Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 451: Which enzyme catalyzes substrate-level phosphorylation?

A. Pyruvate kinase (Correct Answer)
B. Phosphofructokinase
C. Hexokinase
D. ATP synthase

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, independent of the electron transport chain and oxygen. **1. Why Pyruvate Kinase is correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two sites of SLP in glycolysis (the other being Phosphoglycerate kinase). **2. Why the other options are incorrect:** * **Phosphofructokinase (PFK-1) & Hexokinase:** These are regulatory enzymes of glycolysis that **consume** ATP rather than producing it. They catalyze phosphorylation of the substrate (Glucose and Fructose-6-P, respectively) using ATP as the phosphate donor. * **ATP Synthase:** This enzyme produces ATP via **oxidative phosphorylation** in the mitochondria. It utilizes the proton motive force (electrochemical gradient) generated by the Electron Transport Chain, not a direct phosphate transfer from a substrate. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites in Glucose metabolism:** 1. **Glycolysis (2 sites):** Phosphoglycerate kinase and Pyruvate kinase. 2. **TCA Cycle (1 site):** Succinate thiokinase (Succinyl-CoA synthetase), which produces **GTP**. * **Clinical Correlation:** Pyruvate kinase deficiency is the second most common cause of enzyme-deficient **hereditary hemolytic anemia** (after G6PD deficiency). Since RBCs lack mitochondria, they depend entirely on SLP for ATP; deficiency leads to ATP depletion and membrane failure.

Question 452: In the well-fed state, which of the following inhibits Carnitine Palmitoyltransferase 1 (CPT1) on the outer membrane of mitochondria?

A. Malonyl CoA (Correct Answer)
B. Acetyl CoA
C. ADP
D. Glucose

Explanation: ### Explanation **1. Why Malonyl CoA is Correct:** In the **well-fed state**, high levels of glucose lead to an increase in insulin. Insulin promotes fatty acid synthesis by activating **Acetyl CoA Carboxylase (ACC)**, which converts Acetyl CoA into **Malonyl CoA**. Malonyl CoA serves as a critical regulatory molecule; it acts as a potent **allosteric inhibitor of Carnitine Palmitoyltransferase 1 (CPT1)**. By inhibiting CPT1, Malonyl CoA prevents the entry of long-chain fatty acids into the mitochondria for beta-oxidation. This ensures that fatty acid synthesis and fatty acid breakdown do not occur simultaneously (preventing a "futile cycle"). **2. Why the Other Options are Incorrect:** * **B. Acetyl CoA:** While it is a precursor for Malonyl CoA, it does not directly inhibit CPT1. Its primary role in the fed state is to provide the carbon skeleton for lipogenesis. * **C. ADP:** ADP is a marker of low energy status. In a well-fed state, ATP levels are high and ADP levels are low. ADP generally stimulates catabolic pathways (like the TCA cycle), not the inhibition of fatty acid transport. * **D. Glucose:** While glucose availability triggers the hormonal shift (insulin) that leads to CPT1 inhibition, glucose itself does not interact with the CPT1 enzyme. **3. Clinical Pearls & High-Yield Facts:** * **Rate-Limiting Step:** CPT1 is the rate-limiting enzyme for **Beta-oxidation**. * **Location:** CPT1 is located on the **outer** mitochondrial membrane, while CPT2 is on the **inner** membrane. * **Hormonal Control:** Glucagon decreases Malonyl CoA levels (by inhibiting ACC), thereby relieving the inhibition on CPT1 and stimulating fatty acid oxidation during fasting. * **Carnitine Shuttle:** This process is essential for long-chain fatty acids; however, short and medium-chain fatty acids can bypass this shuttle and enter the mitochondria directly.

Question 453: Pyruvate can be converted directly into all of the following except?

A. Phosphoenol pyruvate (Correct Answer)
B. Alanine
C. Acetyl CoA
D. Lactate

Explanation: **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.

Question 454: In a well-fed state, what is the major metabolic fate of glucose-6-phosphate in tissues?

A. Hydrolysis to glucose
B. Conversion to glycogen (Correct Answer)
C. Isomerization to fructose-6-phosphate
D. Conversion to ribulose-5-phosphate

Explanation: **Explanation:** In a **well-fed state**, the body is characterized by high blood glucose levels and a high **insulin-to-glucagon ratio**. Glucose enters cells (like the liver and muscles) and is immediately phosphorylated to **Glucose-6-Phosphate (G6P)** by hexokinase/glucokinase. 1. **Why Option B is Correct:** Under the influence of insulin, the enzyme **Glycogen Synthase** is activated (via dephosphorylation). G6P is converted to Glucose-1-Phosphate and then incorporated into glycogen for storage. This is the primary "storage" fate of glucose when energy demands are met. 2. **Why Options A, C, and D are Incorrect:** * **Option A:** Hydrolysis to glucose (via Glucose-6-Phosphatase) occurs primarily in the **fasting state** to maintain blood glucose; it is inhibited by insulin. * **Option C:** While G6P isomerizes to Fructose-6-Phosphate during glycolysis, in a well-fed state with high ATP levels, glycolysis is regulated. Glycogen synthesis is prioritized for long-term energy storage. * **Option D:** Conversion to Ribulose-5-phosphate (HMP Shunt) does occur, but it is a minor pathway compared to the bulk storage of glucose as glycogen in the liver and muscle. **High-Yield NEET-PG Pearls:** * **Glucokinase vs. Hexokinase:** Glucokinase (Liver/B-cells) has a high $K_m$ and high $V_{max}$, allowing it to handle large glucose loads in the fed state. * **Key Regulatory Step:** Insulin stimulates **Protein Phosphatase-1**, which activates Glycogen Synthase. * **Tissue Specificity:** Muscle glycogen is used for local contraction, while liver glycogen maintains systemic blood glucose.

Question 455: A 12-year-old boy presents with severe polydipsia and polyuria. Laboratory examination reveals a purple ring when a test was performed on his urine. What is the most likely source of this compound positive in this patient?

A. Gluconeogenesis
B. Fatty acid breakdown (Correct Answer)
C. Protein breakdown
D. Side chain of cholesterol

Explanation: **Explanation:** The clinical presentation of severe polydipsia and polyuria in a 12-year-old boy is highly suggestive of **Type 1 Diabetes Mellitus**. The "purple ring" mentioned refers to **Rothera’s Test**, which is used to detect ketone bodies (specifically acetone and acetoacetate) in the urine. **1. Why Fatty Acid Breakdown is Correct:** In the absence of insulin, the body cannot utilize glucose and instead shifts to massive lipolysis. Large amounts of free fatty acids are released from adipose tissue and undergo **Beta-oxidation** in the liver. This results in an excess of Acetyl-CoA, which exceeds the capacity of the TCA cycle and is diverted toward **Ketogenesis**. The resulting ketone bodies (Acetoacetate, Beta-hydroxybutyrate, and Acetone) cause the positive Rothera's test. **2. Why Incorrect Options are Wrong:** * **Gluconeogenesis:** While active in diabetes, this process produces glucose, not ketone bodies. Glucose is detected by Benedict’s test (orange-red precipitate), not a purple ring. * **Protein Breakdown:** While muscle wasting occurs in uncontrolled diabetes, the primary product of amino acid catabolism is urea. * **Side chain of cholesterol:** The cleavage of the cholesterol side chain produces pregnenolone (the precursor for steroid hormones), not ketone bodies. **Clinical Pearls for NEET-PG:** * **Rothera’s Test:** Detects Acetoacetate and Acetone (not Beta-hydroxybutyrate). Reagent used: Sodium nitroprusside. * **Gerhardt’s Test:** Uses Ferric chloride to detect acetoacetate. * **Ketone Body Synthesis:** Occurs in the **mitochondria** of liver cells. The rate-limiting enzyme is **HMG-CoA Synthase**. * **Utilization:** Ketone bodies are used by extrahepatic tissues (brain, heart) but **cannot** be used by the liver due to the absence of the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase).

Question 456: What is typically not found in a patient with fructose intolerance?

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

Explanation: ### Explanation The question refers to **Hereditary Fructose Intolerance (HFI)**, an autosomal recessive disorder caused by a deficiency of **Aldolase B**. **1. Why Fructose is the Correct Answer:** In HFI, the deficiency of Aldolase B leads to the intracellular accumulation of **Fructose-1-Phosphate (F-1-P)** in the liver, kidney, and small intestine. This accumulation "traps" inorganic phosphate, leading to ATP depletion. The resulting metabolic crisis inhibits gluconeogenesis and glycogenolysis, causing severe **hypoglycemia** following fructose ingestion. Crucially, the renal proximal tubules are affected (Fanconi-like syndrome), leading to the excretion of various sugars in the urine (**mellituria**). However, because F-1-P is trapped inside the cells and cannot be converted back to fructose or further metabolized, **fructose itself is typically absent from the urine** (or found in negligible amounts) compared to other reducing sugars. Instead, the laboratory hallmark is the presence of non-glucose reducing sugars, but the metabolic block specifically prevents the systemic circulation/excretion of free fructose. **2. Analysis of Incorrect Options:** * **A. Glucose:** Hypoglycemia is a hallmark; however, glucose can still be found in urine if there is significant proximal tubular damage (glycosuria). * **B. Galactose & D. Maltose:** In HFI, the secondary dysfunction of the proximal renal tubules leads to generalized **aminoaciduria and mellituria**. Patients often excrete other reducing sugars like galactose and maltose due to the "leaky" nature of the damaged tubules. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** Aldolase B (converts F-1-P to DHAP and Glyceraldehyde). * **Clinical Presentation:** Symptoms (vomiting, jaundice, hypoglycemia, seizures) appear only **after weaning** from breast milk (when sucrose/fructose is introduced). * **Diagnosis:** Reducing sugars in urine (Clinitest positive) but glucose oxidase test (Dipstick) negative. * **Treatment:** Strict avoidance of Fructose, Sucrose, and Sorbitol. * **Contrast:** Essential Fructosuria (Fructokinase deficiency) is asymptomatic and *does* present with fructose in the urine.

Question 457: Which enzyme catalyzes substrate-level phosphorylation in the citric acid cycle?

A. Pyruvate kinase
B. Phosphoglycerate kinase
C. Malate dehydrogenase
D. Succinate thiokinase (Correct Answer)

Explanation: ### Explanation **1. Why Succinate Thiokinase is Correct:** In the Citric Acid Cycle (TCA cycle), **Succinate thiokinase** (also known as **Succinyl-CoA synthetase**) catalyzes the conversion of Succinyl-CoA to Succinate. This reaction is unique because it involves the cleavage of a high-energy thioester bond, which provides sufficient energy to drive the phosphorylation of GDP to GTP (or ADP to ATP). This process is called **substrate-level phosphorylation** because the phosphate group is transferred directly from a substrate to a nucleoside diphosphate without the involvement of the electron transport chain or oxygen. **2. Analysis of Incorrect Options:** * **Pyruvate kinase (Option A):** Catalyzes substrate-level phosphorylation (PEP to Pyruvate), but it occurs in **Glycolysis**, not the TCA cycle. * **Phosphoglycerate kinase (Option B):** Also catalyzes substrate-level phosphorylation (1,3-BPG to 3-Phosphoglycerate), but this is also a step in **Glycolysis**. * **Malate dehydrogenase (Option C):** This enzyme catalyzes the oxidation of Malate to Oxaloacetate. It generates **NADH**, which leads to ATP production via oxidative phosphorylation, not substrate-level phosphorylation. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Location:** Succinate thiokinase is the **only** enzyme in the TCA cycle that performs substrate-level phosphorylation. * **Tissue Specificity:** There are two isoforms: the **GTP-specific** isoform is found primarily in the liver and kidneys (gluconeogenic tissues), while the **ATP-specific** isoform is found in the heart and skeletal muscle. * **Energy Yield:** One turn of the TCA cycle produces **1 GTP** (via substrate-level phosphorylation), **3 NADH**, and **1 FADH₂**, totaling **10 ATP** equivalents per Acetyl-CoA. * **Arsenite Inhibition:** Note that the α-Ketoglutarate dehydrogenase complex (the step immediately preceding Succinate thiokinase) is inhibited by Arsenite.

Question 458: Excess of which of the following can result in cataract?

A. Sugar alcohol (Correct Answer)
B. Glucose
C. Sugar amines
D. Galactose

Explanation: **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).

Question 459: Muscle glycogen stores can NOT be used to provide glucose into the circulation due to lack of which enzyme?

A. Glucose-6-phosphate dehydrogenase
B. Glutamate dehydrogenase
C. Glucose-6-phosphatase (Correct Answer)
D. Glucokinase

Explanation: **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.

Question 460: Overingestion of fructose leads to which of the following conditions?

A. Hypertriglyceridemia (Correct Answer)
B. Hypouricemia
C. Hyperphosphatemia
D. Hypoglycemia

Explanation: **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.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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