Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 211: In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. Which enzymic step is key to this process?

A. The decarboxylation of a hydroxy acid
B. The decarboxylation of an alpha-ketoacid (Correct Answer)
C. The decarboxylation of a beta-ketoacid
D. The oxidation of a keto group to a carboxylic acid

Explanation: ### 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.

Question 212: What is the source of energy in the Krebs cycle?

A. NAD
B. NADP
C. NADPH
D. NADH (Correct Answer)

Explanation: ### Explanation The **Krebs cycle (TCA cycle)** is the central metabolic pathway for the oxidation of Acetyl-CoA. Its primary purpose is to harvest high-energy electrons from carbon fuels and transfer them to carrier molecules. **Why NADH is the correct answer:** During one turn of the Krebs cycle, three molecules of **NAD+** are reduced to **NADH** (at the Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and Malate dehydrogenase steps). These NADH molecules serve as the primary "energy currency" or mobile electron carriers. They transport high-potential electrons to the **Electron Transport Chain (ETC)** in the inner mitochondrial membrane. Through oxidative phosphorylation, each NADH molecule yields approximately **2.5 ATP**, making it the major source of potential energy generated within the cycle. **Analysis of Incorrect Options:** * **NAD (NAD+):** This is the oxidized form of the coenzyme. It acts as an electron *acceptor*, not a source of energy. It must be reduced to NADH to carry energy. * **NADP+:** This is the oxidized form of Nicotinamide adenine dinucleotide phosphate. It is not a primary coenzyme in the Krebs cycle; it is more commonly associated with the Pentose Phosphate Pathway (PPP). * **NADPH:** This molecule is primarily used for **reductive biosynthesis** (e.g., fatty acid and cholesterol synthesis) and maintaining antioxidant status (glutathione reduction). It is not the product of the Krebs cycle used for ATP generation. **High-Yield NEET-PG Pearls:** * **Total Yield:** One turn of the TCA cycle produces **3 NADH, 1 FADH2, and 1 GTP** (equivalent to 10 ATP). * **Rate-limiting step:** Isocitrate dehydrogenase (inhibited by ATP and NADH). * **Only Membrane-bound Enzyme:** Succinate dehydrogenase (also part of ETC Complex II). * **Clinical Correlation:** Thiamine (B1) deficiency inhibits α-ketoglutarate dehydrogenase, leading to impaired ATP production, commonly seen in Wernicke-Korsakoff syndrome.

Question 213: What is the net gain of ATP molecules during glycolysis?

A. 5
B. 7 (Correct Answer)
C. 15
D. 20

Explanation: **Explanation:** The net gain of ATP during glycolysis depends on whether we are calculating **Substrate Level Phosphorylation (SLP)** alone or including the **Respiratory Chain (Oxidative Phosphorylation)**. For NEET-PG, unless "anaerobic" is specified, we calculate the yield under aerobic conditions. **Why 7 is the correct answer:** In the payoff phase of glycolysis, 4 ATP are produced via SLP (at Phosphoglycerate kinase and Pyruvate kinase steps). However, 2 ATP are consumed in the preparatory phase (at Hexokinase and PFK-1 steps), leaving a net of **2 ATP**. Additionally, 2 molecules of **NADH** are produced (at Glyceraldehyde-3-phosphate dehydrogenase step). In the aerobic pathway, these NADH enter the mitochondria. Using the current biochemical standard (Malate-Aspartate Shuttle), 1 NADH yields 2.5 ATP. * Calculation: 2 ATP (Net SLP) + 5 ATP (from 2 NADH) = **7 ATP**. *(Note: If using the Glycerol-3-phosphate shuttle, the yield is 1.5 ATP per NADH, totaling 5 ATP, but 7 is the standard high-yield answer for aerobic glycolysis).* **Analysis of Incorrect Options:** * **Option A (5):** This represents the net gain if the Glycerol-3-phosphate shuttle is used (2 ATP + 3 ATP from NADH). * **Option C (15) & D (20):** These values do not correspond to glycolysis alone; they are more reflective of the total ATP yield from the complete oxidation of one molecule of pyruvate or acetyl-CoA in the TCA cycle. **Clinical Pearls & High-Yield Facts:** * **Anaerobic Glycolysis:** The net gain is always **2 ATP**. NADH is not shuttled to the ETC but is used to reduce pyruvate to lactate. * **Rate Limiting Enzyme:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, a bypass occurs where 2,3-BPG is formed. This results in **zero net ATP** gain from glycolysis because the ATP-producing PGK step is skipped. * **Essential for RBCs:** Mature erythrocytes lack mitochondria and depend entirely on anaerobic glycolysis (2 ATP) for survival.

Question 214: What is the end product of one stage of fermentation?

A. Formic acid
B. Pyruvate
C. Lactate (Correct Answer)
D. Ethanol

Explanation: **Explanation:** The question refers to the process of **anaerobic glycolysis**, commonly known as fermentation in a biological context. In humans, this occurs primarily in erythrocytes (which lack mitochondria) and exercising skeletal muscle. **1. Why Lactate is Correct:** During glycolysis, glucose is converted to pyruvate, producing 2 ATP and reducing NAD+ to NADH. For glycolysis to continue in the absence of oxygen, NAD+ must be regenerated. The enzyme **Lactate Dehydrogenase (LDH)** reduces pyruvate to **Lactate**, simultaneously oxidizing NADH back to NAD+. This allows the cell to maintain a continuous supply of NAD+ for the glyceraldehyde-3-phosphate dehydrogenase reaction, ensuring ATP production continues under anaerobic conditions. **2. Analysis of Incorrect Options:** * **Pyruvate (B):** This is the end product of *aerobic* glycolysis. In fermentation, pyruvate is merely an intermediate that is further reduced to lactate. * **Ethanol (D):** This is the end product of fermentation in yeast and some microorganisms (via pyruvate decarboxylase), but it is not the standard pathway in human metabolism. * **Formic acid (A):** This is a byproduct of metabolism in certain bacteria (mixed acid fermentation) and a toxic metabolite of methanol, but it is not a product of human carbohydrate fermentation. **Clinical Pearls for NEET-PG:** * **Cori Cycle:** Lactate produced in muscles travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Lactic Acidosis:** Occurs when there is a failure of oxidative phosphorylation (e.g., shock, hypoxia), leading to excessive lactate accumulation and a drop in blood pH. * **RBC Metabolism:** Since RBCs lack mitochondria, they depend *entirely* on anaerobic glycolysis (lactate production) for energy.

Question 215: The reaction Oxaloacetate + Acetyl CoA -> Citrate + CoASH is:

A. Reversible
B. Irreversible (Correct Answer)
C. Can be reversed by catalase
D. Competitive

Explanation: ### 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.

Question 216: Spermatozoa obtain their nutrition from which of the following substances?

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

Explanation: **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.

Question 217: How many dehydrogenases are present in the Krebs cycle?

A. 3
B. 2
C. 4 (Correct Answer)
D. 5

Explanation: The Krebs cycle (TCA cycle) is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. To answer this question, one must identify the specific enzymatic steps where oxidation-reduction reactions occur. ### Why Option C (4) is Correct There are exactly **four dehydrogenase enzymes** in the Krebs cycle that catalyze the removal of hydrogen atoms (electrons), which are then transferred to electron carriers (NAD⁺ or FAD): 1. **Isocitrate Dehydrogenase:** Converts Isocitrate to α-Ketoglutarate (Produces **NADH**). This is the rate-limiting step. 2. **α-Ketoglutarate Dehydrogenase Complex:** Converts α-Ketoglutarate to Succinyl-CoA (Produces **NADH**). 3. **Succinate Dehydrogenase:** Converts Succinate to Fumarate (Produces **FADH₂**). *Note: This enzyme is also Complex II of the Electron Transport Chain.* 4. **Malate Dehydrogenase:** Converts Malate to Oxaloacetate (Produces **NADH**). ### Why Other Options are Incorrect * **Options A & B (3 & 2):** These are undercounts. Students often forget Malate Dehydrogenase or Succinate Dehydrogenase because the latter uses FAD instead of NAD⁺. * **Option D (5):** This is a common distractor. While **Pyruvate Dehydrogenase (PDH)** is a crucial enzyme that links glycolysis to the TCA cycle, it is technically a "link reaction" enzyme and is **not** considered part of the Krebs cycle itself. ### High-Yield NEET-PG Pearls * **Location:** All TCA enzymes are in the mitochondrial matrix except **Succinate Dehydrogenase**, which is located on the **inner mitochondrial membrane**. * **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). * **Inhibitor:** Fluoroacetate inhibits Aconitase, while Arsenite inhibits the α-Ketoglutarate Dehydrogenase complex.

Question 218: Glycogen phosphorylase catalyzes the first step in glycogen degradation. Which cofactor is required by glycogen phosphorylase?

A. Pyridoxine (Correct Answer)
B. Vitamin B complex
C. Nickel
D. Cobalt

Explanation: **Explanation:** **Glycogen phosphorylase** is the rate-limiting enzyme of glycogenolysis. It catalyzes the phosphorolytic cleavage of glycogen to produce glucose-1-phosphate by breaking the α-1,4-glycosidic bonds. 1. **Why Pyridoxine is Correct:** Glycogen phosphorylase requires **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin B6 (Pyridoxine)**, as an essential cofactor. Unlike its role in transamination reactions where the aldehyde group is active, in glycogen phosphorylase, the **5'-phosphate group** of PLP acts as a general acid-base catalyst to facilitate the attack of inorganic phosphate on the glycosidic bond. Interestingly, more than 80% of the body’s total Vitamin B6 is stored in skeletal muscle, bound to this enzyme. 2. **Why Other Options are Incorrect:** * **Vitamin B complex:** While PLP is part of the B complex, "Pyridoxine" is the specific and most accurate answer required for this biochemical reaction. * **Nickel:** This is a cofactor for the enzyme **Urease** (found in *H. pylori*) but has no role in human carbohydrate metabolism. * **Cobalt:** This is a central component of **Vitamin B12 (Cobalamin)**, which is a cofactor for methionine synthase and methylmalonyl-CoA mutase, not glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **McArdle Disease (GSD Type V):** Caused by a deficiency of muscle glycogen phosphorylase. Patients present with exercise intolerance, muscle cramps, and myoglobinuria. * **Hers Disease (GSD Type VI):** Caused by a deficiency of liver glycogen phosphorylase, leading to hepatomegaly and mild fasting hypoglycemia. * **Key Regulation:** The enzyme is activated by **phosphorylation** (via phosphorylase kinase) and allosterically activated by **AMP** in the muscle.

Question 219: Which monosaccharide is best absorbed?

A. Glucose (Correct Answer)
B. Mannose
C. Fructose
D. Lactose

Explanation: **Explanation:** The absorption of monosaccharides in the small intestine occurs via two primary mechanisms: **Active Transport** and **Facilitated Diffusion**. **Why Glucose is the correct answer:** Glucose (and Galactose) are absorbed most rapidly because they utilize **Secondary Active Transport** via the **SGLT-1 (Sodium-Glucose Co-transporter 1)**. This mechanism is "active" because it leverages the sodium gradient maintained by the Na⁺-K⁺ ATPase pump. Because it is energy-dependent and works against a concentration gradient, it ensures near-complete and rapid absorption, making Glucose the most efficiently absorbed monosaccharide. **Analysis of Incorrect Options:** * **B. Mannose:** Absorbed much more slowly than glucose, primarily through simple diffusion. * **C. Fructose:** Absorbed via **Facilitated Diffusion** using the **GLUT-5** transporter. Since this process is passive (down a concentration gradient) and does not use energy, it is slower than the active transport of glucose. * **D. Lactose:** This is a **disaccharide**, not a monosaccharide. It must first be hydrolyzed into glucose and galactose by the enzyme lactase before absorption can occur. **NEET-PG High-Yield Pearls:** * **Rate of Absorption:** The order of absorption rate is: **Galactose > Glucose > Fructose > Mannose**. (Note: While Galactose is technically slightly faster, Glucose is often the standard answer in exams when Galactose is not listed). * **Transporters:** * **SGLT-1:** Glucose/Galactose (Apical membrane). * **GLUT-5:** Fructose (Apical membrane). * **GLUT-2:** All three (Glucose, Galactose, Fructose) exit the basolateral membrane into the portal circulation. * **Clinical Correlation:** Oral Rehydration Solution (ORS) utilizes the SGLT-1 mechanism; sodium absorption is enhanced in the presence of glucose.

Question 220: What is the most common enzyme defect in galactosemia?

A. Galactokinase
B. Uridyl transferase (Correct Answer)
C. 4-epimerase
D. None of the above

Explanation: **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).

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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