Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 421: Which type of enzyme is a transaminase?

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

Explanation: **Explanation:** **1. Why Transferase is Correct:** Transaminases (also known as aminotransferases) belong to **Class 2: Transferases**. These enzymes catalyze the transfer of a functional group from one molecule (the donor) to another (the acceptor). Specifically, transaminases transfer an **amino group (-NH₂)** from an amino acid to a keto acid (usually α-ketoglutarate), converting the keto acid into a new amino acid. This process requires **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as an essential coenzyme. **2. Why Other Options are Incorrect:** * **Oxidoreductases (Class 1):** Catalyze oxidation-reduction reactions (e.g., Dehydrogenases). While transamination is linked to the urea cycle, it does not involve the direct transfer of electrons or hydrogen in this specific step. * **Isomerases (Class 5):** Catalyze structural rearrangements within a single molecule (e.g., Phosphohexose isomerase). They do not transfer groups between different molecules. * **Ligases (Class 6):** Catalyze the joining of two molecules using ATP energy (e.g., DNA ligase, Pyruvate carboxylase). Transaminases do not require ATP for the transfer. **3. Clinical Pearls for NEET-PG:** * **Diagnostic Markers:** AST (Aspartate Transaminase) and ALT (Alanine Transaminase) are critical biomarkers for liver injury. ALT is more specific for liver pathology, while AST is also found in cardiac and skeletal muscle. * **Coenzyme Dependency:** A common high-yield question involves **Vitamin B6 (Pyridoxine)** deficiency, which impairs transamination, leading to neurological symptoms and sideroblastic anemia. * **Mechanism:** Transaminases utilize a "Ping-Pong" (Double Displacement) kinetic mechanism.

Question 422: Hormone substrate concentration affects the velocity of enzymatic action. This is based upon which of the following?

A. Zimmermann reaction
B. Salkowski reaction
C. Michaelis-Menten equation (Correct Answer)
D. Liebermann-Burchard reaction

Explanation: ### Explanation **1. Why Michaelis-Menten Equation is Correct:** The **Michaelis-Menten equation** ($V = \frac{V_{max} [S]}{K_m + [S]}$) describes the relationship between the rate of an enzymatic reaction (velocity, $V$) and the substrate concentration ($[S]$). It explains how, at low substrate concentrations, the velocity is directly proportional to $[S]$ (first-order kinetics), while at high concentrations, the enzyme becomes saturated, reaching a maximum velocity ($V_{max}$) independent of $[S]$ (zero-order kinetics). This principle is fundamental to understanding how hormones and metabolic substrates regulate biochemical pathways in the body. **2. Analysis of Incorrect Options:** * **Zimmermann Reaction:** This is a chemical test used for the detection and estimation of **17-ketosteroids** (androgens) in urine. It produces a violet/purple color. * **Salkowski Reaction:** A classic colorimetric test used to detect the presence of **cholesterol**. When chloroform and concentrated sulfuric acid are added to a cholesterol solution, a reddish-brown color develops in the lower layer. * **Liebermann-Burchard Reaction:** Another test for **cholesterol** (often used in clinical labs). It involves acetic anhydride and sulfuric acid, producing a characteristic emerald green color. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **$K_m$ (Michaelis Constant):** Defined as the substrate concentration at which the velocity is half of $V_{max}$. It reflects the **affinity** of the enzyme for its substrate (Inversely proportional: Low $K_m$ = High affinity). * **Lineweaver-Burk Plot:** A double-reciprocal plot ($1/V$ vs $1/[S]$) used to determine $V_{max}$ and $K_m$ more accurately. * **Enzyme Inhibition:** * *Competitive:* $K_m$ increases, $V_{max}$ remains unchanged. * *Non-competitive:* $K_m$ remains unchanged, $V_{max}$ decreases. * **Glucokinase vs. Hexokinase:** A classic application of $K_m$. Glucokinase (liver) has a high $K_m$ (low affinity), allowing it to function only when blood glucose levels are high, whereas Hexokinase (extra-hepatic) has a low $K_m$ (high affinity) to ensure glucose uptake even during fasting.

Question 423: Erythrocyte transketolase activity is associated with which vitamin?

A. Riboflavin
B. Thiamine (Correct Answer)
C. Folic acid
D. Niacin

Explanation: **Explanation:** **Thiamine (Vitamin B1)** is the correct answer because its active form, **Thiamine Pyrophosphate (TPP)**, serves as an essential coenzyme for the enzyme **Transketolase**. This enzyme is a key component of the non-oxidative phase of the **Pentose Phosphate Pathway (HMP Shunt)**, facilitating the transfer of two-carbon units between sugars. **Why other options are incorrect:** * **Riboflavin (B2):** Functions as a precursor for FAD and FMN, primarily involved in redox reactions (e.g., Glutathione reductase, Succinate dehydrogenase). * **Folic acid (B9):** Acts as a carrier of one-carbon units (THF) for DNA synthesis and amino acid metabolism. * **Niacin (B3):** Precursor for NAD and NADP, involved in electron transfer reactions (e.g., Lactate dehydrogenase). **Clinical Pearls for NEET-PG:** 1. **Diagnostic Utility:** Measuring **Erythrocyte Transketolase Activity (ETKA)** is the most reliable biochemical test to diagnose Thiamine deficiency. An increase in enzyme activity after adding TPP in vitro (the "TPP effect") confirms the deficiency. 2. **Key TPP-Dependent Enzymes:** Remember the mnemonic **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase. (Also includes Branched-chain ketoacid dehydrogenase). 3. **Clinical Correlation:** Thiamine deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff Syndrome**, often seen in chronic alcoholics. Chronic alcohol consumption inhibits the intestinal absorption of thiamine.

Question 424: All of the following are examples of competitive enzyme inhibition except?

A. Cyclooxygenase by Aspirin (Correct Answer)
B. Thymidylate synthase by 5-Fluorouracil
C. Coagulation cascade by Dicumarol
D. Dihydrofolate reductase by Methotrexate

Explanation: **Explanation** The correct answer is **A. Cyclooxygenase by Aspirin**. In **competitive inhibition**, the inhibitor binds reversibly to the active site of the enzyme, competing with the substrate. However, **Aspirin** is a classic example of **Irreversible Inhibition**. It works by covalently acetylating a specific serine residue (Serine 529 in COX-1) at the active site of the Cyclooxygenase enzyme. This permanent modification prevents the substrate (arachidonic acid) from binding for the entire lifespan of the enzyme/cell (e.g., the 7–10 day lifespan of a platelet). **Analysis of other options:** * **B. Thymidylate synthase by 5-Fluorouracil:** This is a "Suicide Inhibition" (a specialized form of irreversible inhibition). However, in many standard textbooks and competitive exams, it is categorized under the broad umbrella of antimetabolites that compete for the active site. *Note: If both "Irreversible" and "Competitive" are options, 5-FU is suicide/irreversible.* * **C. Coagulation cascade by Dicumarol:** Dicumarol (and Warfarin) acts as a competitive inhibitor of **Vitamin K Epoxide Reductase**. It structurally resembles Vitamin K and competes for the same binding site. * **D. Dihydrofolate reductase (DHFR) by Methotrexate:** Methotrexate is a structural analog of folic acid. It binds to the active site of DHFR with a much higher affinity than the natural substrate, acting as a classic competitive inhibitor. **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** $V_{max}$ remains unchanged; $K_m$ increases. Can be overcome by increasing substrate concentration. * **Non-competitive Inhibition:** $V_{max}$ decreases; $K_m$ remains unchanged. * **Suicide Inhibition Examples:** Allopurinol (on Xanthine Oxidase), Aspirin (on COX), Penicillin (on Transpeptidase), and 5-Fluorouracil. * **Statin drugs** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA Reductase.

Question 425: The blood-brain barrier is formed by which type of glial cells?

A. Oligodendrocytes
B. Astrocytes (Correct Answer)
C. Microglial cells
D. Schwann cells

Explanation: **Explanation:** The **Blood-Brain Barrier (BBB)** is a highly selective semipermeable border that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system (CNS). **Why Astrocytes are correct:** Astrocytes are the most abundant glial cells in the CNS. They possess specialized processes called **"end-feet" (podocytes)** that wrap around the endothelial cells of brain capillaries. These end-feet secrete paracrine factors that induce and maintain the **tight junctions** between endothelial cells, which are the structural basis of the BBB. While the endothelial cells themselves form the physical barrier, astrocytes are essential for its formation, maintenance, and physiological regulation. **Why other options are incorrect:** * **Oligodendrocytes:** These are responsible for the **myelination** of axons within the CNS (one cell can myelinate multiple axons). * **Microglial cells:** These are the resident **macrophages** of the CNS, acting as the primary immune defense. They are derived from the mesoderm (monocyte-macrophage lineage). * **Schwann cells:** These provide **myelination in the Peripheral Nervous System (PNS)**. Unlike oligodendrocytes, one Schwann cell myelinates only a single axon segment. **High-Yield Clinical Pearls for NEET-PG:** * **Components of BBB:** Tight junctions (Zonula occludens) between non-fenestrated endothelial cells, basement membrane, and astrocyte end-feet. * **Areas lacking BBB:** Known as **Circumventricular Organs (CVOs)**, such as the Area Postrema (chemoreceptor trigger zone), Posterior Pituitary, and Pineal gland. * **Marker for Astrocytes:** **GFAP** (Glial Fibrillary Acidic Protein) is a diagnostic marker for astrocytomas.

Question 426: Which of the following enzymes does not require copper for its action?

A. Tyrosinase
B. Superoxide dismutase
C. Carbonic anhydrase (Correct Answer)
D. Ceruloplasmin

Explanation: **Explanation:** The correct answer is **Carbonic anhydrase** because it is a **zinc-containing metalloenzyme**, not a copper-dependent one. ### 1. Why Carbonic Anhydrase is Correct Carbonic anhydrase (found in RBCs and renal tubules) requires **Zinc (Zn²⁺)** as a cofactor to catalyze the reversible hydration of carbon dioxide ($CO_2 + H_2O \rightleftharpoons H_2CO_3$). It is one of the fastest known enzymes and is essential for acid-base balance and $CO_2$ transport. ### 2. Analysis of Incorrect Options (Copper-containing Enzymes) * **Tyrosinase:** A copper-containing enzyme essential for melanin synthesis. It converts Tyrosine to DOPA and then to Dopaquinone. Deficiency leads to **Albinism**. * **Superoxide dismutase (SOD):** The cytoplasmic form (Cu-Zn SOD) requires both **Copper** and Zinc to scavenge free radicals. (Note: The mitochondrial form requires Manganese). * **Ceruloplasmin:** This is a ferroxidase enzyme that carries 95% of plasma copper. It is vital for iron metabolism (converting $Fe^{2+}$ to $Fe^{3+}$) and is deficient in **Wilson’s Disease**. ### 3. High-Yield Clinical Pearls for NEET-PG * **Other Copper Enzymes:** Cytochrome c oxidase (Complex IV of ETC), Lysyl oxidase (collagen cross-linking), and Dopamine $\beta$-hydroxylase. * **Zinc Enzymes:** Alcohol dehydrogenase, Carboxypeptidase, DNA/RNA polymerase, and Alkaline phosphatase (ALP). * **Menkes Kinky Hair Syndrome:** Due to impaired copper absorption/transport (ATP7A mutation), leading to deficiency of copper-dependent enzymes (e.g., Lysyl oxidase deficiency causes brittle hair and connective tissue issues).

Question 427: What is the treatment for Multiple Carboxylase deficiency?

A. Biotin (Correct Answer)
B. Pyridoxine
C. Thiamine
D. Folic acid

Explanation: **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is a metabolic disorder caused by the inability to utilize **Biotin (Vitamin B7)** effectively. Biotin serves as an essential coenzyme for four major carboxylase enzymes in humans: 1. **Pyruvate Carboxylase** (Gluconeogenesis) 2. **Acetyl-CoA Carboxylase** (Fatty acid synthesis) 3. **Propionyl-CoA Carboxylase** (Amino acid catabolism) 4. **3-Methylcrotonyl-CoA Carboxylase** (Leucine catabolism) MCD typically results from a deficiency in either **Holocarboxylase synthetase** (which attaches biotin to the enzymes) or **Biotinidase** (which recycles biotin from dietary sources or protein turnover). Since the underlying pathology is the lack of functional biotin-bound enzymes, pharmacological doses of **oral Biotin** bypass the metabolic block, making it the definitive treatment. **Analysis of Incorrect Options:** * **B. Pyridoxine (B6):** Acts as a cofactor for transamination and decarboxylation (e.g., Homocystinuria, Sideroblastic anemia). It has no role in carboxylation. * **C. Thiamine (B1):** A cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase). Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome. * **D. Folic Acid (B9):** Involved in one-carbon metabolism and DNA synthesis. Deficiency leads to megaloblastic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** MCD presents with dermatitis (periorificial rash), alopecia, metabolic acidosis, and neurological symptoms (seizures, hypotonia). * **Biochemical Hallmark:** Elevated organic acids in urine (e.g., 3-hydroxyisovaleric acid). * **Mnemonic:** Remember **"ABC"** enzymes—**A**cetyl-CoA, **B**iotin, **C**arboxylase. Biotin is always the carrier of $CO_2$ in carboxylation reactions.

Question 428: Which of the following is not an intracellular enzyme?

A. Enzymes of oxidative phosphorylation
B. Salivary amylase (Correct Answer)
C. Glycogen synthase
D. None of the above

Explanation: ### Explanation The classification of enzymes based on their site of action is a fundamental concept in biochemistry. Enzymes are categorized into two types: **Intracellular (Endoenzymes)**, which function within the cell where they are produced, and **Extracellular (Exoenzymes)**, which are secreted out of the cell to perform their functions elsewhere. **Why Salivary Amylase is the correct answer:** Salivary amylase (ptyalin) is synthesized by the acinar cells of the salivary glands. However, it is packaged into secretory vesicles and discharged into the oral cavity via ducts. Its primary function—the hydrolysis of starch into maltose—occurs in the mouth, which is an extracellular environment. Therefore, it is an **extracellular enzyme**. **Analysis of Incorrect Options:** * **Enzymes of oxidative phosphorylation:** These are located on the inner mitochondrial membrane (e.g., ATP synthase, Cytochrome c oxidase). Since they function strictly within the mitochondria to generate ATP, they are **intracellular**. * **Glycogen synthase:** This is the key regulatory enzyme for glycogenesis. It is located in the cytosol of liver and muscle cells, where it converts glucose to glycogen. Being functional within the cytoplasm, it is an **intracellular enzyme**. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Significance:** Intracellular enzymes (like ALT, AST, CK-MB, and LDH) are normally present in very low concentrations in the blood. Their elevation in serum is a clinical marker of **cell membrane damage** or necrosis (e.g., Myocardial Infarction or Hepatitis). * **Digestive Enzymes:** Most digestive enzymes (Pepsin, Trypsin, Lipase) are classic examples of extracellular enzymes. * **Lysosomal Enzymes:** While they function inside the cell, they are sequestered within organelles to prevent autolysis. If released into the cytosol, they are often inactivated by the neutral cytosolic pH.

Question 429: Which of the following is the FAD-linked dehydrogenase of the TCA cycle?

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

Explanation: **Explanation:** The correct answer is **Succinate dehydrogenase (SDH)**. In the TCA cycle, most dehydrogenases utilize $NAD^+$ as an electron acceptor. However, Succinate dehydrogenase catalyzes the oxidation of **Succinate to Fumarate**, which involves the reduction of **FAD to $FADH_2$**. **Why Succinate Dehydrogenase?** The free energy change ($\Delta G$) of the succinate-to-fumarate reaction is insufficient to reduce $NAD^+$. Therefore, the enzyme utilizes the more powerful oxidant, **FAD**, which is covalently bound to the enzyme. Uniquely, SDH is the only enzyme of the TCA cycle that is **integral to the inner mitochondrial membrane**, doubling as **Complex II** of the Electron Transport Chain (ETC). **Analysis of Incorrect Options:** * **A. Isocitrate dehydrogenase:** Catalyzes the oxidative decarboxylation of Isocitrate to $\alpha$-ketoglutarate; it is $NAD^+$-linked and is the rate-limiting step of the TCA cycle. * **B. Malate dehydrogenase:** Catalyzes the oxidation of Malate to Oxaloacetate; it is $NAD^+$-linked. * **D. Alpha-ketoglutarate dehydrogenase:** A multi-enzyme complex that converts $\alpha$-ketoglutarate to Succinyl-CoA; it is $NAD^+$-linked (though it uses FAD as a prosthetic group internally, the final electron exit is via $NAD^+$). **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** Succinate dehydrogenase is competitively inhibited by **Malonate** (a classic exam favorite). * **Location:** While all other TCA enzymes are in the mitochondrial matrix, SDH is in the **inner mitochondrial membrane**. * **Marker Enzyme:** SDH is used as a marker enzyme for mitochondria. * **Vitamins:** FAD is derived from **Riboflavin (Vitamin $B_2$)**.

Question 430: The type of enzyme inhibition in which the succinate dehydrogenase reaction is inhibited by malonate is an example of?

A. Noncompetitive
B. Uncompetitive
C. Competitive (Correct Answer)
D. Allosteric

Explanation: ### Explanation **1. Why Competitive Inhibition is Correct:** Competitive inhibition occurs when a substrate and an inhibitor compete for the same **active site** on an enzyme. This is the classic textbook example of competitive inhibition. * **Mechanism:** Malonate is a **structural analog** of succinate (the natural substrate). Both molecules possess two carboxyl groups. Because of this structural similarity, malonate binds to the active site of **Succinate Dehydrogenase (SDH)**, preventing succinate from binding. * **Kinetics:** In competitive inhibition, the **$V_{max}$ remains unchanged** (can be overcome by increasing substrate concentration), while the **$K_m$ increases** (affinity appears to decrease). **2. Why Other Options are Incorrect:** * **Noncompetitive:** The inhibitor binds to a site other than the active site (E or ES complex). It decreases $V_{max}$ but $K_m$ remains unchanged. Malonate specifically targets the active site, so this is incorrect. * **Uncompetitive:** The inhibitor binds only to the **Enzyme-Substrate (ES) complex**. Both $V_{max}$ and $K_m$ decrease. This is rare in single-substrate reactions. * **Allosteric:** This involves binding at a regulatory site (allosteric site) distinct from the active site, causing a conformational change. Malonate’s effect is purely due to its structural mimicry of the substrate at the active site. **3. High-Yield Clinical Pearls for NEET-PG:** * **SDH Significance:** Succinate dehydrogenase is unique because it is the only enzyme that participates in both the **TCA Cycle** and the **Electron Transport Chain (Complex II)**. * **Reversibility:** Competitive inhibition can be reversed by increasing the concentration of the substrate (Succinate). * **Other Competitive Examples:** * **Statins** (HMG-CoA Reductase inhibitors) * **Methanol poisoning** treated with Ethanol (competes for Alcohol Dehydrogenase) * **Sulfonamides** (compete with PABA in bacterial folate synthesis)

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

Practice Questions

Enzyme Inhibition: Competitive and Non-competitive

Practice Questions

Allosteric Regulation

Practice Questions

Coenzymes and Cofactors

Practice Questions

Isoenzymes and Clinical Significance

Practice Questions

Enzyme Regulation: Covalent Modification

Practice Questions

Enzyme Regulation: Zymogen Activation

Practice Questions

Enzyme Induction and Repression

Practice Questions

Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

Practice Questions

Enzyme Therapy and Inhibitors as Drugs

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?