Enzymes Practice Questions

Q: Arrange the following enzyme categories as per increasing order of their enzyme commission (EC) numbers?

Transferases - Hydrolases - Hydratase - Isomerases. To master enzyme classification for NEET-PG, remember the standard mnemonic **"O T H L I L"**, which represents the six major classes of enzymes in their specific numerical order (EC 1 to EC 6). ### 1. Understanding the Correct Sequence (Option B) The Enzyme Commission (EC) numbers are assigned based on the type of reaction catalyzed: * **EC 1: Oxidoreductases** (Redox reactions) * **EC 2: Transferases** (Transfer of functional groups) * **EC 3: Hydrolases** (Cleavage of bonds using water) * **EC 4: Lyases** (Addition/removal of groups to form double bonds; includes **Hydratases**) * **EC 5: Isomerases** (Intramolecular rearrangements) * **EC 6: Ligases** (Joining two molecules using ATP) In the correct option (B), the sequence follows: **Transferases (2) < Hydrolases (3) < Hydratase (a subclass of Lyases, 4) < Isomerases (5)**. This perfectly matches the increasing numerical order. ### 2. Analysis of Incorrect Options * **Option A:** Starts with Isomerases (5), which is higher than Transferases (2). * **Option C:** Starts with Hydratase (4), placing it before Transferases (2) and Hydrolases (3). * **Option D:** Places Hydrolases (3) before Transferases (2). ### 3. High-Yield Clinical Pearls for NEET-PG * **Hydratase vs. Hydrolase:** Do not confuse them. **Hydrolases (EC 3)** use water to break a bond (e.g., Pepsin), while **Hydratases (EC 4)** add or remove water without breaking the primary skeleton (e.g., Enolase, Fumarase). * **Kinases:** These belong to **Transferases (EC 2)** because they transfer a phosphate group from ATP. * **Dehydrogenases:** These are the most common **Oxidoreductases (EC 1)**. * **Transaminases (ALT/AST):** These are **Transferases (EC 2)** and are vital markers for liver injury.

Q: All of the following statements about functional enzymes are TRUE, EXCEPT:

Substrates for functional enzymes are absent in blood.. ### Explanation The question asks for the **incorrect** statement regarding functional enzymes. **1. Why Option C is the Correct Answer (The False Statement):** Functional plasma enzymes are those that have a specific physiological role in the blood. For these enzymes to perform their functions, their **substrates must be present in the blood**. For example, the substrate for thrombin (fibrinogen) is always present in the plasma to facilitate coagulation. Therefore, stating that substrates are absent is factually incorrect. **2. Analysis of Other Options:** * **Option A (True):** Prothrombin (Factor II) is a classic example of a functional enzyme. Other examples include Lipoprotein Lipase (LPL) and enzymes of the complement system. * **Option B (True):** Functional enzymes are actively secreted into the blood by organs (like the liver) and maintain a **higher concentration in the plasma** than in the tissues. This is the opposite of *non-functional* enzymes (like ALT or CK), which are intracellular and only appear in high plasma levels during tissue damage. * **Option D (True):** Since most functional enzymes (like clotting factors) are synthesized in the **liver**, their activity and concentration significantly **decrease in liver disease**, leading to clinical issues like coagulopathy. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Synthesized in the liver, active in plasma, substrates present in blood. Examples: Clotting factors, Pseudocholinesterase, Lipoprotein lipase. * **Non-Functional Enzymes:** No physiological role in plasma, high concentration in tissues, low in plasma. Their rise indicates **cell death or membrane damage**. Examples: LDH, AST, ALT, Amylase. * **Diagnostic Tip:** A decrease in functional enzymes (e.g., low Prothrombin Time/INR) is a sensitive indicator of liver biosynthetic failure.

Q: Which enzyme is used in the treatment of acute myocardial infarction?

Urokinase. **Explanation:** **Urokinase** is the correct answer because it belongs to a class of drugs known as **thrombolytics** or "clot busters." In the context of an acute myocardial infarction (AMI), the primary pathology is the occlusion of a coronary artery by a thrombus. Urokinase acts as a serine protease that directly converts **plasminogen to plasmin**. Plasmin then degrades the fibrin meshwork of the thrombus, restoring blood flow to the ischemic myocardium (reperfusion). **Analysis of Incorrect Options:** * **Papain:** Derived from papaya, this proteolytic enzyme is primarily used for wound debridement (removing dead tissue) and as a digestive aid, not for systemic thrombolysis. * **Asparaginase:** This enzyme is used as a chemotherapeutic agent, specifically in **Acute Lymphoblastic Leukemia (ALL)**. It breaks down asparagine; since leukemic cells cannot synthesize asparagine, they undergo apoptosis. * **Serratiopeptidase:** A proteolytic enzyme used to reduce inflammation and edema in conditions like sinusitis or post-traumatic swelling. It has no role in treating acute coronary syndromes. **Clinical Pearls for NEET-PG:** * **Thrombolytic Generations:** * *1st Gen:* Streptokinase (non-specific, antigenic), Urokinase. * *2nd/3rd Gen:* Alteplase (tPA), Reteplase, Tenecteplase (fibrin-specific, preferred in modern practice). * **Mechanism:** All thrombolytics ultimately increase **plasmin** levels. * **Contraindications:** Always check for history of hemorrhagic stroke, active internal bleeding, or recent major surgery before administration.

Q: Which of the following does not cause an increase in serum amylase?

Cardiac failure. **Explanation:** Serum amylase is a key biochemical marker primarily used to diagnose pancreatic disorders, but its elevation can occur due to non-pancreatic causes. **Why Cardiac Failure is the Correct Answer:** Cardiac failure does not typically cause an elevation in serum amylase. While severe congestive heart failure can lead to "shock liver" (elevated transaminases), it does not involve the salivary glands or the pancreas, nor does it impair the renal clearance of amylase significantly enough to cause hyperamylasemia. **Analysis of Incorrect Options:** * **Pancreatitis:** This is the most common cause. Inflammation leads to the leakage of amylase from pancreatic acinar cells into the systemic circulation (usually >3x the upper limit of normal). * **Renal Failure:** Amylase is a small molecule filtered by the kidneys. In renal insufficiency, the decreased glomerular filtration rate (GFR) leads to reduced clearance, causing a persistent, moderate rise in serum amylase levels. * **Carcinoma Lung:** Certain tumors, particularly small cell lung cancer and non-small cell lung cancer, can produce amylase ectopically (ectopic hyperamylasemia). **NEET-PG High-Yield Pearls:** 1. **Macroamylasemia:** A condition where amylase binds to Immunoglobulins (IgA/IgG), forming a complex too large to be filtered by the kidney. Result: **High serum amylase but low urinary amylase.** 2. **Lipase vs. Amylase:** Lipase is more specific for acute pancreatitis and remains elevated longer (7–14 days) than amylase (2–5 days). 3. **Other causes of high amylase:** Mumps (parotitis), ectopic pregnancy, and perforated peptic ulcer.

Q: Which enzyme catalyzes the addition of water to a C-C bond?

Hydrolase. ### Explanation The correct answer is **C. Hydrolase**. **1. Why Hydrolase is Correct:** Hydrolases are a major class of enzymes (EC 3) that catalyze the cleavage of various chemical bonds (such as C-O, C-N, or C-C) by the **addition of water**. This process is known as hydrolysis. In the context of a C-C bond, a hydrolase breaks the bond by incorporating a water molecule, typically resulting in two smaller molecules. Common examples include digestive enzymes like peptidases and lipases. **2. Why the Other Options are Incorrect:** * **A. Hydroxylase:** These belong to the Oxidoreductase class. They catalyze the addition of a hydroxyl group (-OH) to a substrate, usually requiring molecular oxygen ($O_2$) rather than water ($H_2O$). * **B. Dehydrogenase:** These are Oxidoreductases that catalyze the removal of hydrogen atoms from a substrate, transferring them to electron carriers like $NAD^+$ or $FAD$. They are involved in redox reactions, not bond cleavage via water. * **D. Hydratase:** While the name sounds similar, hydratases belong to the **Lyase** class. They add water to a double bond (e.g., Fumarase adding water to Fumarate to form Malate) **without** breaking the bond to split the molecule. Hydrolases split the molecule; Hydratases do not. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **EC Classification:** Remember the mnemonic **OTH LIL** (Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase). * **Hydrolase vs. Lyase:** This is a common trap. **Hydrolases** use water to *break* a bond (Hydrolysis). **Lyases** (like Hydratases) add water to a *double bond* or remove groups to form double bonds without hydrolysis. * **Key Example:** Acetylcholinesterase is a clinically significant hydrolase; its inhibition is the basis of Organophosphate poisoning.

Q: Which element is required for the function of the enzyme group kinases?

Mg2+. **Explanation:** **Why Mg²⁺ is the correct answer:** Kinases are a class of enzymes that catalyze the transfer of a phosphate group from a high-energy molecule (usually ATP) to a specific substrate (phosphorylation). The true substrate for these enzymes is not just ATP, but a **Mg²⁺-ATP complex**. Magnesium ions coordinate with the negatively charged oxygen atoms of the phosphate groups on ATP, neutralizing their charge. This stabilization facilitates a nucleophilic attack by the substrate, making the terminal phosphate group more susceptible to transfer. Without Mg²⁺, the electrostatic repulsion would prevent the enzyme from positioning the ATP correctly. **Analysis of Incorrect Options:** * **Mn²⁺ (Manganese):** While Mn²⁺ can sometimes substitute for Mg²⁺ in vitro, its primary physiological role is as a cofactor for enzymes like **Pyruvate Carboxylase** and Arginase. * **Fe²⁺ (Iron):** Iron is essential for oxygen transport (Hemoglobin) and redox reactions. It is a key component of **Cytochromes** in the Electron Transport Chain and enzymes like Catalase. * **Cu²⁺ (Copper):** Copper is a cofactor for "oxidative" enzymes such as **Cytochrome c Oxidase** (Complex IV), Lysyl Oxidase (collagen cross-linking), and Tyrosinase (melanin synthesis). **High-Yield Clinical Pearls for NEET-PG:** * **Hexokinase & Glucokinase:** These are the most discussed kinases in biochemistry; both require Mg²⁺ for the first step of glycolysis. * **Hypomagnesemia:** Low magnesium levels can lead to refractory hypocalcemia and hypokalemia because Mg²⁺ is required for PTH secretion and the function of the Na⁺/K⁺-ATPase pump. * **Enzyme Trick:** If a question mentions **ATP-utilizing enzymes**, the answer is almost always **Magnesium (Mg²⁺)**.

Q: Which of the following enzymes is NOT typically elevated in liver disorders?

Lipase. **Explanation:** The correct answer is **Lipase**. **Why Lipase is the correct answer:** Lipase is a digestive enzyme primarily synthesized and secreted by the **pancreas** into the duodenum to facilitate the hydrolysis of dietary fats. While trace amounts are found in other tissues, its clinical utility is almost exclusively as a highly specific marker for **acute pancreatitis**. It is not produced by hepatocytes or biliary epithelial cells; therefore, its levels remain normal in primary liver disorders like hepatitis or cirrhosis. **Analysis of Incorrect Options:** * **ALT (Alanine Aminotransferase):** This is the most specific marker for hepatocellular injury. It is found primarily in the cytosol of hepatocytes. * **AST (Aspartate Aminotransferase):** While also found in cardiac and skeletal muscle, AST is significantly elevated in liver cell necrosis. A characteristic high-yield fact is the **AST:ALT ratio > 2:1**, which is highly suggestive of alcoholic liver disease. * **ALP (Alkaline Phosphatase):** This enzyme is located on the canalicular membranes of hepatocytes. It is the hallmark marker for **cholestasis** (biliary obstruction) and infiltrative liver diseases. **NEET-PG High-Yield Pearls:** 1. **Specificity:** ALT is more specific for the liver than AST. Lipase is more specific for the pancreas than Amylase. 2. **De Ritis Ratio:** An AST/ALT ratio 2 indicates alcohol-induced damage. 3. **GGT (Gamma-Glutamyl Transferase):** Often tested alongside ALP; if both are elevated, the source of ALP is confirmed to be hepatic rather than bone. 4. **Half-life:** ALT has a longer half-life (approx. 47 hours) compared to AST (approx. 17 hours).

Question 1

Arrange the following enzyme categories as per increasing order of their enzyme commission (EC) numbers?

Accepted Answer

Transferases - Hydrolases - Hydratase - Isomerases

Answer

Isomerases - Hydratase - Transferases - Hydrolases

Answer

Hydratase - Hydrolases - Transferases - Isomerases

Answer

Hydrolases - Transferases - Isomerases - Hydratase

Question 2

All of the following statements about functional enzymes are TRUE, EXCEPT:

Accepted Answer

Substrates for functional enzymes are absent in blood.

Answer

Prothrombin is a functional enzyme.

Answer

Functional enzymes are present in plasma in higher concentration than in tissue.

Answer

Functional enzyme activity is decreased in liver disease.

Question 3

Which enzyme is used in the treatment of acute myocardial infarction?

Accepted Answer

Urokinase

Answer

Papain

Answer

Asparginase

Answer

Serratiopeptidase

Question 4

Which of the following is a peroxisomal free radical scavenger?

Accepted Answer

Catalase

Answer

Superoxide dismutase

Answer

Glutathione peroxidase

Answer

Selenium

Question 5

Which enzyme is responsible for the respiratory burst reaction?

Accepted Answer

NADPH-oxidase

Answer

Dehydrogenase

Answer

Peroxidase

Answer

Hydroxylase

Question 6

Which of the following does not cause an increase in serum amylase?

Accepted Answer

Cardiac failure

Answer

Pancreatitis

Answer

Renal failure

Answer

Carcinoma lung

Question 7

Which enzyme catalyzes the addition of water to a C-C bond?

Accepted Answer

Hydrolase

Answer

Hydroxylase

Answer

Dehydrogenase

Answer

Hydratase

Question 8

Which element is required for the function of the enzyme group kinases?

Accepted Answer

Mg2+

Answer

Mn2+

Answer

Fe2+

Answer

Cu2+

Question 9

Which of the following enzymes is NOT typically elevated in liver disorders?

Accepted Answer

Lipase

Answer

Alkaline phosphatase (ALP)

Answer

Aspartate aminotransferase (AST)

Answer

Alanine aminotransferase (ALT)

Question 10

Competitive inhibition is characterized by all of the following, except?

Accepted Answer

Irreversible

Answer

Inhibitor is a structural analogue of the substrate

Answer

Inhibitor binds to the active site

Answer

Increases Km value

Enzymes — MCQs

Enzymes — MCQs

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