Enzymes Practice Questions

Q: The conversion of glutamate to glutamine is catalysed by which type of enzyme?

Ligase. **Explanation:** The conversion of glutamate to glutamine is catalyzed by the enzyme **Glutamine Synthetase**. This reaction involves the condensation of glutamate and ammonia, which requires the consumption of energy in the form of **ATP hydrolysis**. 1. **Why Ligase is correct:** According to the IUBMB classification, **Ligases (Class 6)** are enzymes that catalyze the joining of two molecules coupled with the breakdown of a high-energy phosphate bond (like ATP). Since Glutamine Synthetase joins glutamate and ammonia using ATP, it is a classic example of a ligase. 2. **Why other options are incorrect:** * **Lyases:** Catalyze the cleavage of bonds (C-C, C-O, C-N) by means other than hydrolysis or oxidation, often forming double bonds. * **Transferases:** Transfer a functional group (e.g., methyl or phosphate) from one substrate to another. * **Oxidoreductases:** Catalyze oxidation-reduction reactions (e.g., Dehydrogenases). **High-Yield Clinical Pearls for NEET-PG:** * **Ammonia Detoxification:** This reaction is the primary mechanism for ammonia detoxification in the **brain**. Glutamine is non-toxic and can safely cross the blood-brain barrier to be transported to the liver. * **Glutaminase vs. Synthetase:** Do not confuse these. *Glutaminase* (a Hydrolase) breaks down glutamine into glutamate and ammonia in the kidneys and liver, whereas *Glutamine Synthetase* (a Ligase) creates glutamine. * **Mnemonic:** "Ligases join things together like **Glue** (Glutamine Synthetase)."

Q: What is a marker of peroxisomes?

Uric acid oxidase. **Explanation:** **Peroxisomes** (also known as microbodies) are membrane-bound organelles involved in various metabolic pathways, including the oxidation of long-chain fatty acids and the detoxification of reactive oxygen species. **Why Uric Acid Oxidase is correct:** Uric acid oxidase (Urate oxidase) is a classic marker enzyme for peroxisomes. It is involved in the catabolism of purines, converting uric acid into allantoin. In many species, this enzyme is so concentrated that it forms a **crystalline nucleoid** core visible under an electron microscope within the peroxisome. While humans lack a functional urate oxidase enzyme (leading to higher uric acid levels), it remains the definitive biochemical marker for identifying peroxisomal fractions in laboratory settings. **Analysis of Incorrect Options:** * **A. Glutamate dehydrogenase:** This is a marker enzyme for the **Mitochondrial matrix**. It plays a key role in nitrogen metabolism. * **B. Glucose-6-phosphatase:** This is the marker enzyme for the **Endoplasmic Reticulum (ER)**. It is crucial for gluconeogenesis and glycogenolysis (absent in Von Gierke’s disease). * **C. 5' - nucleotidase:** This is a marker for the **Plasma membrane**. Clinically, it is also used as a marker for hepatobiliary disease/cholestasis. **High-Yield Clinical Pearls for NEET-PG:** * **Other Peroxisomal Markers:** Catalase (breaks down $H_2O_2$), D-amino acid oxidase. * **Functions:** $\beta$-oxidation of Very Long Chain Fatty Acids (VLCFA), plasmalogen synthesis (essential for myelin), and bile acid synthesis. * **Clinical Correlation:** **Zellweger Syndrome** is an autosomal recessive "empty peroxisome" defect (PEX gene mutation) leading to the accumulation of VLCFA, characterized by hypotonia, seizures, and hepatomegaly.

Q: All of the following enzymes are increased in myocardial infarction, EXCEPT:

Isocitrate dehydrogenase. ### Explanation In the context of Myocardial Infarction (MI), cardiac enzymes are released into the bloodstream due to the necrosis of myocardial cells and subsequent leakage of their cytoplasmic contents. **Why Isocitrate Dehydrogenase (ICD) is the correct answer:** Isocitrate dehydrogenase is an enzyme of the Citric Acid (TCA) cycle. While it is present in various tissues, its clinical utility is primarily associated with **liver diseases** (specifically acute hepatitis), not cardiac injury. It does not show a significant or diagnostic rise following an MI, making it the "except" in this list. **Analysis of incorrect options (Enzymes that DO increase in MI):** * **CK (Creatine Kinase):** Specifically the **CK-MB** isoenzyme is a classic marker for MI. It rises within 4–6 hours, peaks at 24 hours, and returns to baseline within 48–72 hours. * **AST (Aspartate Aminotransferase):** Historically part of the "cardiac profile," AST rises within 6–12 hours of infarction. However, it is non-specific as it is also found in the liver and skeletal muscle. * **LDH (Lactate Dehydrogenase):** LDH levels begin to rise 24–48 hours after an MI and remain elevated for 7–10 days. A "flipped pattern" (LDH1 > LDH2) is characteristic of myocardial damage. **Clinical Pearls for NEET-PG:** * **Gold Standard:** **Cardiac Troponins (I and T)** are the most sensitive and specific markers for MI, surpassing all the enzymes listed above. * **Earliest Marker:** **Myoglobin** is the earliest marker to rise (1–3 hours) but lacks specificity. * **Re-infarction:** **CK-MB** is the investigation of choice to detect a second MI occurring shortly after the first, because it returns to baseline quickly (unlike Troponins, which stay elevated for up to 2 weeks).

Q: Flipped pattern of LDH is seen in which of the following conditions?

Acute myocardial infarction. **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme with five isoenzymes (LDH1 to LDH5). In a healthy individual, the concentration of **LDH2 (H3M1)** is higher than **LDH1 (H4)**. **1. Why Acute Myocardial Infarction (AMI) is correct:** LDH1 is primarily found in cardiac muscle and RBCs. Following an AMI, damaged myocardial cells release large amounts of LDH1 into the bloodstream. When the levels of LDH1 rise to exceed LDH2, the normal ratio is reversed (LDH1 > LDH2). This phenomenon is known as the **"Flipped Pattern."** While Troponins are the current gold standard for AMI, the LDH flip remains a classic biochemical marker, typically appearing 24–48 hours after the event and persisting for 7–10 days. **2. Why other options are incorrect:** * **Muscular Dystrophy:** Characterized by an increase in **LDH5** (found in skeletal muscle). * **Lymphoma:** Associated with a general increase in LDH levels (often LDH2, LDH3, and LDH4) due to high cell turnover, but not specifically a flipped LDH1/LDH2 ratio. * **Hemolytic Anemia:** While LDH1 increases (as it is present in RBCs), the "flipped pattern" is a term classically used in medical literature to describe the diagnostic shift specifically associated with **myocardial injury**. **High-Yield Clinical Pearls for NEET-PG:** * **LDH Isoenzymes:** LDH1 (Heart/RBC), LDH2 (Reticuloendothelial), LDH3 (Lungs), LDH4 (Kidney/Pancreas), LDH5 (Liver/Skeletal Muscle). * **Total LDH:** A non-specific marker of tissue injury or inflammation. * **Mega-tip:** If a question mentions "Flipped LDH" and both AMI and Hemolytic Anemia are options, **AMI** is the preferred clinical association for this specific terminology.

Q: What is the cofactor for Glycogen phosphorylase in Glycogenolysis?

Pyridoxal phosphate. **Explanation:** **1. Why Pyridoxal Phosphate (PLP) is correct:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. It catalyzes the phosphorolytic cleavage of glycogen to produce glucose-1-phosphate. Unlike most enzymes that use PLP for transamination or decarboxylation, glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor for its catalytic activity. Specifically, the **phosphate group** of PLP acts as a general acid-base catalyst, promoting the attack of inorganic phosphate on the glycosidic bond. This is a unique mechanism where the aldehyde group of PLP (usually the active site) is not directly involved in the catalysis. **2. Why other options are incorrect:** * **Thiamine pyrophosphate (TPP):** This is the active form of Vitamin B1. it is a cofactor for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase) and the HMP shunt (Transketolase). * **Citrate:** This is an intermediate of the TCA cycle and acts as an allosteric **inhibitor** of Phosphofructokinase-1 (PFK-1) in glycolysis, not a cofactor. * **FAD:** Flavin adenine dinucleotide (Vitamin B2 derivative) acts as an electron carrier in redox reactions, such as those catalyzed by Succinate dehydrogenase. **3. 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. * **Regulation:** Glycogen phosphorylase is activated by **phosphorylation** (via phosphorylase kinase) and allosterically activated by **AMP** in the muscle. * **B6 Fact:** Over 80% of the body's Vitamin B6 is stored in the muscle, bound to glycogen phosphorylase.

Q: All of the following can be attributed to enzymatic catalysis, EXCEPT:

Solvation of active site. **Explanation:** Enzymes function by lowering the activation energy of a reaction, thereby increasing the reaction rate. The correct answer is **Option B (Solvation of active site)** because enzymes actually achieve catalysis through **desolvation**. 1. **Why Option B is correct:** In an aqueous environment, substrates are surrounded by a "hydration shell" of water molecules. For a reaction to occur, this water must be removed so the substrate can interact directly with the enzyme. Enzymes displace these water molecules (**Desolvation**), which replaces the strong hydrogen bonds between the substrate and water with weaker, more specific interactions between the substrate and the enzyme's active site. This increases the reactivity of the substrate. 2. **Why other options are incorrect:** * **Entropy reduction (A):** Enzymes hold substrates in a specific orientation and proximity. This reduces the random translational and rotational motion (entropy) of the substrates, making the formation of the transition state more favorable. * **Reduction of activation energy (C):** This is the fundamental mechanism of all catalysts. By stabilizing the transition state, enzymes lower the energy barrier required for the reaction to proceed. * **Catalysis by strain (D):** Also known as the "Induced Fit" or "Induced Strain" model, the enzyme undergoes conformational changes that physically strain the bonds of the substrate, pushing it toward the transition state. **High-Yield NEET-PG Pearls:** * **Transition State Stabilization:** The most important way enzymes lower activation energy is by having a higher affinity for the *transition state* than for the substrate itself. * **Acid-Base Catalysis:** Often involves amino acids like **Histidine** (due to its pKa near physiological pH) acting as proton donors or acceptors. * **Covalent Catalysis:** Involves the formation of a transient covalent bond (e.g., Serine proteases).

Q: Which trace element is essential for the function of glutathione peroxidase?

Selenium (Se). **Explanation:** **Glutathione Peroxidase (GPx)** is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. The correct answer is **Selenium (Se)** because GPx is a **selenoprotein**. It contains the unique amino acid **selenocysteine** at its active site, which is often referred to as the "21st amino acid." Selenium is essential for the catalytic activity of the enzyme; without it, the body cannot effectively neutralize peroxides, leading to oxidative stress. **Analysis of Incorrect Options:** * **Copper (Cu):** While copper is a vital cofactor, it is associated with enzymes like **Superoxide Dismutase (Cytosolic Cu-Zn SOD)**, Cytochrome c Oxidase, and Tyrosinase, but not GPx. * **Iron (Fe):** Iron is the cofactor for **Catalase** (which also breaks down $H_2O_2$) and various cytochromes. It is not the primary trace element for GPx. * **Mercury (Hg):** Mercury is a heavy metal toxin. It actually **inhibits** selenium-dependent enzymes by binding to selenium with high affinity, thereby reducing antioxidant defenses. **Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy caused by Selenium deficiency, leading to decreased GPx activity. * **Selenocysteine:** Encoded by the **UGA stop codon** through a specialized recoding mechanism involving the SECIS element. * **Glutathione Reductase:** Do not confuse GPx with Glutathione Reductase, which requires **Riboflavin (Vitamin B2)** as a cofactor (FAD) and **NADPH** from the HMP shunt to regenerate reduced glutathione.

Q: Which of the following is an inhibitor of dihydrofolate reductase?

Methotrexate. ### Explanation **Correct Option: C. Methotrexate** Methotrexate is a structural analogue of folic acid. It acts as a **competitive inhibitor** of the enzyme **Dihydrofolate Reductase (DHFR)**. This enzyme is responsible for converting dihydrofolate (DHF) into tetrahydrofolate (THF), the active form of folic acid required for one-carbon metabolism and DNA synthesis (specifically the conversion of dUMP to dTMP). By inhibiting DHFR, methotrexate depletes the pool of THF, leading to the arrest of DNA synthesis and cell death, which explains its use as a potent anticancer and immunosuppressant drug. **Analysis of Incorrect Options:** * **A. Phenytoin:** This anticonvulsant causes folate deficiency not by enzyme inhibition, but by **inhibiting the intestinal absorption** of dietary folates (folate polyglutamates). * **B. Alcohol:** Ethanol interferes with folate metabolism primarily by impairing its **enterohepatic circulation** and increasing renal excretion, rather than direct DHFR inhibition. * **C. Yeast:** Yeast is actually a **rich dietary source of folic acid** (folates), making it a treatment/supplement rather than an inhibitor. **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** Methotrexate toxicity is managed with **Leucovorin (Folinic acid)**, which bypasses the blocked DHFR enzyme ("Leucovorin Rescue"). * **Other DHFR Inhibitors:** * **Trimethoprim:** Selective for bacterial DHFR. * **Pyrimethamine:** Selective for protozoal DHFR (used in Malaria/Toxoplasmosis). * **Side Effect:** A common side effect of DHFR inhibitors is **Megaloblastic Anemia** due to impaired DNA synthesis in RBC precursors.

Question 1

The conversion of glutamate to glutamine is catalysed by which type of enzyme?

Accepted Answer

Ligase

Answer

Lyase

Answer

Transferase

Answer

Oxidoreductase

Question 2

What is a marker of peroxisomes?

Accepted Answer

Uric acid oxidase

Answer

Glutamate dehydrogenase

Answer

Glucose-6-phosphate

Answer

5' - nucleotidase

Question 3

All of the following enzymes are increased in myocardial infarction, EXCEPT:

Accepted Answer

Isocitrate dehydrogenase

Answer

LDH

Answer

CK

Answer

Aspartate aminotransferase

Question 4

Flipped pattern of LDH is seen in which of the following conditions?

Accepted Answer

Acute myocardial infarction

Answer

Muscular dystrophy

Answer

Lymphoma

Answer

Hemolytic anemia

Question 5

What is the cofactor for Glycogen phosphorylase in Glycogenolysis?

Accepted Answer

Pyridoxal phosphate

Answer

Thiamine pyrophosphate

Answer

Citrate

Answer

FAD

Question 6

The provided graph illustrates the effect of substrate concentration on the initial velocity of an enzyme-catalyzed reaction. Identify the INCORRECT statement regarding this graph.

Accepted Answer

At point C, only a small amount of the enzyme is present as the enzyme-substrate complex.

Answer

The curve is hyperbolic in shape.

Answer

The value represented by '?' on the x-axis is the Km of the enzyme.

Answer

At point C, the initial velocity (Vi) is independent of substrate concentration.

Question 7

Which of the following statements is true about allosteric inhibition?

Accepted Answer

Citrate and ATP inhibiting PFK-1 is a classical example of allosteric inhibition.

Answer

Allosteric inhibitors are substrates of the enzyme.

Answer

Allosteric inhibitors bind directly to the active site of the enzyme.

Answer

Allosteric regulation is only homotropic.

Question 8

All of the following can be attributed to enzymatic catalysis, EXCEPT:

Accepted Answer

Solvation of active site

Answer

Entropy reduction

Answer

Reduction of activation energy

Answer

Catalysis by strain

Question 9

Which trace element is essential for the function of glutathione peroxidase?

Accepted Answer

Selenium (Se)

Answer

Copper (Cu)

Answer

Iron (Fe)

Answer

Mercury (Hg)

Question 10

Which of the following is an inhibitor of dihydrofolate reductase?

Accepted Answer

Methotrexate

Answer

Phenytoin

Answer

Alcohol

Answer

Yeast

Enzymes — MCQs

Enzymes — MCQs

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