Enzymes Practice Questions

Q: Which is the earliest reliable enzyme to show an increase in acute myocardial infarction (AMI)?

CPK-MB. **Explanation:** In the context of Acute Myocardial Infarction (AMI), the timing of cardiac marker elevation is a high-yield topic for NEET-PG. **Why CPK-MB is the correct answer:** Creatine Phosphokinase-MB (CPK-MB) is a cardiac-specific isoenzyme. It is considered the **earliest reliable enzyme** because it begins to rise within **4–6 hours** of chest pain, peaks at 18–24 hours, and returns to baseline within 48–72 hours. While Troponins (proteins, not enzymes) are the gold standard for diagnosis, CPK-MB remains the enzyme of choice for detecting **re-infarction** due to its rapid clearance from the blood. **Analysis of Incorrect Options:** * **LDH (Lactate Dehydrogenase):** This is a late marker. It begins to rise after 24 hours, peaks at 3–6 days, and remains elevated for up to 2 weeks. It is used for late diagnosis of MI. * **SGOT (AST):** Aspartate Aminotransferase was the first marker used historically, but it lacks specificity (found in liver and muscle) and rises later than CPK-MB (8–12 hours). * **CK-MM:** This isoenzyme is primarily found in skeletal muscle. While it rises in muscle injury, it is not specific to cardiac tissue and is not used to diagnose AMI. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Marker Overall:** Myoglobin (rises in 1–2 hours), but it is non-specific. * **Gold Standard/Most Specific:** Cardiac Troponins (I and T). * **Marker for Re-infarction:** CPK-MB (because it returns to normal quickly). * **LDH Flip:** In AMI, LDH-1 becomes higher than LDH-2 (normally LDH-2 > LDH-1).

Q: Which class of enzymes catalyzes the transfer of electrons?

Oxidoreductases. ### Explanation **1. Why Oxidoreductases is Correct:** Oxidoreductases (EC Class 1) are enzymes that catalyze **oxidation-reduction (redox) reactions**. Oxidation involves the loss of electrons (or hydrogen), while reduction involves the gain of electrons. These enzymes facilitate the transfer of electrons from a reductant (electron donor) to an oxidant (electron acceptor). They typically utilize cofactors like **NAD⁺/NADH, FAD/FADH₂, or NADP⁺**. Common examples include dehydrogenases, oxidases, and reductases. **2. Why the Other Options are Incorrect:** * **Transferases (EC 2):** These enzymes catalyze the transfer of a **functional group** (e.g., methyl, phosphate, or amino groups) from one molecule to another. They do not primarily involve electron transfer. *Example: Hexokinase (transfers phosphate).* * **Lyases (EC 4):** These enzymes catalyze the **cleavage of bonds** (C-C, C-O, C-N) by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds. *Example: Aldolase.* * **Ligases (EC 6):** These enzymes catalyze the **joining of two large molecules** by forming new chemical bonds, a process that requires energy input, usually from **ATP hydrolysis**. *Example: DNA Ligase, Pyruvate carboxylase.* **3. NEET-PG High-Yield Pearls:** * **IUBMB Classification:** Remember the mnemonic **"O.T.H.L.I.L."** to recall the six classes in order: **O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases. * **Dehydrogenases:** These are the most clinically significant oxidoreductases in the TCA cycle and Glycolysis (e.g., Lactate Dehydrogenase/LDH). * **Clinical Correlation:** LDH isoenzymes are diagnostic markers; LDH-1 is elevated in myocardial infarction, while LDH-5 is elevated in liver disease.

Q: What is true regarding isozymes?

Forms of the same enzymes that catalyze the same reaction. **Explanation:** **Isozymes (or Isoenzymes)** are physically distinct forms of the same enzyme. The core concept is that they possess **different amino acid sequences** (encoded by different gene loci) but **catalyze the same chemical reaction**. 1. **Why Option B is correct:** Isozymes are "iso-functional." While they differ in their primary structure, physical properties (like electrophoretic mobility), and kinetic parameters ($K_m$ and $V_{max}$), they act upon the same substrate to produce the same product. This allows for fine-tuned metabolic regulation in different tissues or organelles. 2. **Why incorrect options are wrong:** * **Options A & C:** If enzymes catalyze different reactions, they are simply different enzymes (e.g., Hexokinase vs. Glucose-6-Phosphatase), not isozymes. * **Option D:** Isozymes are considered variants of the *same* enzyme family, not entirely "different" enzymes, as they share the same EC (Enzyme Commission) number. **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Dehydrogenase (LDH):** A tetramer with 5 isoforms. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is found in skeletal muscle and liver. A "flipped pattern" (LDH-1 > LDH-2) is a classic marker for Myocardial Infarction. * **Creatine Kinase (CK):** A dimer with 3 isoforms. **CK-MB** is specific for cardiac muscle; **CK-MM** for skeletal muscle; **CK-BB** for the brain. * **Hexokinase vs. Glucokinase:** These are functional isozymes. Glucokinase (Hexokinase IV) is found in the liver/pancreas and has a **high $K_m$** (low affinity), allowing it to respond to high post-prandial glucose levels.

Q: Which of the following statements regarding isoenzymes is false?

Isoenzymes have the same Km values. ### Explanation **Isoenzymes** (or isozymes) are physically distinct forms of the same enzyme. The core concept is that while they perform the exact same biochemical function, they are structurally different because they are encoded by different genes or gene loci. #### Why Option D is the Correct (False) Statement: Isoenzymes differ in their **amino acid sequences**, which results in different three-dimensional conformations at their active sites. Consequently, they possess **different kinetic properties**, specifically different **Km (Michaelis constant)** and **Vmax** values. * *Example:* **Glucokinase** (Liver) has a high Km for glucose (low affinity), while **Hexokinase** (Muscle) has a low Km (high affinity). This allows the liver to only process glucose when blood levels are high. #### Why Other Options are Incorrect (True Statements): * **A. Tissue Localization:** Isoenzymes are often tissue-specific. For instance, LDH-1 is predominant in the heart, while LDH-5 is found in the liver and skeletal muscle. * **B. Catalyze the same reaction:** By definition, isoenzymes catalyze the same chemical transformation (e.g., all LDH isoenzymes convert pyruvate to lactate). * **C. Separated by electrophoresis:** Because they have different amino acid compositions, they carry different net charges. This allows them to be separated based on their electrophoretic mobility. #### High-Yield Clinical Pearls for NEET-PG: 1. **LDH (Lactate Dehydrogenase):** A tetramer with 5 isoforms. **"Flipped Pattern"** (LDH1 > LDH2) is a classic (though older) marker for Myocardial Infarction. 2. **CK (Creatine Kinase):** * **CK-MB:** Cardiac muscle (Marker for MI). * **CK-MM:** Skeletal muscle. * **CK-BB:** Brain. 3. **Alkaline Phosphatase (ALP):** Isoenzymes help differentiate the source of pathology (e.g., **Regan isoenzyme** is a carcinofetal marker seen in some cancers).

Q: Which organ injury is associated with elevated levels of LDH isoenzyme-5?

Liver and muscles. **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme composed of two subunits: **H (Heart)** and **M (Muscle)**. These combine to form five isoenzymes (LDH1 to LDH5), which are tissue-specific. **Why Liver and Muscles are correct:** **LDH-5 (M4)** consists of four M subunits. It is primarily found in tissues that function under anaerobic conditions or have high glycolytic activity, specifically **skeletal muscle** and the **liver parenchyma**. An elevation in LDH-5 is a highly sensitive marker for hepatocellular injury (e.g., hepatitis) or skeletal muscle damage (e.g., muscular dystrophy or strenuous exercise). **Analysis of Incorrect Options:** * **Lungs:** Associated with **LDH-3 (H2M2)**. Elevations are seen in pulmonary embolism or pneumonia. * **Brain:** Primarily contains **LDH-1 and LDH-2**, though LDH-3 is also present. Brain injury or infarct typically shows a rise in these fractions. * **Heart:** Rich in **LDH-1 (H4)**. In myocardial infarction, LDH-1 levels rise and exceed LDH-2 levels (known as the **"Flipped Ratio"**). **High-Yield Clinical Pearls for NEET-PG:** * **LDH-1 (H4):** Heart and RBCs (Elevated in MI and Hemolytic anemia). * **LDH-2 (H3M1):** Reticuloendothelial system (Normal predominant serum fraction). * **LDH-4 (HM3):** Kidney and Pancreas. * **LDH-X (LDH-6):** Found in the mid-piece of spermatozoa; used as a marker for germ cell tumors. * **Total LDH:** A non-specific marker of cell turnover; significantly elevated in **Megaloblastic anemia** and **Pneumocystis jirovecii** pneumonia.

Q: Which of the following is an autocatalytic enzyme?

Pepsinogen. ### Explanation **Correct Answer: C. Pepsinogen** **Mechanism of Autocatalysis:** Autocatalysis refers to a process where the product of a reaction acts as a catalyst for its own formation. **Pepsinogen**, the inactive zymogen secreted by gastric chief cells, is initially activated into **pepsin** by the low pH (HCl) of the stomach. Once a small amount of pepsin is formed, it acts proteolytically on remaining pepsinogen molecules to rapidly convert them into more pepsin. This positive feedback loop ensures rapid protein digestion in the stomach. **Analysis of Incorrect Options:** * **A, B, and D (Proelastase, Procarboxypeptidase, Chymotrypsinogen):** These are all pancreatic zymogens. Unlike pepsinogen, they are not autocatalytic. Their activation follows a specific cascade: 1. The duodenal enzyme **Enteropeptidase (Enterokinase)** first converts Trypsinogen to **Trypsin**. 2. Trypsin then acts as the common activator for all other pancreatic zymogens (Proelastase → Elastase; Procarboxypeptidase → Carboxypeptidase; Chymotrypsinogen → Chymotrypsin). *Note: Trypsinogen itself is also considered autocatalytic because once trypsin is formed, it can activate more trypsinogen.* **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Inactive enzyme precursors that require covalent modification (proteolysis) for activation. This prevents autodigestion of the secreting organs (e.g., the pancreas). * **Trypsin:** Often called the "master switch" of pancreatic digestion. * **Enteropeptidase Deficiency:** Leads to severe protein malabsorption because no pancreatic enzymes can be activated. * **Acute Pancreatitis:** Occurs when trypsin is prematurely activated within the pancreas, leading to autodigestion.

Question 1

Acid phosphatase is associated with which of the following cell types?

Accepted Answer

Monocytes

Answer

T-Lymphocyte

Answer

B-lymphocyte

Answer

Myelocyte

Question 2

Which is the earliest reliable enzyme to show an increase in acute myocardial infarction (AMI)?

Accepted Answer

CPK-MB

Answer

LDH

Answer

SGOT

Answer

CK-MM

Question 3

Which class of enzymes catalyzes the transfer of electrons?

Accepted Answer

Oxidoreductases

Answer

Transferases

Answer

Lyases

Answer

Ligases

Question 4

Which of the following statements about Cytochrome P450 is FALSE?

Accepted Answer

Peroxidase converts H2O2 at the expense of electron acceptors.

Answer

It is exclusively seen in the ER of cells of the liver and intestines.

Answer

Catalase uses H2O2 as both electron acceptor and donor.

Answer

Superoxide ion is the most toxic form.

Question 5

What is true regarding isozymes?

Accepted Answer

Forms of the same enzymes that catalyze the same reaction

Answer

Forms of the same enzymes that catalyze different reactions

Answer

Forms of different enzymes that catalyze different reactions

Answer

Forms of different enzymes that catalyze the same reaction

Question 6

Which of the following statements regarding isoenzymes is false?

Accepted Answer

Isoenzymes have the same Km values

Answer

They differ in tissue localization

Answer

They catalyze the same reaction

Answer

They can be separated by electrophoresis

Question 7

In non-competitive inhibition, where do inhibitors bind?

Accepted Answer

The inhibitor binds to the enzyme at a site distinct from the substrate binding site.

Answer

The inhibitor binds to the enzyme at the substrate binding site.

Answer

The inhibitor can bind to either the substrate binding site or a distinct site.

Answer

None of the above.

Question 8

Which organ injury is associated with elevated levels of LDH isoenzyme-5?

Accepted Answer

Liver and muscles

Answer

Lungs

Answer

Brain

Answer

Heart

Question 9

Which of the following is an autocatalytic enzyme?

Accepted Answer

Pepsinogen

Answer

Proelastase

Answer

Procarboxylase

Answer

Chymotrypsinogen

Question 10

Which of the following is NOT a feature of competitive inhibition?

Accepted Answer

Inhibitor and substrate can bind simultaneously to the same enzyme molecule.

Answer

Inhibitor is a structural analogue of the substrate.

Answer

Km increases.

Answer

Vmax is unaffected.

Enzymes — MCQs

Enzymes — MCQs

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