Carbohydrate Metabolism Practice Questions

Q: In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. Which enzymic step is key to this process?

The decarboxylation of an alpha-ketoacid. ### Explanation **1. Why the Correct Answer is Right:** The reaction that produces **Succinyl CoA** in the Citric Acid Cycle (TCA) is the conversion of **$\alpha$-ketoglutarate** to Succinyl CoA, catalyzed by the **$\alpha$-ketoglutarate dehydrogenase complex**. * **Substrate Chemistry:** $\alpha$-ketoglutarate is an **$\alpha$-ketoacid** (it has a ketone group on the carbon atom adjacent to the carboxylic acid group). * **Mechanism:** This step is an **oxidative decarboxylation**. The enzyme complex removes a carboxyl group (as $CO_2$) and attaches Coenzyme A. This process is chemically analogous to the conversion of Pyruvate (another $\alpha$-ketoacid) to Acetyl CoA. **2. Why the Incorrect Options are Wrong:** * **Option A (Hydroxy acid):** Isocitrate is a hydroxy acid. While its decarboxylation produces $CO_2$, it forms $\alpha$-ketoglutarate, not Succinyl CoA. * **Option C (Beta-ketoacid):** Decarboxylation of $\beta$-ketoacids occurs during **ketogenesis** (e.g., acetoacetate to acetone) or in the HMP shunt, but not in the specific step forming Succinyl CoA. * **Option D (Oxidation of keto to carboxylic acid):** While oxidation occurs, the primary "key" chemical event that releases $CO_2$ while forming the thioester bond of Succinyl CoA is the decarboxylation of the $\alpha$-keto structure. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Co-factors:** The $\alpha$-ketoglutarate dehydrogenase complex requires five co-factors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**inda). * **Inhibition:** This enzyme is inhibited by **Arsenite**, which binds to the SH groups of lipoic acid. * **Rate-limiting:** This is one of the three irreversible, regulatory steps of the TCA cycle. * **Symmetry:** This is the second of two $CO_2$ releasing steps in the cycle.

Q: The reaction Oxaloacetate + Acetyl CoA -> Citrate + CoASH is:

Irreversible. ### Explanation **Underlying Concept:** The reaction **Oxaloacetate + Acetyl CoA → Citrate** is the first step of the Citric Acid Cycle (TCA cycle), catalyzed by the enzyme **Citrate Synthase**. This is a condensation reaction that involves the hydrolysis of a high-energy thioester bond in Acetyl CoA. The cleavage of this bond releases a significant amount of free energy ($\Delta G^\circ = -7.7 \text{ kcal/mol}$), making the reaction **highly exergonic**. In cellular conditions, this large negative free energy change ensures the reaction proceeds in only one direction, effectively committing the Acetyl group to the cycle. **Analysis of Options:** * **A. Reversible:** Incorrect. Reversible reactions typically have a $\Delta G$ near zero. The high energy release from thioester bond cleavage makes this step a major "rate-limiting" and irreversible regulatory point. * **C. Can be reversed by catalase:** Incorrect. Catalase is an antioxidant enzyme involved in the breakdown of hydrogen peroxide ($H_2O_2$) into water and oxygen; it has no role in the TCA cycle or Citrate synthesis. * **D. Competitive:** Incorrect. This term refers to a type of enzyme inhibition, not a classification of a chemical reaction's directionality. **High-Yield Facts for NEET-PG:** * **Regulatory Steps:** There are three irreversible steps in the TCA cycle: 1. Citrate Synthase (Step 1) 2. Isocitrate Dehydrogenase (Step 3) – *The rate-limiting step.* 3. $\alpha$-Ketoglutarate Dehydrogenase complex (Step 4). * **Inhibitors:** Citrate synthase is inhibited by its products (Citrate and NADH) and ATP. * **Fluoroacetate:** Known as "suicide inhibitor," it is converted to fluorocitrate, which inhibits aconitase, halting the TCA cycle.

Q: Spermatozoa obtain their nutrition from which of the following substances?

Fructose. **Explanation:** **Why Fructose is the Correct Answer:** Spermatozoa utilize **fructose** as their primary source of energy for motility. Fructose is secreted in high concentrations by the **seminal vesicles**. This is a unique metabolic adaptation because, unlike most cells in the body that prefer glucose, sperm cells utilize fructose via the **polyol pathway** (also known as the sorbitol pathway). In this pathway, glucose is converted to sorbitol by aldose reductase and then to fructose by sorbitol dehydrogenase. This ensures a dedicated energy supply for sperm that does not compete with the glucose requirements of other tissues. **Analysis of Incorrect Options:** * **A. Glucose:** While glucose is the universal fuel for most cells (like the brain and RBCs), it is not the primary nutrient in seminal fluid. * **C. Galactose:** This sugar is primarily involved in lactose synthesis (in mammary glands) and the formation of glycolipids/glycoproteins; it is not a significant energy source for sperm. * **D. Starch:** This is a complex plant polysaccharide. Human cells cannot utilize starch directly for energy without prior digestion into glucose in the gastrointestinal tract. **High-Yield Clinical Pearls for NEET-PG:** * **The Polyol Pathway:** Occurs mainly in the seminal vesicles. In diabetic patients, this same pathway in the lens of the eye leads to sorbitol accumulation, causing osmotic damage and **cataracts**. * **Forensic Significance:** The presence of fructose in a sample is used in forensic medicine (e.g., rape kits) to confirm the presence of semen, as fructose is specifically secreted by seminal vesicles. * **Diagnostic Utility:** Absence of fructose in the semen (Azoospermia) suggests **bilateral congenital absence of the vas deferens** or obstruction of the ejaculatory ducts.

Q: What is the most common enzyme defect in galactosemia?

Uridyl transferase. **Explanation:** Galactosemia is an autosomal recessive disorder of galactose metabolism. The correct answer is **Uridyl transferase** (specifically, Galactose-1-phosphate uridyltransferase or GALT). **1. Why Uridyl Transferase is Correct:** Deficiency of the **GALT** enzyme causes **Classic Galactosemia**, which is the most common and clinically severe form of the disease. In this condition, Galactose-1-phosphate and galactitol accumulate in tissues (liver, brain, and renal tubules), leading to severe manifestations like hepatomegaly, jaundice, and mental retardation shortly after starting milk feeds. **2. Analysis of Incorrect Options:** * **A. Galactokinase (GALK):** Deficiency leads to "Non-classic Galactosemia." It is less common and presents primarily with early-onset cataracts due to galactitol accumulation in the lens, but lacks the severe systemic toxicity seen in the GALT deficiency. * **C. 4-epimerase (GALE):** UDP-galactose-4-epimerase deficiency is the rarest form. It can exist in a benign peripheral form (limited to RBCs) or a rare systemic form resembling classic galactosemia. **3. NEET-PG High-Yield Pearls:** * **Screening Test:** Benedict’s test is positive (reducing sugar in urine), but the Glucose Oxidase (dipstick) test is negative. * **Cataracts:** Formed due to the conversion of excess galactose to **Galactitol** by the enzyme *Aldose Reductase*. * **Key Association:** Infants with classic galactosemia are at a significantly increased risk of **E. coli neonatal sepsis**. * **Management:** Immediate exclusion of lactose and galactose from the diet (stop breastfeeding, switch to soy milk).

Q: Which of the following metabolic disorders causes post-prandial hypoglycemia?

Hereditary fructose intolerance. **Explanation:** **Hereditary Fructose Intolerance (HFI)** is caused by a deficiency of **Aldolase B**. When a patient ingests fructose (or sucrose), fructose-1-phosphate (F1P) accumulates in the liver. This accumulation "traps" intracellular inorganic phosphate, leading to ATP depletion. High levels of F1P and low phosphate levels **allosterically inhibit glycogen phosphorylase** (preventing glycogenolysis) and interfere with **gluconeogenesis**. Consequently, after a meal containing fructose, the liver cannot release glucose, leading to severe **post-prandial hypoglycemia**. **Analysis of Incorrect Options:** * **GSD Type I (Von Gierke Disease):** Characterized by **fasting hypoglycemia** because the deficiency of Glucose-6-Phosphatase prevents the liver from releasing glucose derived from both glycogenolysis and gluconeogenesis during periods of starvation. * **GSD Type III (Cori Disease):** Caused by debranching enzyme deficiency. It presents with **fasting hypoglycemia**, though typically milder than Type I, as gluconeogenesis remains intact. * **Fanconi-Bickel Syndrome (GSD Type XI):** Caused by a **GLUT-2 mutation**. While it involves glycogen accumulation, it typically presents with fasting hypoglycemia and post-prandial hyperglycemia (due to impaired glucose uptake/transport). **High-Yield Clinical Pearls for NEET-PG:** * **HFI Presentation:** Symptoms (vomiting, jaundice, hypoglycemia) appear only **after weaning** from breast milk (when fruits/sucrose are introduced). * **Urine Test:** Patients with HFI show a **positive Benedict’s test** (reducing sugar) but a **negative glucose oxidase test** (dipstick), indicating a non-glucose reducing sugar (fructose). * **Treatment:** Strict avoidance of fructose, sucrose, and sorbitol.

Q: After overnight fasting, in which of the following cells are glucose transporter levels reduced?

Adipocytes. **Explanation:** The regulation of glucose uptake depends on the specific type of glucose transporter (GLUT) expressed on the cell membrane. **1. Why Adipocytes are correct:** Adipocytes (and skeletal muscle) primarily express **GLUT-4**, which is the only **insulin-dependent** glucose transporter. In a fasting state, insulin levels are low. This causes GLUT-4 transporters to be sequestered from the cell surface and internalized into intracellular vesicles, significantly reducing their levels on the plasma membrane. This mechanism conserves glucose for glucose-dependent tissues like the brain. **2. Why the other options are incorrect:** * **Brain cells (A):** Express **GLUT-1 and GLUT-3**. These are insulin-independent and have a low Km (high affinity), ensuring constant glucose uptake even during hypoglycemia. * **Hepatocytes (B):** Express **GLUT-2**. This is an insulin-independent, high-capacity transporter that allows bidirectional glucose flux. Its presence on the membrane does not decrease during fasting; rather, it facilitates glycogenolysis and gluconeogenesis to release glucose into the blood. * **RBCs (D):** Express **GLUT-1**. RBCs lack mitochondria and rely solely on anaerobic glycolysis; therefore, they require constant, insulin-independent glucose access. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the "Insulin-Responsive" transporter found in **Adipose tissue and Striated muscle** (Skeletal & Cardiac). * **Exercise** can trigger GLUT-4 translocation to the membrane in muscles independent of insulin (important for managing Diabetes). * **GLUT-2** acts as a "Glucose Sensor" in Pancreatic Beta-cells. * **GLUT-5** is unique as it primarily transports **Fructose** (found in the small intestine and spermatozoa).

Q: Which GLUT transporter is responsible for the secretion of insulin from beta cells of the pancreas?

GLUT2. **Explanation:** The correct answer is **GLUT2**. In pancreatic beta cells, GLUT2 acts as a "glucose sensor." It is a high-capacity, high-Km (low affinity) transporter, meaning its rate of glucose transport is proportional to blood glucose levels. When blood glucose rises after a meal, GLUT2 facilitates the rapid entry of glucose into the beta cells. This glucose is then phosphorylated by **Glucokinase**, leading to ATP production, closure of ATP-sensitive K+ channels, depolarization, and the subsequent release of insulin. **Analysis of Incorrect Options:** * **GLUT1:** Found in most tissues, including RBCs and the blood-brain barrier. It provides basal glucose uptake but is not the primary trigger for insulin secretion. * **GLUT3:** Primarily found in neurons. It has a very low Km (high affinity), ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** The only **insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane of skeletal muscle and adipose tissue only in the presence of insulin. * **Note:** While humans primarily use GLUT1/3 for beta-cell sensing, GLUT2 remains the classic textbook and exam answer for the "glucose sensor" mechanism in medical boards. **High-Yield Clinical Pearls for NEET-PG:** * **Bidirectional Transport:** GLUT2 is unique because it allows bidirectional transport, crucial for glucose release from the liver during gluconeogenesis. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a mutation in the GLUT2 gene. * **SGLT vs. GLUT:** Remember that SGLTs (Sodium-Glucose Linked Transporters) use active transport (secondary), while GLUTs use **facilitated diffusion**.

Q: The metabolic defect in hereditary fructose intolerance is due to deficiency of which enzyme?

Aldolase-B. **Explanation:** **Hereditary Fructose Intolerance (HFI)** is an autosomal recessive disorder caused by a deficiency of **Aldolase-B**. In the liver, fructose is first converted to Fructose-1-Phosphate (F1P) by fructokinase. Aldolase-B is responsible for cleaving F1P into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde. When Aldolase-B is deficient, **Fructose-1-Phosphate accumulates** intracellularly. This "traps" inorganic phosphate, leading to ATP depletion. The lack of ATP inhibits gluconeogenesis and glycogenolysis, resulting in severe postprandial hypoglycemia, jaundice, and vomiting following the ingestion of fructose, sucrose, or sorbitol. **Analysis of Incorrect Options:** * **A. Fructokinase:** Deficiency causes **Essential Fructosuria**. This is a benign, asymptomatic condition where fructose is excreted in the urine because it cannot be "trapped" in cells. * **C. Xylitol dehydrogenase:** Deficiency of L-xylulose reductase (part of the uronic acid pathway) leads to **Essential Pentosuria**, characterized by the excretion of L-xylulose in urine. * **D. Phosphofructokinase:** This is the rate-limiting enzyme of glycolysis. Its deficiency (Tarui disease) leads to Glycogen Storage Disease Type VII, characterized by exercise-induced muscle cramps and hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Trapping" Mechanism:** Accumulation of F1P is the toxic event in HFI, similar to Galactose-1-Phosphate accumulation in Classic Galactosemia. * **Dietary Management:** Treatment involves strict avoidance of **fructose, sucrose** (glucose + fructose), and **sorbitol** (which converts to fructose). * **Clinical Presentation:** Symptoms typically appear when an infant is weaned from breast milk and introduced to fruit juices or formulas containing sucrose.

Question 1

What is the rate-limiting step of glycogenolysis?

Accepted Answer

Phosphorylase

Answer

Glucan transferase

Answer

Debranching enzyme

Answer

Glucose-6-phosphatase

Question 2

A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Serum analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency in which of the following enzymes?

Accepted Answer

Pyruvate dehydrogenase

Answer

Alanine aminotransferase

Answer

Glutamate dehydrogenase

Answer

Lactate dehydrogenase

Question 3

In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. Which enzymic step is key to this process?

Accepted Answer

The decarboxylation of an alpha-ketoacid

Answer

The decarboxylation of a hydroxy acid

Answer

The decarboxylation of a beta-ketoacid

Answer

The oxidation of a keto group to a carboxylic acid

Question 4

The reaction Oxaloacetate + Acetyl CoA -> Citrate + CoASH is:

Accepted Answer

Irreversible

Answer

Reversible

Answer

Can be reversed by catalase

Answer

Competitive

Question 5

Spermatozoa obtain their nutrition from which of the following substances?

Accepted Answer

Fructose

Answer

Glucose

Answer

Galactose

Answer

Starch

Question 6

What is the most common enzyme defect in galactosemia?

Accepted Answer

Uridyl transferase

Answer

Galactokinase

Answer

4-epimerase

Answer

None of the above

Question 7

Which of the following metabolic disorders causes post-prandial hypoglycemia?

Accepted Answer

Hereditary fructose intolerance

Answer

Glycogen storage disease type I

Answer

Glycogen storage disease type III

Answer

Fanconi-Bickel syndrome

Question 8

After overnight fasting, in which of the following cells are glucose transporter levels reduced?

Accepted Answer

Adipocytes

Answer

Brain cells

Answer

Hepatocytes

Answer

RBCs

Question 9

Which GLUT transporter is responsible for the secretion of insulin from beta cells of the pancreas?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 10

The metabolic defect in hereditary fructose intolerance is due to deficiency of which enzyme?

Accepted Answer

Aldolase-B

Answer

Fructokinase

Answer

Xylitol dehydrogenase

Answer

Phosphofructokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?