Enzymes Practice Questions

Q: Which type of enzyme is a transaminase?

Transferase. **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.

Q: Erythrocyte transketolase activity is associated with which vitamin?

Thiamine. **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.

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

Cyclooxygenase by Aspirin. **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.

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

Astrocytes. **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.

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

Carbonic anhydrase. **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).

Q: What is the treatment for Multiple Carboxylase deficiency?

Biotin. **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.

Q: Which of the following is not an intracellular enzyme?

Salivary amylase. ### 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.

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

Succinate dehydrogenase. **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 1

Which type of enzyme is a transaminase?

Accepted Answer

Transferase

Answer

Oxidoreductase

Answer

Isomerase

Answer

Ligase

Question 2

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

Accepted Answer

Michaelis-Menten equation

Answer

Zimmermann reaction

Answer

Salkowski reaction

Answer

Liebermann-Burchard reaction

Question 3

Erythrocyte transketolase activity is associated with which vitamin?

Accepted Answer

Thiamine

Answer

Riboflavin

Answer

Folic acid

Answer

Niacin

Question 4

All of the following are examples of competitive enzyme inhibition except?

Accepted Answer

Cyclooxygenase by Aspirin

Answer

Thymidylate synthase by 5-Fluorouracil

Answer

Coagulation cascade by Dicumarol

Answer

Dihydrofolate reductase by Methotrexate

Question 5

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

Accepted Answer

Astrocytes

Answer

Oligodendrocytes

Answer

Microglial cells

Answer

Schwann cells

Question 6

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

Accepted Answer

Carbonic anhydrase

Answer

Tyrosinase

Answer

Superoxide dismutase

Answer

Ceruloplasmin

Question 7

What is the treatment for Multiple Carboxylase deficiency?

Accepted Answer

Biotin

Answer

Pyridoxine

Answer

Thiamine

Answer

Folic acid

Question 8

Which of the following is not an intracellular enzyme?

Accepted Answer

Salivary amylase

Answer

Enzymes of oxidative phosphorylation

Answer

Glycogen synthase

Answer

None of the above

Question 9

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

Accepted Answer

Succinate dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Question 10

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

Accepted Answer

Competitive

Answer

Noncompetitive

Answer

Uncompetitive

Answer

Allosteric

Enzymes — MCQs

Enzymes — MCQs

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