Enzymes Practice Questions

Q: Which enzyme is diagnostic of myocardial infarction in a case of hypothyroidism?

CPK-MB. **Explanation:** The correct answer is **CPK-MB** (Creatine Phosphokinase-MB). **Why CPK-MB is the correct answer:** In patients with **hypothyroidism**, there is a baseline elevation of total CPK (specifically the CPK-MM isoform) due to increased muscle membrane permeability and decreased clearance of the enzyme. Because total CPK is already high in hypothyroid patients, it cannot be used to diagnose a Myocardial Infarction (MI). However, the **CPK-MB isoenzyme** is specific to cardiac muscle. Even in the presence of high total CPK, a rise in the MB fraction (typically >5% of total CPK) remains a highly specific and diagnostic marker for acute myocardial injury in these patients. **Analysis of Incorrect Options:** * **A. SGOT (AST):** While AST rises in MI, it is found in the liver, skeletal muscle, and RBCs. It lacks the cardiac specificity required, especially when skeletal muscle involvement is suspected in hypothyroidism. * **B. LDH:** LDH is a late marker for MI. Like CPK, total LDH can be elevated in various conditions (including hypothyroidism and hemolysis), making it less specific than CPK-MB for acute diagnosis. * **C. Aldolase:** This enzyme is a marker for skeletal muscle diseases (like muscular dystrophy) and has no diagnostic role in myocardial infarction. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While CPK-MB is the answer here, **Cardiac Troponins (I and T)** are the overall most sensitive and specific markers for MI. * **Time Frame:** CPK-MB rises within 4–6 hours, peaks at 24 hours, and returns to normal within 48–72 hours. It is the marker of choice for detecting **re-infarction**. * **Hypothyroidism Fact:** Always remember that **elevated CPK-MM** is a common biochemical finding in hypothyroid myopathy.

Q: In competitive inhibition, what happens to the substrate affinity to the enzyme?

Decreases. In competitive inhibition, the inhibitor structurally resembles the substrate and competes for the same **active site** on the enzyme. ### Why the Correct Answer is Right (Option A) The term **"affinity"** refers to how strongly an enzyme binds to its substrate, which is inversely represented by the **Michaelis constant ($K_m$)**. * In competitive inhibition, the presence of the inhibitor makes it harder for the substrate to bind to the active site. * To achieve the same reaction velocity ($1/2 V_{max}$), a much higher concentration of substrate is required to "outcompete" the inhibitor. * Since $K_m$ increases, the **affinity decreases**. ### Why Other Options are Wrong * **Option B (Increases):** Affinity never increases in the presence of an inhibitor. An increase in affinity would mean a decrease in $K_m$, which occurs in certain types of allosteric activation, not inhibition. * **Option D (Does not change):** This occurs in **Non-competitive inhibition**. In that case, the inhibitor binds to an allosteric site, not the active site; therefore, the binding ability (affinity/$K_m$) of the substrate remains unchanged, but the maximal velocity ($V_{max}$) decreases. ### NEET-PG High-Yield Pearls 1. **The $V_{max}$ Rule:** In competitive inhibition, $V_{max}$ remains **unchanged** because the inhibition can be completely reversed by increasing substrate concentration. 2. **Lineweaver-Burk Plot:** The lines for inhibited and uninhibited reactions intersect on the **Y-axis** ($1/V_{max}$ is constant). 3. **Clinical Example:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA reductase. **Methanol poisoning** is treated with Ethanol because they compete for the active site of Alcohol Dehydrogenase.

Q: Aspartate transaminase, an enzyme, is most abundant in which of the following organs?

Heart. **Explanation:** **Aspartate Transaminase (AST)**, formerly known as Serum Glutamic Oxaloacetic Transaminase (SGOT), is a pyridoxal phosphate (Vitamin B6)-dependent enzyme. It catalyzes the reversible transfer of an amino group between aspartate and alpha-ketoglutarate to form oxaloacetate and glutamate. **Why Heart is Correct:** AST is found in high concentrations in tissues with high metabolic activity. The **heart (myocardium)** contains the highest concentration of AST per unit of tissue, followed closely by the liver and skeletal muscle. Historically, AST was the first cardiac biomarker used to diagnose Myocardial Infarction (MI). Following an MI, AST levels rise within 6–8 hours, peak at 24–48 hours, and return to baseline within 4–6 days. **Why Other Options are Incorrect:** * **Brain:** While AST is present in the brain, its concentration is significantly lower than in cardiac or hepatic tissues. * **Spleen:** The spleen contains minimal amounts of AST; it is not a primary source for this enzyme. * **Retina:** The retina relies on specific metabolic enzymes, but AST is not found there in clinically significant or "most abundant" quantities. **High-Yield Clinical Pearls for NEET-PG:** 1. **Tissue Distribution:** Heart > Liver > Skeletal Muscle > Kidney > Pancreas > RBCs. 2. **De Ritis Ratio:** The AST/ALT ratio. A ratio **>2:1** is highly suggestive of **Alcoholic Liver Disease**, as alcohol is a mitochondrial toxin and AST has both mitochondrial and cytosolic isoenzymes. 3. **Hemolysis:** Since AST is present in RBCs, a hemolyzed blood sample will show falsely elevated AST levels. 4. **Current Status:** In modern practice, AST has been replaced by **Troponins (I and T)** for MI diagnosis due to their superior cardiac specificity.

Q: Which of the following enzymes does NOT possess antioxidant property?

Phosphorylase. **Explanation:** The correct answer is **Phosphorylase**. This question tests the distinction between enzymes involved in metabolic pathways and those involved in the cellular defense against oxidative stress. **1. Why Phosphorylase is the correct answer:** Phosphorylase (specifically Glycogen Phosphorylase) is a key enzyme in **glycogenolysis**. It catalyzes the rate-limiting step of breaking down glycogen into glucose-1-phosphate by adding an inorganic phosphate. It has no role in neutralizing Reactive Oxygen Species (ROS) or protecting the cell from oxidative damage. **2. Why the other options are incorrect:** The other three enzymes constitute the primary **enzymatic antioxidant defense system** of the body: * **Superoxide Dismutase (SOD):** This is the "first line" of defense. It converts the highly reactive superoxide radical ($O_2^{\bullet-}$) into hydrogen peroxide ($H_2O_2$) and oxygen. * **Catalase:** Located primarily in peroxisomes, it catalyzes the decomposition of $H_2O_2$ into water and oxygen, preventing the formation of the toxic hydroxyl radical. * **Glutathione Peroxidase (GPx):** This selenium-dependent enzyme reduces $H_2O_2$ and lipid hydroperoxides to water and alcohols, respectively, using reduced glutathione (GSH) as a donor. **High-Yield Clinical Pearls for NEET-PG:** * **Selenium Connection:** Glutathione peroxidase requires **Selenium** as a cofactor (in the form of selenocysteine). Deficiency can lead to Keshan disease. * **SOD Isoforms:** Cytosolic SOD requires **Copper and Zinc**, while mitochondrial SOD requires **Manganese**. * **G6PD Deficiency:** This is clinically relevant because G6PD is essential for generating **NADPH**, which is required by Glutathione Reductase to regenerate GSH, maintaining the antioxidant capacity of RBCs.

Q: Peroxidases belong to which enzyme group?

Oxidase-Reductase. **Explanation:** **1. Why Oxidase-Reductase is Correct:** Peroxidases belong to the **Oxidoreductase (EC 1)** class of enzymes. These enzymes catalyze oxidation-reduction reactions, where electrons are transferred from a donor (reductant) to an acceptor (oxidant). Specifically, peroxidases catalyze the breakdown of hydrogen peroxide ($H_2O_2$) or organic peroxides, using them as electron acceptors to oxidize various substrates. A classic example is **Glutathione Peroxidase**, which reduces $H_2O_2$ to water while oxidizing glutathione (GSH to GSSG), protecting cells from oxidative damage. **2. Why the Other Options are Incorrect:** * **Lipases (EC 3):** These are a sub-class of hydrolases that specifically catalyze the hydrolysis of ester bonds in lipid substrates (e.g., Triacylglycerol lipase). * **Hydrolases (EC 3):** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. Examples include digestive enzymes like pepsin and urease. * **Transferases (EC 2):** These enzymes transfer a functional group (e.g., methyl, phosphate, or amino groups) from one molecule to another. Examples include Kinases and Transaminases (ALT/AST). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Heme Protein:** Most peroxidases (like Myeloperoxidase) contain **heme** as a prosthetic group. * **Myeloperoxidase (MPO):** Found in neutrophil granules; it produces hypochlorous acid (HOCl) to kill bacteria. Its deficiency leads to impaired microbial killing. * **Glutathione Peroxidase:** This is a **Selenium-dependent** enzyme. Selenium deficiency can lead to Keshan disease (cardiomyopathy). * **Catalase:** A specific type of peroxidase that converts $H_2O_2$ into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$), protecting the cell from reactive oxygen species (ROS).

Q: Which of the following enzymes acts by a 'Ping-Pong' mechanism?

Alanine amino transferase. ### Explanation **1. Why Alanine Aminotransferase (ALT) is Correct:** Alanine Aminotransferase (ALT) follows the **Ping-Pong (Double-Displacement) mechanism**. In this mechanism, the first substrate (Alanine) binds to the enzyme and transfers its amino group to the prosthetic group, **Pyridoxal Phosphate (PLP)**, forming Pyridoxamine Phosphate (PMP). The first product (Pyruvate) is released *before* the second substrate (α-Ketoglutarate) binds. The second substrate then binds, picks up the amino group from PMP, and is released as the second product (Glutamate). The key feature is that the enzyme exists in a modified intermediate state (PMP-enzyme) between the release of the first product and the binding of the second substrate. **2. Why the Other Options are Incorrect:** * **Hexokinase:** Follows a **Random Bi-Bi mechanism**. Substrates (Glucose and ATP) can bind in any order, and both must be present on the enzyme before any product is released. * **Lactate Dehydrogenase (LDH):** Follows an **Ordered Bi-Bi mechanism**. There is a mandatory sequence: NADH must bind first before Pyruvate can bind, and Lactate must be released before NAD+ can leave. * **Pyruvate Dehydrogenase (PDH):** This is a multi-enzyme complex involving five cofactors. While it involves a series of transfers, it is classified as a complex oxidative decarboxylation rather than a classic Ping-Pong bisubstrate kinetic model. **3. High-Yield Clinical Pearls for NEET-PG:** * **PLP Dependency:** All aminotransferases (ALT, AST) require **Vitamin B6 (Pyridoxine)** as a cofactor. * **Ping-Pong Rule:** If a question mentions "transamination" or "covalent enzyme intermediate," think Ping-Pong mechanism. * **Diagnostic Marker:** ALT is more specific for liver cell damage than AST, as AST is also found in cardiac and skeletal muscle.

Q: Xanthine oxidase is associated with which co-factor?

Molybdenum. ### Explanation **Correct Answer: D. Molybdenum** **Concept:** Xanthine oxidase (XO) is a complex metalloenzyme that plays a critical role in the catabolism of purines. It catalyzes the oxidation of hypoxanthine to xanthine and xanthine to **uric acid**. The enzyme requires **Molybdenum** (in the form of a molybdopterin cofactor), **Iron** (Fe-S clusters), and **FAD** to function. Molybdenum is essential for the oxygen atom transfer during the hydroxylation of the substrate. **Why other options are incorrect:** * **Zinc:** Associated with enzymes like Carbonic anhydrase, Alcohol dehydrogenase, Carboxypeptidase, and DNA polymerase. * **Copper:** A cofactor for Cytochrome c oxidase, Tyrosinase, Lysyl oxidase, and Superoxide dismutase (cytosolic). * **Selenium:** Essential for **Glutathione peroxidase** (antioxidant defense) and Deiodinase (thyroid hormone metabolism). **High-Yield Clinical Pearls for NEET-PG:** * **Gout Connection:** **Allopurinol**, the drug of choice for chronic gout, is a suicide inhibitor of Xanthine oxidase. It reduces the production of uric acid. * **Molybdenum-dependent enzymes:** Apart from Xanthine oxidase, Molybdenum is also a cofactor for **Sulfite oxidase** and **Aldehyde oxidase**. * **Genetic Deficiency:** Rare hereditary deficiency of Xanthine oxidase leads to **Xanthinuria**, characterized by low serum uric acid levels and potential xanthine stones in the urinary tract. * **Free Radical Production:** Xanthine oxidase is a significant source of Superoxide ($O_2^-$) radicals, especially during reperfusion injury following ischemia.

Q: Which of the following is a functional enzyme?

Prothrombin. ### Explanation In biochemistry, plasma enzymes are categorized into two groups: **Functional** and **Non-functional** plasma enzymes. **1. Why Prothrombin is the Correct Answer:** **Functional plasma enzymes** are those that are actively present in the blood at all times and perform their primary physiological function within the circulation. **Prothrombin** (Factor II) is synthesized in the liver and secreted into the plasma, where it plays a critical role in the blood coagulation cascade. Other examples include pseudocholinesterase, lipoprotein lipase, and various clotting factors. These enzymes are present in higher concentrations in plasma than in tissues. **2. Why the Other Options are Incorrect:** Options A, B, and D are **Non-functional plasma enzymes**. These enzymes have no known physiological function in the blood. They are normally present in very low concentrations in the plasma because they perform their work intracellularly or within specific tracts (like the GI tract). * **LDH (Lactate Dehydrogenase):** An intracellular enzyme; elevated levels indicate tissue damage (e.g., myocardial infarction or hemolysis). * **Amylase:** Secreted by the pancreas and salivary glands into the digestive tract. Its presence in high amounts in the blood indicates pathology like acute pancreatitis. * **Acid Phosphatase:** Primarily found in the prostate; elevated levels serve as a marker for prostatic carcinoma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Substrates are always present in the blood. Their deficiency usually indicates liver dysfunction (decreased synthesis). * **Non-functional Enzymes:** Their elevation in plasma is a diagnostic marker for organ-specific damage (e.g., ALT/AST for liver, CK-MB for heart). * **Key Distinction:** Functional enzymes are synthesized in the liver and act *in* the blood; Non-functional enzymes act *inside* cells and leak into the blood during injury.

Q: Transaldolase is a type of enzyme that belongs to which class?

Transferase. **Explanation:** **1. Why Transferase is Correct:** Transaldolase is a key enzyme in the **non-oxidative phase of the Pentose Phosphate Pathway (PPP)**. Its primary function is to catalyze the transfer of a three-carbon dihydroxyacetone unit from a ketose (Sedoheptulose-7-phosphate) to an aldose (Glyceraldehyde-3-phosphate). According to the IUBMB enzyme classification, enzymes that move a functional group from one substrate to another belong to **Class 2: Transferases**. Specifically, transaldolase facilitates the interconversion of sugars to generate ribose-5-phosphate and NADPH. **2. Why Other Options are Incorrect:** * **Hydrolases (Class 3):** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. Transaldolase does not involve water in its mechanism. * **Lyases (Class 4):** These enzymes catalyze the cleavage of bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds (e.g., Aldolase in glycolysis). While the names are similar, Transaldolase transfers a group rather than simply splitting a molecule. * **Ligases (Class 6):** These enzymes catalyze the joining of two large molecules, usually coupled with the **hydrolysis of ATP** (e.g., Pyruvate carboxylase). Transaldolase does not require ATP. **3. High-Yield Clinical Pearls for NEET-PG:** * **PPP Link:** Transaldolase and Transketolase (also a transferase) link the PPP to Glycolysis. * **Cofactor Distinction:** Unlike Transketolase, which requires **Thiamine Pyrophosphate (TPP)** as a cofactor, Transaldolase does **not** require any cofactor. * **Clinical Significance:** Transaldolase deficiency is a rare metabolic disorder presenting with liver dysfunction, hepatosplenomegaly, and hemolytic anemia. * **Mnemonic:** Remember the "Three T's" of the PPP: **T**ransketolase and **T**ransaldolase are **T**ransferases.

Question 1

Which enzyme is diagnostic of myocardial infarction in a case of hypothyroidism?

Accepted Answer

CPK-MB

Answer

SGOT

Answer

LDH

Answer

Aldolase

Question 2

In competitive inhibition, what happens to the substrate affinity to the enzyme?

Accepted Answer

Decreases

Answer

Increases

Answer

Any of the above

Answer

Does not change

Question 3

Aspartate transaminase, an enzyme, is most abundant in which of the following organs?

Accepted Answer

Heart

Answer

Brain

Answer

Spleen

Answer

Retina

Question 4

Which of the following enzymes does NOT possess antioxidant property?

Accepted Answer

Phosphorylase

Answer

Catalase

Answer

Glutathione peroxidase

Answer

Superoxide dismutase

Question 5

Peroxidases belong to which enzyme group?

Accepted Answer

Oxidase-Reductase

Answer

Lipase

Answer

Hydrolase

Answer

Transferase

Question 6

Which of the following enzymes acts by a 'Ping-Pong' mechanism?

Accepted Answer

Alanine amino transferase

Answer

Hexokinase

Answer

Lactate Dehydrogenase

Answer

Pyruvate Dehydrogenase

Question 7

Which of the following conditions can cause a raised serum amylase level?

Accepted Answer

Appendicitis

Answer

Blocked salivary duct

Answer

Ruptured ectopic pregnancy

Answer

Pancreatitis

Question 8

Xanthine oxidase is associated with which co-factor?

Accepted Answer

Molybdenum

Answer

Zinc

Answer

Copper

Answer

Selenium

Question 9

Which of the following is a functional enzyme?

Accepted Answer

Prothrombin

Answer

LDH

Answer

Amylase

Answer

Acid phosphatase

Question 10

Transaldolase is a type of enzyme that belongs to which class?

Accepted Answer

Transferase

Answer

Hydrolase

Answer

Lyase

Answer

Ligase

Enzymes — MCQs

Enzymes — MCQs

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