Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 41: Enolase is inhibited by which of the following substances?

A. Chloride
B. Cyanide
C. Fluoride (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Enolase** is a key glycolytic enzyme that catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). **Why Fluoride is the Correct Answer:** Fluoride is a potent **competitive inhibitor** of enolase. The mechanism involves fluoride ions reacting with magnesium and phosphate to form a **magnesium-fluorophosphate complex**. Since enolase requires $Mg^{2+}$ as a cofactor for its catalytic activity, the formation of this complex displaces the magnesium from the enzyme's active site, effectively halting glycolysis. **Analysis of Incorrect Options:** * **Chloride:** Chloride ions are actually **activators** of certain enzymes, most notably **Salivary Amylase**. They do not inhibit enolase. * **Cyanide:** Cyanide is a potent inhibitor of the electron transport chain, specifically targeting **Cytochrome Oxidase (Complex IV)**. It does not directly affect the glycolytic enzyme enolase. **High-Yield Clinical Pearls for NEET-PG:** 1. **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **vacutainers containing Sodium Fluoride (NaF)** (Grey-top bulbs). This prevents "in vitro" glycolysis by RBCs and WBCs, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of draw. 2. **Anticoagulant Pairing:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. 3. **Enzyme Classification:** Enolase belongs to the **Lyase** class of enzymes (EC 4). 4. **Reversibility:** The reaction catalyzed by enolase is reversible under physiological conditions.

Question 42: Alkaline phosphatase contains which of the following trace metals?

A. Cobalt
B. Zinc (Correct Answer)
C. Iron
D. Copper

Explanation: **Explanation:** **Alkaline Phosphatase (ALP)** is a metalloenzyme that requires **Zinc (Zn²⁺)** for its catalytic activity and structural integrity. Specifically, each monomer of the enzyme contains two zinc ions and one magnesium ion. Zinc acts as a cofactor that stabilizes the enzyme’s active site, allowing it to hydrolyze phosphate esters at an alkaline pH. **Analysis of Options:** * **Zinc (Correct):** Zinc is the primary metal constituent of ALP. It is also a cofactor for other high-yield enzymes like Carbonic Anhydrase, Alcohol Dehydrogenase, and DNA/RNA Polymerase. * **Cobalt:** While cobalt is a central component of **Vitamin B12 (Cobalamin)**, it is not found in ALP. * **Iron:** Iron is found in **Heme-containing enzymes** (Cytochromes, Catalase, Peroxidase) and non-heme proteins like Aconitase. * **Copper:** Copper is a cofactor for enzymes involved in redox reactions, such as **Cytochrome c oxidase**, Superoxide Dismutase (cytosolic), and Lysyl Oxidase. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Significance:** ALP levels are significantly elevated in **Obstructive Jaundice** (cholestasis) and **Bone diseases** with increased osteoblastic activity (e.g., Paget’s disease, Rickets). * **Heat Stability Test:** Used to differentiate ALP isoenzymes. The **Placental** isoenzyme (Regan) is the most heat-stable, while the **Bone** isoenzyme is the most heat-labile (*Mnemonic: "Bone is Burned"*). * **Zinc Deficiency:** Can lead to decreased ALP activity, along with clinical features like acrodermatitis enteropathica, poor wound healing, and hypogeusia (loss of taste).

Question 43: What is the predominant isozyme of lactate dehydrogenase (LDH) in the heart?

A. LD-1 (Correct Answer)
B. LD-2
C. LD-3
D. LD-5

Explanation: **Explanation:** Lactate dehydrogenase (LDH) is a tetrameric enzyme composed of two types of subunits: **H (Heart)** and **M (Muscle)**. These subunits combine in five different ways to form tissue-specific isozymes. **1. Why LD-1 is correct:** LD-1 consists of four H subunits (**H₄**). It is the predominant isozyme found in the **heart** and **erythrocytes** (RBCs). Because the heart relies on aerobic metabolism, LD-1 is specialized to favor the conversion of lactate to pyruvate for energy production. **2. Analysis of Incorrect Options:** * **LD-2 (H₃M₁):** Predominantly found in the **Reticuloendothelial system** and serum. In a healthy individual, LD-2 is the most abundant isozyme in the blood. * **LD-3 (H₂M₂):** Predominantly found in the **Lungs** and lymphoid tissue. * **LD-5 (M₄):** Predominantly found in the **Liver** and **Skeletal Muscle**. It favors the conversion of pyruvate to lactate under anaerobic conditions. **3. Clinical Pearls for NEET-PG:** * **LDH Flip:** Normally, serum LD-2 > LD-1. However, in **Myocardial Infarction (MI)** or **Hemolytic Anemia**, LD-1 levels rise above LD-2. This reversal is known as the "Flipped LDH" pattern. * **Diagnostic Timing:** LDH levels begin to rise 12–24 hours after an MI, peak at 48–72 hours, and remain elevated for 7–10 days. This makes it a useful marker for **late diagnosis of MI** (though Troponins are now the gold standard). * **LD-4 (HM₃):** Primarily found in the kidneys and pancreas.

Question 44: Which enzyme is responsible for the conversion of testosterone to dihydrotestosterone?

A. Conversion of cholesterol to pregnenolone and enhancing steroidogenesis
B. Conversion of testosterone to dihydrotestosterone (Correct Answer)
C. Aromatization of testosterone to estradiol
D. Increasing the synthesis of LH

Explanation: **Explanation:** The enzyme responsible for the conversion of **Testosterone** to **Dihydrotestosterone (DHT)** is **5-alpha reductase**. DHT is a more potent androgen than testosterone and is essential for the development of male external genitalia and prostate growth. * **Why Option B is correct:** 5-alpha reductase reduces the double bond between C4 and C5 of the testosterone molecule, producing DHT. This occurs primarily in peripheral tissues like the skin, prostate, and seminal vesicles. * **Why Option A is incorrect:** This describes the **Cholesterol Side-Chain Cleavage enzyme (Desmolase/CYP11A1)**. It is the rate-limiting step in steroidogenesis, stimulated by ACTH in the adrenals and LH in the gonads. * **Why Option C is incorrect:** This is the function of **Aromatase (CYP19A1)**. It converts androgens (testosterone and androstenedione) into estrogens (estradiol and estrone), primarily in the ovaries and adipose tissue. * **Why Option D is incorrect:** LH synthesis is regulated by **GnRH** from the hypothalamus and negative feedback from sex steroids; it is not an enzymatic conversion process of testosterone. **High-Yield Clinical Pearls for NEET-PG:** 1. **5-alpha reductase deficiency:** Results in male pseudohermaphroditism (ambiguous external genitalia at birth but virilization at puberty). 2. **Pharmacology Link:** **Finasteride** and **Dutasteride** are 5-alpha reductase inhibitors used to treat Benign Prostatic Hyperplasia (BPH) and male pattern baldness. 3. **Aromatase Inhibitors:** Drugs like **Anastrozole** and **Letrozole** are used in the treatment of ER-positive breast cancer in postmenopausal women.

Question 45: What is the classification of tyrosinase?

A. Oxidase (Correct Answer)
B. Transferase
C. Lyase
D. Isomerase

Explanation: **Explanation:** **Tyrosinase** is a copper-containing enzyme that belongs to the class of **Oxidoreductases** (EC 1). Specifically, it acts as an **oxidase** because it uses molecular oxygen ($O_2$) as an electron acceptor to catalyze the rate-limiting steps in melanin synthesis: the hydroxylation of Tyrosine to L-DOPA and the subsequent oxidation of L-DOPA to Dopaquinone. **Why the other options are incorrect:** * **Transferases:** These enzymes catalyze the transfer of a functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. Tyrosinase does not transfer groups; it facilitates redox reactions. * **Lyases:** These enzymes catalyze the cleavage of C-C, C-O, or C-N bonds by means other than hydrolysis or oxidation, often forming double bonds. * **Isomerases:** These enzymes catalyze structural rearrangements within a single molecule (e.g., converting glucose to fructose). **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Albinism:** A congenital deficiency or absence of the enzyme **Tyrosinase** leads to Oculocutaneous Albinism (Type 1), characterized by a lack of melanin in the skin, hair, and eyes. 2. **Cofactor:** Tyrosinase requires **Copper ($Cu^{2+}$)** for its activity. This is why copper deficiency can sometimes lead to hypopigmentation. 3. **Melanoma Marker:** Tyrosinase is often used as a specific immunohistochemical marker for identifying malignant melanoma cells. 4. **Pathway:** Tyrosine $\xrightarrow{\text{Tyrosinase}}$ DOPA $\xrightarrow{\text{Tyrosinase}}$ Dopaquinone $\rightarrow$ Melanin.

Question 46: Which enzyme class cleaves a C-C bond?

A. Lyase (Correct Answer)
B. Ligase
C. Transferase
D. Isomerase

Explanation: ### Explanation Enzymes are classified into six major classes based on the type of reaction they catalyze (EC classification). **1. Why Lyase is Correct:** **Lyases (Class 4)** catalyze the cleavage of **C-C, C-O, C-N**, and other bonds by means other than hydrolysis or oxidation. This process often results in the formation of a double bond or the addition of a group to a double bond. A classic example is **Aldolase**, which cleaves Fructose 1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate (cleaving a C-C bond) during glycolysis. **2. Why the Other Options are Incorrect:** * **Ligases (Class 6):** These enzymes catalyze the **joining** (ligation) of two molecules, coupled with the hydrolysis of a high-energy phosphate bond (like ATP). They form bonds rather than cleaving them (e.g., Pyruvate carboxylase). * **Transferases (Class 2):** These catalyze the transfer of a functional group (e.g., methyl, phosphate, or amino group) from one substrate to another (e.g., Hexokinase). * **Isomerases (Class 5):** These catalyze structural rearrangements within a single molecule (interconversion of optical, geometric, or positional isomers) without changing the molecular formula (e.g., Phosphohexose isomerase). **3. NEET-PG High-Yield Pearls:** * **Mnemonic (OTH LIL):** **O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases. * **Hydrolases vs. Lyases:** Both cleave bonds, but Hydrolases (Class 3) use **water** to break the bond, whereas Lyases do not. * **Decarboxylases** are a clinically important subclass of Lyases (e.g., Histidine decarboxylase forming Histamine). * **Synthases** belong to Lyases (do not require ATP), while **Synthetases** belong to Ligases (require ATP).

Question 47: Deficiency of the enzyme phytase results in which of the following conditions?

A. Zellweger syndrome
B. Low's syndrome
C. Refsum's disease (Correct Answer)
D. Tay-Sachs disease

Explanation: **Explanation:** The correct answer is **Refsum’s disease**. This condition is an autosomal recessive lipid storage disorder caused by a deficiency in **Phytanoyl-CoA hydroxylase** (also known as **Phytase** or Alpha-hydroxylase). **Underlying Concept:** Phytanic acid is a branched-chain fatty acid derived from chlorophyll in the diet (found in dairy and ruminant fats). Because it has a methyl group at the beta-carbon, it cannot undergo standard beta-oxidation. It must first undergo **Alpha-oxidation** in the **peroxisomes** to remove the alpha-carbon. A deficiency in phytase leads to the toxic accumulation of phytanic acid in tissues and serum, particularly affecting the nervous system and retina. **Analysis of Incorrect Options:** * **A. Zellweger Syndrome:** This is a "peroxisome biogenesis disorder" where peroxisomes are absent or non-functional. While it affects multiple pathways (including alpha and beta oxidation), it is a generalized defect rather than a specific deficiency of the phytase enzyme alone. * **B. Lowe’s Syndrome:** Also known as Oculocerebrorenal syndrome, this is an X-linked disorder caused by a deficiency in an inositol polyphosphate 5-phosphatase, affecting the Golgi apparatus, not phytanic acid metabolism. * **C. Tay-Sachs Disease:** This is a lysosomal storage disease caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Tetrad of Refsum’s:** Retinitis pigmentosa, Peripheral neuropathy, Cerebellar ataxia, and Nerve deafness. * **Treatment:** Dietary restriction of green leafy vegetables (chlorophyll) and ruminant meats/dairy. * **Key Enzyme:** Phytanoyl-CoA hydroxylase (Alpha-oxidation pathway). * **Location:** Peroxisomes.

Question 48: Pyridoxal phosphate acts as a coenzyme for which of the following enzymes?

A. Alanine aminotransferase
B. Transketolase (Correct Answer)
C. ALA synthase
D. Cystathionine synthase

Explanation: **Explanation** The question asks for the enzyme that utilizes **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, as a coenzyme. **Correct Answer: B. Transketolase** Actually, there is a conceptual error in the provided key. **Transketolase** requires **Thiamine Pyrophosphate (TPP/Vitamin B1)**, not PLP. However, in the context of standard biochemistry, PLP is the essential cofactor for transamination, decarboxylation, and certain cleavage reactions. **Analysis of Options:** * **A. Alanine Aminotransferase (ALT):** This enzyme requires **PLP**. It catalyzes the transfer of an amino group from alanine to alpha-ketoglutarate. * **C. ALA Synthase:** This is the rate-limiting enzyme of heme synthesis and is **PLP-dependent**. Deficiency leads to sideroblastic anemia. * **D. Cystathionine Synthase:** This enzyme in the transsulfuration pathway requires **PLP**. Deficiency leads to Homocystinuria. *Note: If the question intended to identify which enzyme does NOT use PLP, Transketolase would be the correct "odd one out" as it uses TPP.* **High-Yield Clinical Pearls for NEET-PG:** 1. **PLP (B6) Functions:** Essential for all **Transaminases** (AST/ALT), **Decarboxylases** (DOPA decarboxylase, Glutamate decarboxylase), and **Glycogen Phosphorylase**. 2. **Isoniazid (INH) Connection:** INH therapy for TB can induce B6 deficiency by binding to PLP, leading to peripheral neuropathy and sideroblastic anemia. 3. **Transketolase:** Always associate this with the **HMP Shunt** and **Thiamine (B1)**. Measuring erythrocyte transketolase activity is the gold standard for diagnosing Thiamine deficiency (Wernicke-Korsakoff).

Question 49: Hexokinase is a type of enzyme classified as which of the following?

A. Transferase (Correct Answer)
B. Reductase
C. Oxidoreductase
D. Oxidase

Explanation: **Explanation:** **Why Transferase is correct:** Enzymes are classified into six major classes by the IUBMB (International Union of Biochemistry and Molecular Biology). **Transferases (Class 2)** are enzymes that catalyze the transfer of a functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. **Hexokinase** catalyzes the first step of glycolysis, where a phosphoryl group is transferred from ATP (the donor) to a hexose sugar like glucose (the acceptor) to form Glucose-6-Phosphate. Since it involves the transfer of a phosphate group, it is a classic example of a transferase (specifically, a phosphotransferase). **Why other options are incorrect:** * **Oxidoreductases (Class 1):** These catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen (e.g., Dehydrogenases). Hexokinase does not involve a change in oxidation states. * **Reductases and Oxidases:** These are specific subclasses of Oxidoreductases. **Oxidases** use oxygen as an electron acceptor, while **Reductases** catalyze the reduction of a substrate. Neither mechanism applies to the phosphorylation of glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Hexokinase vs. Glucokinase:** Hexokinase is found in most extrahepatic tissues, has a **low Km** (high affinity for glucose), and is inhibited by its product (G6P). Glucokinase (Hexokinase IV) is found in the liver and pancreatic beta cells, has a **high Km** (low affinity), and is not inhibited by G6P. * **Irreversible Step:** The reaction catalyzed by Hexokinase is one of the three irreversible, rate-limiting steps of glycolysis. * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerize **L**igases (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase).

Question 50: Which of the following is not a ligase?

A. Aldolase
B. Enolase
C. Decarboxylase
D. Acetyl CoA Carboxylase (Correct Answer)

Explanation: **Explanation:** Enzymes are classified into six major classes (IUBMB classification). **Ligases (Class 6)** are enzymes that catalyze the joining of two molecules, usually coupled with the hydrolysis of a high-energy phosphate bond (like ATP). **Why Acetyl CoA Carboxylase is the Correct Answer:** Most **Carboxylases** (including Acetyl CoA Carboxylase, Pyruvate Carboxylase, and Propionyl CoA Carboxylase) are classic examples of **Ligases**. They utilize ATP to join a CO₂ molecule to a substrate. Acetyl CoA Carboxylase is the rate-limiting enzyme in fatty acid synthesis, converting Acetyl CoA to Malonyl CoA using ATP and Biotin as a cofactor. **Analysis of Incorrect Options:** * **A. Aldolase:** This is a **Lyase (Class 4)**. It catalyzes the cleavage of Fructose 1,6-bisphosphate into DHAP and Glyceraldehyde 3-phosphate in glycolysis without using water or redox reactions. * **B. Enolase:** This is also a **Lyase**. It catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). * **C. Decarboxylase:** These are **Lyases**. Unlike carboxylases, they remove a CO₂ group without requiring ATP energy input (e.g., Histidine decarboxylase). **High-Yield NEET-PG Pearls:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerases **L**igases (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Biotin (B7) Dependency:** All carboxylases (Ligases) require Biotin as a cofactor, except for Vitamin K-dependent γ-carboxylation of clotting factors. * **Synthetase vs. Synthase:** "Synthetase" implies a Ligase (requires ATP), whereas "Synthase" is usually a Lyase or Transferase (does not require ATP).

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Enzymes — MCQs

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