Carbohydrate Metabolism Practice Questions

Q: Dehydrogenases of the HMP shunt are specific for which coenzyme?

Nicotinamide adenine dinucleotide phosphate (NADP+). **Explanation:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway, occurs in the cytosol and is primarily an anabolic pathway. The rate-limiting step and the subsequent oxidative step are catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and **6-Phosphogluconate Dehydrogenase**, respectively. Both of these enzymes are strictly specific for **NADP+** as their coenzyme, reducing it to **NADPH**. Unlike NADH, which is used in the electron transport chain for ATP production, NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids and steroids. 2. **Antioxidant Defense:** Maintaining glutathione in its reduced state to protect cells against reactive oxygen species (ROS), particularly in RBCs. **Analysis of Incorrect Options:** * **A. Thiamine pyrophosphate (TPP):** This is a derivative of Vitamin B1. While it is a coenzyme in the HMP shunt, it is used by **Transketolase** (non-oxidative phase), not by dehydrogenases. * **C & D. FMN and FAD:** These are flavin-derived coenzymes (Vitamin B2) typically involved in redox reactions within the mitochondria (e.g., TCA cycle or Complex I/II of ETC). They are not utilized by the oxidative enzymes of the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to hemolytic anemia because RBCs cannot generate NADPH, leaving them vulnerable to oxidative stress (e.g., fava beans, primaquine, infections) and resulting in **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is most active in tissues requiring high NADPH, such as the adrenal cortex (steroidogenesis), liver (fatty acid synthesis), and lactating mammary glands. * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the gold standard for diagnosing **Thiamine deficiency**.

Q: Glucagon receptors are NOT found in which organ?

Cornea. **Explanation:** Glucagon is a peptide hormone primarily known for its role in glucose homeostasis. It acts via **G-protein coupled receptors (GPCRs)** that activate the Adenylate Cyclase-cAMP pathway. **Why Cornea is the Correct Answer:** Glucagon receptors are primarily localized in tissues involved in fuel mobilization and metabolic regulation. The **cornea** is an avascular, specialized ocular tissue that relies on atmospheric oxygen and glucose from the aqueous humor. It does not possess glucagon receptors because it does not participate in systemic glucose regulation or glycogenolysis. **Analysis of Other Options:** * **Kidney:** Glucagon receptors are present in the renal cortex and distal tubules. Glucagon plays a role in stimulating gluconeogenesis in the kidney and influencing electrolyte excretion (e.g., increasing glomerular filtration rate). * **Stomach:** Receptors are found in the gastric mucosa. Glucagon inhibits gastric acid secretion and reduces gastrointestinal motility. * **Adrenal Gland:** Glucagon receptors are present in the adrenal medulla, where glucagon can stimulate the release of catecholamines (epinephrine and norepinephrine). **High-Yield NEET-PG Pearls:** 1. **Primary Site:** The **Liver** has the highest density of glucagon receptors (stimulating glycogenolysis and gluconeogenesis). 2. **Muscle Paradox:** Glucagon receptors are **NOT** found in **Skeletal Muscle**. This is a frequent exam trap; muscle glycogen is used only for local contraction and cannot be released into the blood because muscle lacks Glucose-6-Phosphatase. 3. **Adipose Tissue:** Glucagon receptors are present here to stimulate lipolysis via Hormone-Sensitive Lipase (HSL). 4. **Clinical Correlation:** Glucagon is used as an antidote for **Beta-blocker overdose** because it bypasses the beta-receptor to increase cAMP in cardiac myocytes, improving heart rate and contractility.

Q: NADPH+ and H+ are 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 is the primary source of NADPH in the body, which is essential for reductive biosynthesis (e.g., fatty acid and steroid synthesis) and maintaining the pool of 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+** as the coenzyme, not NADPH. * **Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD):** A key enzyme in glycolysis that converts G-3-P to 1,3-bisphosphoglycerate, generating **NADH**. * **Alcohol dehydrogenase:** Responsible for ethanol metabolism in the cytosol, it converts ethanol to acetaldehyde using **NAD+** as an electron acceptor to produce **NADH**. **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 the RBCs cannot generate enough NADPH to maintain reduced glutathione. * **Bite Cells & Heinz Bodies:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is highly active in the adrenal cortex, liver, mammary glands, and RBCs. * **Rule of Thumb:** Enzymes using **NAD+** are generally involved in **catabolic** pathways (energy production), while those using **NADP+** are involved in **anabolic** pathways (biosynthesis) or detoxification.

Q: Pyruvate can be converted directly into all of the following except?

Phosphoenol pyruvate. **Explanation:** The conversion of **Pyruvate to Phosphoenolpyruvate (PEP)** is a key regulatory step in gluconeogenesis. It is a **two-step process** because the direct conversion of pyruvate to PEP is energetically unfavorable (the reaction catalyzed by Pyruvate Kinase in glycolysis is irreversible). 1. Pyruvate is first carboxylated to **Oxaloacetate** by *Pyruvate Carboxylase* (in the mitochondria). 2. Oxaloacetate is then converted to **PEP** by *PEP Carboxykinase* (PEPCK). **Analysis of Incorrect Options:** * **Alanine:** Pyruvate can be directly transaminated to Alanine via the enzyme **Alanine Transaminase (ALT)**. This is a crucial part of the Cahill cycle (Glucose-Alanine cycle). * **Acetyl CoA:** Pyruvate undergoes oxidative decarboxylation to form Acetyl CoA via the **Pyruvate Dehydrogenase (PDH) complex**. This links glycolysis to the TCA cycle. * **Lactate:** Under anaerobic conditions, Pyruvate is directly reduced to Lactate by **Lactate Dehydrogenase (LDH)**, regenerating $NAD^+$ for glycolysis. **NEET-PG High-Yield Pearls:** * **Pyruvate Carboxylase** requires **Biotin** as a cofactor and is activated by **Acetyl CoA**. * **The "Four Fates of Pyruvate":** 1. Acetyl CoA (Energy), 2. Oxaloacetate (Gluconeogenesis), 3. Alanine (Protein metabolism), 4. Lactate (Anaerobic glycolysis). * **Clinical Correlation:** Deficiency in the PDH complex leads to **Lactic Acidosis** and neurological dysfunction because pyruvate is shunted toward lactate production.

Q: Excess of which of the following can result in cataract?

Sugar alcohol. **Explanation:** The development of cataracts in metabolic disorders is primarily attributed to the **Polyol Pathway**. When blood glucose or galactose levels are chronically elevated, the enzyme **Aldose Reductase** converts these sugars into their corresponding **sugar alcohols** (polyols). 1. **Mechanism (The "Why"):** Glucose is reduced to **Sorbitol**, and Galactose is reduced to **Dulcitol (Galactitol)**. These sugar alcohols are polar and cannot easily diffuse out of the lens cells. Since they accumulate, they create a strong **osmotic gradient**, drawing water into the lens. This causes swelling, lens fiber disruption, and protein denaturation, leading to opacity (cataract). **Analysis of Options:** * **A. Sugar alcohol (Correct):** As explained, Sorbitol and Dulcitol are the direct osmotic agents responsible for lens damage. * **B. Glucose:** While high glucose (Diabetes) triggers the process, it is the *conversion* to sorbitol that causes the cataract, not the glucose molecule itself. * **C. Sugar amines:** These (like glucosamine) are structural components of glycosaminoglycans and are not implicated in osmotic cataract formation. * **D. Galactose:** Similar to glucose, galactose is the precursor. While galactosemia causes cataracts, it does so via its sugar alcohol derivative, Dulcitol. **NEET-PG High-Yield Pearls:** * **Enzyme involved:** Aldose Reductase (uses NADPH). * **Classic Presentation:** "Oil droplet cataract" is seen in Galactosemia (deficiency of GALK or GALT). * **Sorbitol Metabolism:** In most tissues, Sorbitol is converted to Fructose by **Sorbitol Dehydrogenase**. However, the **Lens, Retina, Kidney, and Schwann cells** lack this enzyme, making them highly susceptible to sorbitol-mediated damage (explaining cataracts, retinopathy, and neuropathy in diabetics).

Q: Muscle glycogen stores can NOT be used to provide glucose into the circulation due to lack of which enzyme?

Glucose-6-phosphatase. **Explanation:** The correct answer is **Glucose-6-phosphatase**. **1. Why Glucose-6-phosphatase is the correct answer:** Glycogenolysis in both the liver and muscle produces **Glucose-6-phosphate (G6P)**. In the liver, the enzyme **Glucose-6-phosphatase** cleaves the phosphate group from G6P to form free glucose, which can then be transported out of the cell into the bloodstream to maintain blood glucose levels. However, **skeletal muscle lacks Glucose-6-phosphatase**. Consequently, the G6P produced in muscles is trapped within the myocytes and must enter the glycolytic pathway to generate ATP for local muscular contraction rather than contributing to systemic blood glucose. **2. Analysis of Incorrect Options:** * **A. Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt, responsible for generating NADPH and ribose-5-phosphate. It is not involved in releasing free glucose. * **B. Glutamate dehydrogenase:** This enzyme is involved in amino acid metabolism (oxidative deamination), converting glutamate to alpha-ketoglutarate. * **D. Glucokinase:** This enzyme (found in the liver and pancreatic beta cells) catalyzes the phosphorylation of glucose to G6P. It is the functional opposite of Glucose-6-phosphatase. **3. NEET-PG High-Yield Pearls:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperlactatemia. * **Muscle Glycogen:** Serves as a fuel reserve for synthesis of ATP during muscle contraction. * **Liver Glycogen:** Serves as a glucose reserve for the maintenance of blood glucose during fasting. * **Cori Cycle:** Muscle glycogen can indirectly contribute to blood glucose by being converted to lactate, which travels to the liver to undergo gluconeogenesis.

Q: Overingestion of fructose leads to which of the following conditions?

Hypertriglyceridemia. **Explanation:** The correct answer is **Hypertriglyceridemia**. Unlike glucose, fructose metabolism in the liver bypasses the major rate-limiting step of glycolysis—the enzyme **Phosphofructokinase-1 (PFK-1)**. Fructose is rapidly phosphorylated by **fructokinase** to fructose-1-phosphate and subsequently cleaved into trioses (DHAP and glyceraldehyde). This unregulated flux floods the liver with acetyl-CoA, which serves as a substrate for **de novo lipogenesis**. This leads to increased synthesis of fatty acids and VLDL, ultimately resulting in hypertriglyceridemia and non-alcoholic fatty liver disease (NAFLD). **Analysis of Incorrect Options:** * **B. Hypouricemia:** Incorrect. Fructose ingestion causes **Hyperuricemia**. Rapid phosphorylation of fructose depletes intracellular ATP and inorganic phosphate, stimulating the purine degradation pathway, which increases uric acid production. * **C. Hyperphosphatemia:** Incorrect. It causes **Hypophosphatemia**. The "trapping" of inorganic phosphate in the form of fructose-1-phosphate reduces serum phosphate levels. * **D. Hypoglycemia:** Incorrect. While hereditary fructose intolerance (aldolase B deficiency) causes severe hypoglycemia, simple overingestion in a healthy individual typically leads to metabolic derangements like insulin resistance rather than acute hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fructosuria:** Deficiency of **Fructokinase**; a benign, asymptomatic condition where fructose appears in the urine (reducing sugar positive). * **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**; characterized by hypoglycemia, jaundice, and vomiting after ingesting fruit/sucrose. * **Key Enzyme:** Fructokinase has a much higher $V_{max}$ than glucokinase, explaining why the liver clears fructose so rapidly.

Q: In the conversion of lactic acid to glucose (gluconeogenesis), three reactions of the glycolytic pathway are circumvented. Which of the following enzymes do NOT participate in this process?

Pyruvate kinase. **Explanation:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (like lactate). While it shares many enzymes with glycolysis, it must bypass three **irreversible** steps of glycolysis to proceed energetically. These "bottleneck" steps are catalyzed by Hexokinase, Phosphofructokinase-1, and Pyruvate kinase. **Why Pyruvate Kinase is the Correct Answer:** Pyruvate kinase is a **glycolytic enzyme** that converts Phosphoenolpyruvate (PEP) to Pyruvate. In gluconeogenesis, this reaction cannot be reversed directly. Instead, the body bypasses it using a two-step process involving **Pyruvate carboxylase** and **PEP carboxykinase (PEPCK)**. Therefore, Pyruvate kinase does not participate in gluconeogenesis; it is actually inhibited during this process to prevent a futile cycle. **Analysis of Incorrect Options:** * **A. Pyruvate carboxylase:** A key gluconeogenic enzyme that converts pyruvate to oxaloacetate in the mitochondria. It requires **Biotin** as a cofactor. * **B. Phosphoenolpyruvate carboxykinase (PEPCK):** Converts oxaloacetate to PEP, completing the bypass of the pyruvate kinase step. * **D. Glucose-6-phosphatase:** The final bypass enzyme found in the ER of the liver and kidney. It converts Glucose-6-phosphate to free glucose, allowing it to enter the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Key Gluconeogenic Enzymes:** Pyruvate carboxylase, PEPCK, Fructose-1,6-bisphosphatase (the rate-limiting step), and Glucose-6-phosphatase. * **Acetyl-CoA Connection:** Pyruvate carboxylase is **obligatorily activated** by Acetyl-CoA. * **Location:** Gluconeogenesis occurs primarily in the **Liver** (90%) and Kidney (10%). It does not occur in the muscle due to the absence of Glucose-6-phosphatase. * **Cori Cycle:** Lactate produced in muscles travels to the liver to be converted back to glucose via these bypass enzymes.

Question 1

Dehydrogenases of the HMP shunt are specific for which coenzyme?

Accepted Answer

Nicotinamide adenine dinucleotide phosphate (NADP+)

Answer

Thiamine pyrophosphate (TPP)

Answer

Flavin mononucleotide (FMN)

Answer

Flavin adenine dinucleotide (FAD)

Question 2

Glucagon receptors are NOT found in which organ?

Accepted Answer

Cornea

Answer

Kidney

Answer

Stomach

Answer

Adrenal gland

Question 3

NADPH+ and H+ are 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 4

An infant appears normal at birth following an uncomplicated pregnancy and begins to nurse. Within 3 days the infant exhibits vomiting and diarrhea. A week later jaundice is evident, and the infant becomes febrile. On physical examination hepatomegaly is present. Laboratory studies show abnormal reducing substances in the urine, and Escherichia coli is cultured from blood. This infant most likely has a deficiency in which of the following enzymes?

Accepted Answer

Galactose-1-phosphate uridyltransferase

Answer

Glucocerebrosidase

Answer

Phenylalanine hydroxylase

Answer

Sphingomyelinase

Question 5

Pyruvate can be converted directly into all of the following except?

Accepted Answer

Phosphoenol pyruvate

Answer

Alanine

Answer

Acetyl CoA

Answer

Lactate

Question 6

Excess of which of the following can result in cataract?

Accepted Answer

Sugar alcohol

Answer

Glucose

Answer

Sugar amines

Answer

Galactose

Question 7

Muscle glycogen stores can NOT be used to provide glucose into the circulation due to lack of which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Glutamate dehydrogenase

Answer

Glucokinase

Question 8

Overingestion of fructose leads to which of the following conditions?

Accepted Answer

Hypertriglyceridemia

Answer

Hypouricemia

Answer

Hyperphosphatemia

Answer

Hypoglycemia

Question 9

Which of the following is NOT a product of the Pentose Phosphate Pathway?

Accepted Answer

CO2

Answer

Glyceraldehyde 3-phosphate

Answer

NADPH

Answer

Sedoheptulose phosphate

Question 10

In the conversion of lactic acid to glucose (gluconeogenesis), three reactions of the glycolytic pathway are circumvented. Which of the following enzymes do NOT participate in this process?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

Phosphoenol pyruvate carboxy kinase

Answer

Glucose-6-phosphatase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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