Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 441: A solution contains 2x10^-3 mol/L of a weak acid (pK=3.5) and 2x10^-3 mol/L of its conjugate base. Its pH is best approximated by which one of the following?

A. 4.1
B. 3.9
C. 3.5 (Correct Answer)
D. 3.1

Explanation: ### Explanation **1. Why the Correct Answer is Right (The Henderson-Hasselbalch Equation)** The pH of a buffer solution is determined by the ratio of the concentration of the conjugate base ([A⁻]) to the weak acid ([HA]). This relationship is expressed by the **Henderson-Hasselbalch equation**: $$pH = pK_a + \log \frac{[Conjugate\ Base]}{[Weak\ Acid]}$$ In this question: * $[Conjugate\ Base] = 2 \times 10^{-3}\ mol/L$ * $[Weak\ Acid] = 2 \times 10^{-3}\ mol/L$ * $pK_a = 3.5$ Since the concentrations of the acid and base are **equal**, the ratio is 1. Because the $\log(1) = 0$, the equation simplifies to **$pH = pK_a$**. Therefore, the pH is exactly **3.5**. **2. Why the Incorrect Options are Wrong** * **Options A (4.1) and B (3.9):** These values are higher than the $pK_a$. This would only occur if the concentration of the conjugate base was significantly higher than the acid (making the solution more alkaline). * **Option D (3.1):** This value is lower than the $pK_a$. This would only occur if the concentration of the weak acid was higher than the conjugate base (making the solution more acidic). **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Maximum Buffering Capacity:** A buffer is most effective at resisting pH changes when the **$pH = pK_a$**. This occurs when the acid and conjugate base are in equal concentrations. * **Effective Buffer Range:** Generally, a buffer system functions effectively within **$\pm 1$ pH unit** of its $pK_a$. * **Bicarbonate Buffer System:** The most important extracellular buffer in humans. Though its $pK_a$ is 6.1 (far from physiological pH 7.4), it is effective because the body can independently regulate $CO_2$ (via lungs) and $HCO_3^-$ (via kidneys). * **Intracellular Buffer:** Phosphate ($pK_a \approx 6.8$) and Proteins (specifically **Histidine** residues) are the primary intracellular buffers.

Question 442: What cofactor does the enzyme transketolase require?

A. FAD (Flavin adenine dinucleotide)
B. TPP (Thiamine pyrophosphate) (Correct Answer)
C. PLP (Pyridoxal phosphate)
D. FMN (Flavin mononucleotide)

Explanation: **Explanation:** **Transketolase** is a key enzyme in the **Pentose Phosphate Pathway (Hexose Monophosphate Shunt)**. It catalyzes the transfer of a two-carbon unit (ketol group) from a ketose to an aldose. This reaction is strictly dependent on **Thiamine Pyrophosphate (TPP)**, the active form of Vitamin B1, which acts as a prosthetic group to stabilize the carbanion intermediate during the carbon transfer. **Analysis of Options:** * **TPP (Correct):** Besides transketolase, TPP is a vital cofactor for three other major enzyme complexes: Pyruvate Dehydrogenase (PDH), $\alpha$-ketoglutarate dehydrogenase, and Branched-chain $\alpha$-ketoacid dehydrogenase. * **FAD & FMN:** These are derivatives of Vitamin B2 (Riboflavin). They act as redox cofactors in the Electron Transport Chain and for enzymes like Succinate Dehydrogenase. * **PLP:** This is the active form of Vitamin B6. It is primarily involved in **transamination**, decarboxylation, and deamination reactions of amino acids. **Clinical Pearls for NEET-PG:** 1. **Diagnostic Marker:** Measuring **Erythrocyte Transketolase Activity (ETKA)** is the most reliable laboratory method to diagnose **Thiamine deficiency**. An increase in enzyme activity upon adding TPP in vitro (TPP effect >15-25%) confirms the deficiency. 2. **Wernicke-Korsakoff Syndrome:** This condition is caused by thiamine deficiency (often in chronic alcoholics). It is characterized by the triad of ataxia, ophthalmoplegia, and confusion. 3. **Metabolic Link:** Transketolase provides a reversible link between the HMP shunt and Glycolysis (producing Glyceraldehyde-3-P and Fructose-6-P).

Question 443: Which enzyme of the citric acid cycle catalyzes a reaction involving substrate-level phosphorylation?

A. Pyruvate kinase
B. Succinate thiokinase (Correct Answer)
C. Phosphoglycerate kinase
D. All of the above

Explanation: **Explanation:** **1. Why Succinate Thiokinase is Correct:** Succinate thiokinase (also known as **Succinyl-CoA synthetase**) is the only enzyme in the Citric Acid Cycle (TCA cycle) that performs **substrate-level phosphorylation (SLP)**. In this reaction, the high-energy thioester bond of Succinyl-CoA is cleaved to form Succinate. The energy released is used to phosphorylate GDP to **GTP** (which is later converted to ATP). This is a crucial step because it generates high-energy phosphate directly without the requirement of the Electron Transport Chain or Oxygen. **2. Why the Other Options are Incorrect:** * **Pyruvate Kinase (Option A):** While this enzyme does perform substrate-level phosphorylation (converting Phosphoenolpyruvate to Pyruvate), it is a part of **Glycolysis**, not the Citric Acid Cycle. * **Phosphoglycerate Kinase (Option C):** Similarly, this enzyme performs SLP (converting 1,3-bisphosphoglycerate to 3-phosphoglycerate), but it is also an enzyme of the **Glycolytic pathway**. * **Option D:** Incorrect because only Succinate thiokinase fits both criteria: being an SLP enzyme and belonging to the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of SLP:** The synthesis of ATP/GTP from ADP/GDP coupled directly to the dephosphorylation of a high-energy intermediate. * **Total SLP sites:** In one complete turn of the aerobic oxidation of glucose, there are **3 SLP sites**: two in Glycolysis (Phosphoglycerate kinase, Pyruvate kinase) and one in the TCA cycle (Succinate thiokinase). * **TCA Yield:** One turn of the TCA cycle produces **1 GTP (via SLP)**, 3 NADH, and 1 FADH2, totaling 10 ATP equivalents. * **Isoenzymes:** Succinate thiokinase has two isoforms; one is specific for GDP (liver/kidney) and one for ADP (skeletal/heart muscle).

Question 444: The rate-limiting step in the synthesis of cortisol is catalyzed by which enzyme?

A. 21-Hydroxylase
B. 3b-Hydroxysteroid dehydrogenase
C. Cholesterol side-chain cleavage enzyme (Correct Answer)
D. 11b-Hydroxylase

Explanation: **Explanation:** The synthesis of all steroid hormones, including cortisol, begins with cholesterol. The **rate-limiting and committed step** in this pathway is the conversion of cholesterol to **pregnenolone**. This reaction is catalyzed by the **Cholesterol side-chain cleavage enzyme** (also known as **Desmolase** or **P450scc**), located on the inner mitochondrial membrane. This step is stimulated by ACTH (in the adrenal cortex) via cAMP signaling, which increases the transport of cholesterol into the mitochondria by the StAR (Steroidogenic Acute Regulatory) protein. **Analysis of Incorrect Options:** * **21-Hydroxylase:** This enzyme converts progesterone to 11-deoxycorticosterone and 17-OH progesterone to 11-deoxycortisol. While it is the most common enzyme deficient in **Congenital Adrenal Hyperplasia (CAH)**, it is not the rate-limiting step. * **3β-Hydroxysteroid dehydrogenase:** This enzyme converts pregnenolone to progesterone. It is an essential early step but is not the primary regulatory point of the pathway. * **11β-Hydroxylase:** This enzyme catalyzes the final step in cortisol synthesis (converting 11-deoxycortisol to cortisol). Deficiency leads to CAH with associated hypertension due to the buildup of 11-deoxycorticosterone. **High-Yield Clinical Pearls for NEET-PG:** * **StAR Protein:** The actual "bottleneck" of steroidogenesis is the transport of cholesterol into the mitochondria by the StAR protein. * **Mitochondrial Localization:** Only the first step (Desmolase) and the final step (11β-Hydroxylase) occur inside the **mitochondria**; the intervening steps occur in the smooth endoplasmic reticulum. * **Ketoconazole:** This antifungal drug inhibits Desmolase at high doses and can be used to treat Cushing’s syndrome.

Question 445: Which of the following minerals does not act as a prosthetic group in enzymes?

A. Copper
B. Cobalt
C. Selenium
D. Manganese (Correct Answer)

Explanation: ### Explanation The key to answering this question lies in distinguishing between **Prosthetic Groups** and **Metal Activators**. **1. Why Manganese (D) is the correct answer:** Manganese ($Mn^{2+}$) typically acts as a **metal activator**. In biochemistry, metal activators are loosely and reversibly bound to the enzyme (forming a metal-activated enzyme). They are not permanently attached to the protein structure. Examples include **Arginase**, **Pyruvate carboxylase**, and **Superoxide Dismutase (mitochondrial)**. Since it is not covalently or tightly bound as a permanent integral part of the enzyme, it is not classified as a prosthetic group. **2. Why the other options are incorrect:** * **Copper (A):** Acts as a prosthetic group in enzymes like **Cytochrome c oxidase** and **Tyrosinase**. It is tightly bound and essential for the catalytic cycle. * **Cobalt (B):** Is a central prosthetic component of **Vitamin B12 (Cobalamin)**. It is essential for enzymes like **Methylmalonyl-CoA mutase** and **Methionine synthase**. * **Selenium (C):** Is uniquely incorporated as the amino acid **Selenocysteine** (the 21st amino acid) into the polypeptide chain of enzymes like **Glutathione peroxidase** and **Thioredoxin reductase**, making it a permanent prosthetic element. ### High-Yield Clinical Pearls for NEET-PG: * **Prosthetic Group:** A non-protein organic or inorganic cofactor that is **tightly or covalently bound** to the apoenzyme (e.g., Heme in Hemoglobin, FAD in Succinate dehydrogenase). * **Metalloenzymes:** Enzymes where the metal is a prosthetic group (tightly bound). Examples: **Zinc** in Carbonic anhydrase and Alcohol dehydrogenase; **Iron** in Cytochromes. * **Metal-activated enzymes:** Enzymes where the metal is loosely bound. Examples: **Magnesium ($Mg^{2+}$)** is the most common activator for kinases (ATP-utilizing enzymes). * **Molybdenum** is a high-yield prosthetic group for **Xanthine oxidase**.

Question 446: Large doses of EDTA are used in carbonic anhydrase enzyme inactivation. What is the mechanism by which EDTA acts?

A. It chelates with the metal ions of the enzyme (Correct Answer)
B. It combines with the substrate and reacts with the enzyme
C. Combines with the substrate and doesn't react with the enzyme
D. Enzyme-EDTA complex cannot be attached to substrate

Explanation: ### Explanation **1. Why Option A is Correct:** Carbonic anhydrase is a classic example of a **metalloenzyme**. It contains a tightly bound **Zinc ($Zn^{2+}$) ion** at its active site, which is essential for its catalytic activity (it facilitates the formation of hydroxide ions from water). **EDTA (Ethylenediaminetetraacetic acid)** is a potent **chelating agent**. It acts as a "chemical claw" that binds to the divalent metal ions ($Zn^{2+}$) required by the enzyme. By sequestering the Zinc ion, EDTA disrupts the structural integrity and the catalytic mechanism of carbonic anhydrase, leading to its inactivation. **2. Why the Other Options are Incorrect:** * **Options B and C:** EDTA does not interact with the substrate ($CO_2$ or $H_2O$). Its mechanism is strictly focused on the inorganic cofactor (metal ion) of the enzyme, not the organic substrate. * **Option D:** While it is true that the enzyme cannot function, the primary mechanism is not the formation of an "Enzyme-EDTA-Substrate" steric block; rather, it is the removal/sequestration of the essential metal cofactor that renders the enzyme non-functional. **3. High-Yield NEET-PG Clinical Pearls:** * **Metalloenzymes vs. Metal-activated enzymes:** Carbonic anhydrase is a *metalloenzyme* (metal is integral to the structure). Enzymes like Hexokinase ($Mg^{2+}$) are *metal-activated* (metal is loosely bound). * **Carbonic Anhydrase Inhibitors:** While EDTA is used in lab settings, the clinical inhibitor used for Glaucoma and Mountain Sickness is **Acetazolamide** (non-competitive inhibitor). * **Zinc-containing enzymes:** Other high-yield Zinc enzymes include Alcohol Dehydrogenase, Carboxypeptidase, and DNA Polymerase. * **EDTA Clinical Use:** It is the treatment of choice for **Lead poisoning** (as $CaNa_2EDTA$).

Question 447: Which of the following liver enzymes is predominantly mitochondrial?

A. SGOT (AST) (Correct Answer)
B. SGPT (ALT)
C. GGT
D. 5' Nucleotidase

Explanation: **Explanation:** The correct answer is **SGOT (AST)**. In biochemistry and clinical pathology, understanding the subcellular localization of enzymes is crucial for interpreting diagnostic tests. **1. Why SGOT (AST) is correct:** Aspartate Aminotransferase (AST/SGOT) exists as two distinct isoenzymes: **cytosolic (cAST)** and **mitochondrial (mAST)**. In the liver, approximately **80% of AST activity is found within the mitochondria**. Because it is sequestered deep within the mitochondria, significant elevations of AST often indicate severe cellular necrosis or chronic tissue damage (e.g., alcoholic hepatitis), where the mitochondrial membrane is breached. **2. Why the other options are incorrect:** * **SGPT (ALT):** Alanine Aminotransferase is primarily a **cytosolic** enzyme. It is more specific to the liver than AST and is released easily even with minor cell membrane damage. * **GGT (Gamma-glutamyl transferase):** This enzyme is primarily located on the **cell membranes** (microsomal) of cells with high secretory or absorptive activities, such as the biliary epithelium. * **5' Nucleotidase:** This is a **plasma membrane-bound** enzyme. Like GGT, it is a marker for cholestasis (biliary obstruction) rather than deep mitochondrial damage. **High-Yield Clinical Pearls for NEET-PG:** * **De Ritis Ratio (AST/ALT):** If the ratio is **>2:1**, it strongly suggests **Alcoholic Liver Disease**. Alcohol is a mitochondrial toxin that specifically damages the mitochondria, leading to the preferential release of mAST. * **Specificity:** ALT is more liver-specific; AST is also found in cardiac muscle, skeletal muscle, and RBCs. * **Half-life:** ALT has a longer half-life (~47 hours) compared to AST (~17 hours).

Question 448: Which of the following is a copper-containing enzyme?

A. Catalase
B. Cytochrome oxidase (Correct Answer)
C. Lactate dehydrogenase (LDH)
D. None of the above

Explanation: **Explanation:** **Cytochrome oxidase** (also known as Complex IV of the Electron Transport Chain) is the correct answer because it contains **two copper centers ($Cu_A$ and $Cu_B$)** in addition to two heme groups ($a$ and $a_3$). These copper ions are essential for the final step of cellular respiration, where electrons are transferred to oxygen to form water. **Analysis of Options:** * **Catalase:** This is a **heme-containing (iron)** enzyme found in peroxisomes. It protects cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide into water and oxygen. * **Lactate dehydrogenase (LDH):** This is a glycolytic enzyme that converts pyruvate to lactate. It does not require a metal cofactor like copper; instead, it utilizes **NAD+/NADH** as a coenzyme. * **None of the above:** Incorrect, as Cytochrome oxidase is a well-known cuproenzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Other Copper-containing enzymes:** Remember the mnemonic **"C-C-S-S-T-L"**: **C**ytochrome oxidase, **C**eruloplasmin (ferroxidase), **S**uperoxide dismutase (cytosolic Zn-Cu SOD), **S**pice (Tyrosinase for melanin), **T**ryptophan pyrrolase, and **L**ysyl oxidase (essential for collagen cross-linking). * **Menkes Disease:** A defect in copper absorption (ATP7A) leading to "kinky hair" and connective tissue defects due to low **Lysyl oxidase** activity. * **Wilson Disease:** A defect in copper excretion (ATP7B) leading to copper toxicity and low levels of **Ceruloplasmin**. * **Cyanide/CO Poisoning:** Both inhibit **Cytochrome oxidase** by binding to the iron/copper centers, halting the ETC and causing cellular asphyxia.

Question 449: NAD acts as a cofactor for which of the following enzymes?

A. Citrate synthetase
B. Isocitrate dehydrogenase (Correct Answer)
C. Alpha-ketoglutarate dehydrogenase
D. Malate dehydrogenase

Explanation: **Explanation:** The correct answer is **Isocitrate dehydrogenase**. This question tests your knowledge of the Citric Acid Cycle (TCA cycle) and the specific cofactors required for its oxidative steps. **1. Why Isocitrate Dehydrogenase is Correct:** Isocitrate dehydrogenase catalyzes the first oxidative decarboxylation in the TCA cycle, converting Isocitrate to $\alpha$-ketoglutarate. This reaction involves the reduction of **NAD+ to NADH + H+**. It is considered the rate-limiting step of the TCA cycle and is allosterically activated by ADP and inhibited by ATP and NADH. **2. Analysis of Other Options:** * **Citrate Synthetase (Option A):** This enzyme catalyzes the condensation of Acetyl-CoA and Oxaloacetate to form Citrate. It does not involve a redox reaction and therefore does not require NAD+. * **Alpha-ketoglutarate Dehydrogenase (Option C):** This is a multi-enzyme complex that requires **five cofactors**: Thiamine pyrophosphate (TPP), Lipoic acid, CoA, FAD, and NAD+. While it *does* use NAD+, in the context of standard medical examinations, if a question asks for "the" enzyme or focuses on the primary regulatory NAD-linked step, Isocitrate Dehydrogenase is often the preferred classic example. *Note: In many competitive exams, both C and D also use NAD; however, Isocitrate dehydrogenase is the specific key regulatory link.* * **Malate Dehydrogenase (Option D):** This enzyme converts Malate to Oxaloacetate and also uses NAD+ as a cofactor. **Note on Question Ambiguity:** In the TCA cycle, three enzymes use NAD+ (Isocitrate DH, $\alpha$-KG DH, and Malate DH). If this appears as a single-choice question, Isocitrate Dehydrogenase is often prioritized as it is the **rate-limiting step**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for TCA Cofactors:** "Tender Loving Care For No-one" (TPP, Lipoate, CoA, FAD, NAD) for $\alpha$-KG Dehydrogenase and Pyruvate Dehydrogenase. * **Rate-limiting enzyme of TCA:** Isocitrate Dehydrogenase. * **Only membrane-bound enzyme of TCA:** Succinate Dehydrogenase (also part of Complex II of ETC; uses FAD, not NAD).

Question 450: Which enzyme is commonly used in ELISA?

A. Alkaline phosphatase (Correct Answer)
B. Acid phosphatase
C. Glucosidase
D. Glycosyl transferase

Explanation: **Explanation:** ELISA (Enzyme-Linked Immunosorbent Assay) is a biochemical technique used to detect the presence of an antigen or antibody in a sample. The core principle involves an enzyme conjugated to an antibody which reacts with a colorless substrate to produce a measurable colored product (chromogenic reaction). **Why Alkaline Phosphatase (ALP) is correct:** Alkaline Phosphatase and **Horseradish Peroxidase (HRP)** are the two most commonly used enzymes in ELISA. ALP is preferred because of its high catalytic turnover rate, stability at room temperature, and the availability of sensitive substrates like p-nitrophenyl phosphate (pNPP), which yields a distinct yellow color upon reaction. **Why other options are incorrect:** * **Acid Phosphatase:** Unlike ALP, this enzyme functions at an acidic pH, which is not compatible with the physiological buffers (like PBS) required to maintain the structural integrity of antibodies and antigens during the assay. * **Glucosidase:** While used in some metabolic studies, it lacks the high sensitivity and rapid signal amplification required for standard diagnostic ELISA. * **Glycosyl transferase:** This is a biosynthetic enzyme involved in carbohydrate chain formation; it does not produce the easily measurable colorimetric change necessary for a detection assay. **High-Yield Clinical Pearls for NEET-PG:** * **Most common enzymes in ELISA:** Horseradish Peroxidase (HRP) > Alkaline Phosphatase (ALP) > β-galactosidase. * **Substrate for ALP:** p-nitrophenyl phosphate (pNPP). * **Substrate for HRP:** TMB (Tetramethylbenzidine) or OPD (o-phenylenediamine). * **Application:** ELISA is the **screening test** of choice for HIV (Western Blot is the confirmatory test).

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Enzymes — MCQs

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