepted Answer

Xylulose reductase

Answer

Gulonolactone oxidase

Answer

Phosphoglucomutase

Answer

Fructokinase

Question 4

Which of the following is the FAD-linked dehydrogenase of the TCA cycle?

Accepted Answer

Succinate dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Question 5

Which enzyme is NOT used in glycogen metabolism?

Accepted Answer

Glycogen synthase C

Answer

Glycogen phosphorylase B

Answer

Glycogen synthase I

Answer

Glycogen synthase D

Question 6

All are features of Pompe's disease except?

Accepted Answer

Hyperglycemia

Answer

Cardiomyopathy

Answer

Skeletal myopathy

Answer

Hepatomegaly

Question 7

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

Accepted Answer

Phosphofructokinase-1

Answer

Glucose-6-phosphate dehydrogenase

Answer

Glyceraldehyde-3-phosphate dehydrogenase

Answer

Pyruvate kinase

Question 8

Which of the following enzymes is responsible for the release of free glucose from glycogen during glycogenolysis in muscle?

Accepted Answer

Debranching enzyme

Answer

Glycogen phosphorylase

Answer

Glucose-1-phosphatase

Answer

Glucose-6-phosphatase

Question 9

Which of the following is a glycogen storage disease?

Accepted Answer

Von Gierke's disease

Answer

Fabry's disease

Answer

Fragile syndrome

Answer

Krabbe's disease

Question 10

The L or D configuration of a carbohydrate is determined by its stereochemical relationship to which of the following compounds?

Accepted Answer

Glyceraldehyde

Answer

Fructose

Answer

Glycogen

Answer

Glucose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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