Enzymes Practice Questions

Q: True about isoenzymes is/are?

Different Km value. **Explanation:** **Isoenzymes (or Isozymes)** are physical variants of the same enzyme. They catalyze the **same chemical reaction** but differ in their primary structure (amino acid sequence) because they are encoded by different genes or gene loci. 1. **Why Option A is Correct:** Since isoenzymes have different amino acid sequences, their active sites have varying affinities for the substrate. This results in **different $K_m$ (Michaelis constant)** and $V_{max}$ values. For example, **Glucokinase** (Liver) has a high $K_m$ for glucose, while **Hexokinase** (Muscle) has a low $K_m$, allowing them to function differently based on tissue-specific metabolic needs. 2. **Why Other Options are Incorrect:** * **Option B:** Isoenzymes, by definition, act on the **same substrate** to produce the same product. If they acted on different substrates, they would be classified as different enzymes entirely. * **Option C:** Due to differences in their amino acid composition, isoenzymes possess different net charges. This causes them to exhibit **different electrophoretic mobilities**, which is the primary laboratory method used to separate and identify them (e.g., LDH and CK patterns). **High-Yield Clinical Pearls for NEET-PG:** * **LDH (Lactate Dehydrogenase):** Has 5 isoenzymes. **LDH-1** (Heart) vs. **LDH-5** (Liver/Muscle). A "flipped pattern" (LDH1 > LDH2) is a classic marker for Myocardial Infarction. * **CK (Creatine Kinase):** Has 3 isoenzymes: **CK-BB** (Brain), **CK-MB** (Heart), and **CK-MM** (Skeletal Muscle). * **Alkaline Phosphatase (ALP):** Isoenzymes help differentiate the source of pathology (e.g., **Regan isoenzyme** is a heat-stable ALP found in certain cancers).

Q: Which component transfers four protons?

NADH-Q oxidoreductase. **Explanation:** The question refers to the **Electron Transport Chain (ETC)** located in the inner mitochondrial membrane, where the energy from redox reactions is used to pump protons ($H^+$) from the matrix into the intermembrane space. **Why NADH-Q oxidoreductase is correct:** **NADH-Q oxidoreductase (Complex I)** is the largest complex in the ETC. It accepts electrons from NADH and transfers them to Coenzyme Q (Ubiquinone). This exergonic transfer provides sufficient energy to pump exactly **four protons** across the membrane for every pair of electrons transferred. This establishes the electrochemical gradient necessary for ATP synthesis. **Analysis of Incorrect Options:** * **Cytochrome-Q c oxidoreductase (Complex III):** This complex also pumps **four protons** per pair of electrons. However, in standard medical examinations like NEET-PG, if both Complex I and III are listed, Complex I is often the primary focus for this specific metric, though technically both share this property. * **Cytochrome oxidase (Complex IV):** This complex transfers electrons from Cytochrome c to Oxygen. It pumps only **two protons** into the intermembrane space per pair of electrons (or 4 protons per $O_2$ molecule reduced). * **Isocitrate dehydrogenase:** This is an enzyme of the TCA cycle. While it generates NADH, it is not a transmembrane proton pump and does not directly participate in the ETC proton gradient. **High-Yield Clinical Pearls for NEET-PG:** * **P:O Ratio:** NADH (Complex I) yields ~2.5 ATP, while $FADH_2$ (Complex II) yields ~1.5 ATP because Complex II does **not** pump any protons. * **Inhibitors:** Rotenone, Amobarbital (Amytal), and Piericidin A inhibit **Complex I**. * **Complex IV Inhibitors:** Cyanide, Carbon Monoxide (CO), and Sodium Azide (high-yield for forensic/toxicology integration). * **Leber’s Hereditary Optic Neuropathy (LHON):** Often caused by mutations in Complex I subunits.

Q: Alcohol dehydrogenase belongs to which class of enzymes?

Oxidoreductase. **Explanation:** The International Union of Biochemistry (IUB) classifies enzymes into six major classes (EC 1 to EC 6). **Alcohol Dehydrogenase (ADH)** belongs to **Class 1: Oxidoreductases**. **1. Why Oxidoreductase is correct:** Oxidoreductases catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen atoms. Alcohol dehydrogenase facilitates the conversion of primary or secondary alcohols to aldehydes or ketones. In this reaction, the alcohol is oxidized while the coenzyme **NAD+** is reduced to **NADH**. Since it involves a redox reaction, it is fundamentally an oxidoreductase. **2. Analysis of Incorrect Options:** * **B. Dehydrogenase:** While ADH is indeed a dehydrogenase, this is a *sub-class*, not a primary IUB class. In NEET-PG, when asked for the "class," you must select from the six primary categories (Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase). * **C. Hydrolase:** These enzymes (Class 3) catalyze the cleavage of bonds (C-O, C-N, C-C) by the addition of water (e.g., Pepsin, Urease). ADH does not use water to break bonds. * **D. Oxidase:** This is a sub-class of oxidoreductases where oxygen serves as the electron acceptor, often producing water or hydrogen peroxide (e.g., Cytochrome oxidase). ADH uses NAD+, not oxygen, as the primary electron acceptor. **Clinical Pearls for NEET-PG:** * **Metabolism:** ADH is the rate-limiting enzyme in ethanol metabolism, primarily located in the cytosol of hepatocytes. * **Inhibitor:** **Fomepizole** inhibits ADH and is used as an antidote in methanol or ethylene glycol poisoning to prevent the formation of toxic metabolites (formaldehyde/glycolic acid). * **Kinetics:** Alcohol metabolism follows **zero-order kinetics** because ADH becomes saturated at low ethanol concentrations.

Q: All of the following enzymes are active within a cell except:

Trypsin. **Explanation:** The core concept tested here is the distinction between **intracellular enzymes** and **extracellular (secretory) enzymes**. **Why Trypsin is the correct answer:** Trypsin is a digestive protease synthesized in the pancreas as an inactive precursor called **trypsinogen**. It is secreted into the duodenum, where it is activated by enteropeptidase. Because active trypsin is highly proteolytic and would cause **autodigestion** (pancreatitis) if active within the pancreatic acinar cells, it remains inactive (as a zymogen) while inside the cell. Therefore, it is not "active" within the cell. **Why the other options are incorrect:** * **Fumarase:** An essential enzyme of the **TCA cycle** located in the mitochondrial matrix. It must be active within the cell to facilitate cellular respiration. * **Hexokinase:** The first enzyme of **glycolysis**, active in the cytosol of almost all cells. It phosphorylates glucose to glucose-6-phosphate, trapping it inside the cell. * **Alcohol Dehydrogenase:** Primarily located in the cytosol of **hepatocytes**. It is active within the cell to metabolize ethanol into acetaldehyde. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Enzymes secreted in inactive forms (e.g., pepsinogen, chymotrypsinogen) to protect the site of synthesis. * **Pancreatitis:** Occurs when trypsin is prematurely activated within the pancreatic cells, often due to ductal obstruction or alcohol-induced injury. * **Alpha-1 Antitrypsin:** A critical serum protein that inhibits proteases like trypsin and elastase, preventing tissue damage. * **Marker Enzymes:** Remember specific localizations: **ALT/AST** (Cytosol/Mitochondria), **Acid Phosphatase** (Lysosomes), and **Catalase** (Peroxisomes).

Q: Which enzyme is a marker for mitochondria?

Glutamate dehydrogenase. **Explanation:** In cell biology and biochemistry, specific enzymes are localized within particular organelles, serving as "biochemical markers" to identify the purity of subcellular fractions during centrifugation. **1. Why Glutamate Dehydrogenase (GDH) is correct:** Glutamate dehydrogenase is a key enzyme involved in nitrogen metabolism (oxidative deamination). It is located exclusively within the **mitochondrial matrix**. Since it is not found in the cytosol or other organelles, its presence in a cellular fraction confirms the presence of mitochondria. Other common mitochondrial markers include **Succinate Dehydrogenase (SDH)** (inner membrane) and **Cytochrome Oxidase**. **2. Analysis of Incorrect Options:** * **Acid Phosphatase:** This is the classic marker for **Lysosomes**. It is used clinically to detect lysosomal storage diseases and was historically used as a marker for prostatic carcinoma. * **Alkaline Phosphatase:** This is a marker for the **Plasma Membrane** (and also found in the endoplasmic reticulum). Clinically, it is elevated in obstructive jaundice and bone diseases. * **Hexokinase:** This is a key glycolytic enzyme located in the **Cytosol**. Note: While some isoforms can bind to the outer mitochondrial membrane, it is primarily considered a cytosolic marker. **3. High-Yield NEET-PG Clinical Pearls:** * **Marker for Peroxisomes:** Catalase. * **Marker for Golgi Apparatus:** Galactosyl transferase. * **Marker for Nucleus:** DNA Polymerase / RNA Polymerase. * **Marker for Microsomes (ER):** Glucose-6-phosphatase. * **Mitochondrial DNA:** It is circular, double-stranded, and maternally inherited (Mitochondrial Eve concept).

Q: Glutathione reductase contains which of the following prosthetic groups?

FAD. **Explanation:** **Glutathione Reductase** is a critical enzyme in the antioxidant defense system, responsible for maintaining the pool of reduced glutathione (GSH) in the cell. **1. Why FAD is Correct:** Glutathione reductase is a flavoprotein. It utilizes **Flavin Adenine Dinucleotide (FAD)** as a tightly bound prosthetic group. The enzyme catalyzes the reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH) using **NADPH** as a reducing equivalent. During this catalytic cycle, electrons are transferred from NADPH to the FAD prosthetic group, and then to the disulfide bond of GSSG. **2. Why Other Options are Incorrect:** * **NAD (Option B):** While NAD/NADH are common electron carriers, Glutathione Reductase specifically requires **NADPH** (derived from the HMP Shunt) as a co-substrate, not NAD. Furthermore, NADPH acts as a co-enzyme (dissociable), whereas FAD is the permanent prosthetic group. * **ATP (Option C):** ATP is the energy currency of the cell but does not participate in the redox reactions of the glutathione cycle. It is required for the *synthesis* of glutathione (via Glutamate-cysteine ligase), but not for its *regeneration* by the reductase. **3. Clinical Pearls & High-Yield Facts:** * **The HMP Shunt Connection:** The NADPH required for this reaction is primarily supplied by **Glucose-6-Phosphate Dehydrogenase (G6PD)**. * **G6PD Deficiency:** In G6PD deficiency, a lack of NADPH prevents Glutathione Reductase from functioning, leading to oxidative stress, Heinz body formation, and hemolytic anemia. * **Riboflavin Status:** Since FAD is derived from **Vitamin B2 (Riboflavin)**, the activity of erythrocyte glutathione reductase is used as a functional diagnostic marker to assess riboflavin deficiency. * **Active Site:** Besides FAD, the enzyme also contains a **selenocysteine** residue (in the case of glutathione peroxidase) or essential **thiol groups** (cysteine) at its active site.

Q: What is the relative Km of Hexokinase and Glucokinase?

Glucokinase has a higher Km than hexokinase.. **Explanation:** The correct answer is **B: Glucokinase has a higher Km than hexokinase.** **1. Underlying Concept:** The Michaelis constant (**Km**) represents the substrate concentration at which an enzyme works at half its maximum velocity ($V_{max}$). Km is **inversely proportional** to the affinity of the enzyme for its substrate. * **Hexokinase** has a **low Km** (high affinity) for glucose. This allows it to function at maximum capacity even during fasting states, ensuring that tissues like the brain and muscles can utilize glucose even when blood levels are low. * **Glucokinase (Hexokinase IV)** has a **high Km** (low affinity). It only becomes significantly active when blood glucose levels are high (e.g., after a meal). This allows the liver to "buffer" blood glucose by converting it to glycogen only when there is an excess. **2. Analysis of Incorrect Options:** * **Option A:** Incorrect. If hexokinase had a higher Km, it would be unable to trap glucose in tissues during fasting, leading to cellular energy failure. * **Option C:** Incorrect. These are distinct isoenzymes with different kinetic properties ($V_{max}$ and $Km$) suited to their specific physiological roles. * **Option D:** Incorrect. Km is an intrinsic property of the enzyme itself and does not change based on the amount of glucose ingested (though the *rate* of the reaction will change). **3. NEET-PG High-Yield Pearls:** * **Location:** Hexokinase is ubiquitous (all tissues); Glucokinase is primarily in the **Liver** and **Pancreatic Beta-cells**. * **Vmax:** Glucokinase has a **high Vmax**, allowing it to process large amounts of glucose rapidly post-prandially. * **Inhibition:** Hexokinase is inhibited by its product (**Glucose-6-Phosphate**); Glucokinase is **not**. * **Clinical Correlation:** Mutations in the Glucokinase gene are associated with **MODY type 2** (Maturity-Onset Diabetes of the Young).

Question 1

True about isoenzymes is/are?

Accepted Answer

Different Km value

Answer

Act on different substrates

Answer

Same electrophoretic mobility

Answer

All of the above

Question 2

Which of the following is a characteristic of serine proteases?

Accepted Answer

Are characterized by having several active sites per molecule, each containing a serine residue.

Answer

Hydrolyze peptide bonds involving carboxyl groups of serine residues.

Answer

Are inactivated by reacting with one molecule of di-isopropyl-fluorophosphate per molecule of protein.

Answer

Are exopeptidases.

Question 3

Which component transfers four protons?

Accepted Answer

NADH-Q oxidoreductase

Answer

Cytochrome oxidase

Answer

Cytochrome-Q c oxidoreductase

Answer

Isocitrate dehydrogenase

Question 4

Which of the following is NOT a mechanism for regulating enzyme activity?

Accepted Answer

Competitive inhibition

Answer

Covalent modification

Answer

Allosteric activation

Answer

Induction of genes for enzyme synthesis

Question 5

Alcohol dehydrogenase belongs to which class of enzymes?

Accepted Answer

Oxidoreductase

Answer

Dehydrogenase

Answer

Hydrolase

Answer

Oxidase

Question 6

All of the following enzymes are active within a cell except:

Accepted Answer

Trypsin

Answer

Fumarase

Answer

Hexokinase

Answer

Alcohol dehydrogenase

Question 7

Which enzyme is a marker for mitochondria?

Accepted Answer

Glutamate dehydrogenase

Answer

Acid phosphatase

Answer

Alkaline phosphatase

Answer

Hexokinase

Question 8

Glutathione reductase contains which of the following prosthetic groups?

Accepted Answer

FAD

Answer

NAD

Answer

ATP

Answer

None of the above

Question 9

What is the relative Km of Hexokinase and Glucokinase?

Accepted Answer

Glucokinase has a higher Km than hexokinase.

Answer

Hexokinase has a higher Km than glucokinase.

Answer

The Km is the same for both enzymes.

Answer

The Km depends on the glucose ingested.

Question 10

Inhibition of placental alkaline phosphatase by phenylalanine is an example of which type of enzyme inhibition?

Accepted Answer

Uncompetitive inhibition

Answer

Competitive inhibition

Answer

Noncompetitive inhibition

Answer

Allosteric inhibition

Enzymes — MCQs

Enzymes — MCQs

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