Enzymes Practice Questions

Q: This is an example of:

Competitive inhibition. ***Competitive inhibition*** - In the **Lineweaver-Burk plot**, competitive inhibition shows the same **Vmax** (same Y-intercept) but increased apparent **Km** (lines intersect on the Y-axis). - The **competitive inhibitor** competes with substrate for the **active site**, and this effect can be overcome by increasing **substrate concentration**. *Non-competitive inhibition* - Shows the same **Km** but decreased **Vmax** in the Lineweaver-Burk plot, with lines intersecting on the **X-axis**. - The inhibitor binds to an **allosteric site** rather than competing directly with the substrate for the active site. *Both* - The plot characteristics clearly distinguish between competitive and non-competitive inhibition based on **Vmax** and **Km** changes. - The pattern shown is specific to **competitive inhibition** only, not a combination of both mechanisms. *None* - The Lineweaver-Burk plot clearly demonstrates **enzyme inhibition** with characteristic changes in kinetic parameters. - The intersecting lines on the **Y-axis** with unchanged **Vmax** definitively indicates competitive inhibition is occurring.

Q: Which of the following enzymes does not belong to the isomerase class?

Transaminase. **Explanation:** Enzymes are classified into six major functional classes based on the International Union of Biochemistry (IUB) system. **Isomerases (Class 5)** catalyze structural or geometric changes within a single molecule, converting one isomer into another (e.g., D-form to L-form or aldose to ketose). **Why Transaminase is the correct answer:** **Transaminases** (also known as Aminotransferases) belong to the **Transferase (Class 2)** category. They catalyze the transfer of an amino group (–NH₂) from an amino acid to a keto acid (typically α-ketoglutarate). This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory co-factor. Since it involves the transfer of a functional group between two different molecules rather than an internal rearrangement, it is not an isomerase. **Analysis of Incorrect Options:** * **Mutase:** These are isomerases that catalyze the internal transfer of a functional group (like a phosphate) from one position to another within the same molecule (e.g., Phosphoglycerate mutase). * **Racemase:** These catalyze the interconversion of stereoisomers, specifically between D and L enantiomers (e.g., Alanine racemase). * **Phosphohexoisomerase:** This enzyme catalyzes the conversion of Glucose-6-phosphate (an aldose) to Fructose-6-phosphate (a ketose) in glycolysis, a classic example of an isomerase reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase). * **Transaminase Markers:** ALT (SGPT) and AST (SGOT) are critical clinical markers for liver injury. * **Isomerase in Glycolysis:** Triose phosphate isomerase is considered a "catalytically perfect" enzyme.

Q: Which of the following is NOT a marker of mitochondria?

5-nucleotidase. **Explanation:** The correct answer is **B. 5-nucleotidase**. In cell biology, "marker enzymes" are specific enzymes localized exclusively or predominantly within a particular organelle. They are used to identify and assess the purity of organelles during cell fractionation. 1. **Why 5-nucleotidase is the correct answer:** **5-nucleotidase** is a classic marker enzyme for the **plasma membrane**. It is also found in the lysosomes but is never associated with the mitochondria. Its primary role is the conversion of nucleoside 5′-monophosphates to nucleosides. 2. **Analysis of Incorrect Options (Mitochondrial Markers):** * **ATP synthase (Complex V):** Located on the inner mitochondrial membrane; it is the functional marker for oxidative phosphorylation. * **Glutamate dehydrogenase:** A key enzyme for amino acid metabolism located specifically in the **mitochondrial matrix**. * **Creatine kinase (CK):** Specifically, the **mitochondrial isoenzyme of CK (m-CK)** is found in the intermembrane space. It plays a vital role in the "creatine phosphate shuttle" for energy transport. **High-Yield Markers for NEET-PG:** To excel in General Physiology and Biochemistry, remember these specific organelle markers: * **Mitochondria:** Succinate dehydrogenase (Inner membrane—most specific), Cytochrome oxidase, Malate dehydrogenase (Matrix). * **Plasma Membrane:** Na⁺-K⁺ ATPase, Adenylate cyclase, 5-nucleotidase. * **Lysosomes:** Acid phosphatase. * **Golgi Apparatus:** Galactosyltransferase. * **Endoplasmic Reticulum:** Glucose-6-phosphatase (Smooth ER), Cytochrome P450. * **Peroxisomes:** Catalase, Urate oxidase. * **Cytosol:** Lactate dehydrogenase (LDH).

Q: Dehydrogenases use all of the following as coenzymes, except?

Ferriprotoporphyrin. ### Explanation **Core Concept:** Dehydrogenases are a subclass of **oxidoreductases** that catalyze the removal of hydrogen atoms from a substrate, transferring them to a specific electron carrier (coenzyme). These enzymes are highly dependent on nicotinamide and flavin nucleotides to function as hydrogen acceptors. **Why Ferriprotoporphyrin is the Correct Answer:** Ferriprotoporphyrin (also known as **Heme**) is a prosthetic group characterized by a porphyrin ring complexed with iron ($Fe^{3+}$). While it is essential for oxygen transport (hemoglobin) and electron transfer in the respiratory chain (**Cytochromes**), it does **not** act as a coenzyme for dehydrogenases. Dehydrogenases specifically transfer protons ($H^+$) and electrons, whereas heme-containing proteins like Cytochrome C oxidase or Catalase are involved in direct electron transfer or peroxide breakdown. **Analysis of Incorrect Options:** * **NAD+ / NADP+:** These are nicotinamide derivatives (Vitamin B3). NAD+ is a universal coenzyme for dehydrogenases in glycolysis and the TCA cycle (e.g., Lactate dehydrogenase). NADP+ is typically used in reductive biosynthesis (e.g., G6PD). * **FMN / FAD:** These are riboflavin derivatives (Vitamin B2). They act as prosthetic groups for flavoproteins. FAD is the coenzyme for Succinate dehydrogenase, while FMN is used by NADH dehydrogenase (Complex I). **NEET-PG Clinical Pearls:** * **Succinate Dehydrogenase:** A high-yield fact is that it is the only enzyme of the TCA cycle located in the **inner mitochondrial membrane** (also part of Complex II). * **Niacin Deficiency (Pellagra):** Leads to a deficiency in NAD+/NADP+, impairing dehydrogenase activity across multiple metabolic pathways. * **Riboflavin Deficiency:** Affects FMN/FAD-dependent enzymes, often clinically presenting as cheilosis and glossitis.

Q: Zinc is a component of which of the following enzymes, except?

Hexokinase. **Explanation:** The correct answer is **Hexokinase**. In biochemistry, enzymes often require metal ions for catalytic activity. These are categorized as metalloenzymes (tightly bound) or metal-activated enzymes (loosely bound). **1. Why Hexokinase is the correct answer:** Hexokinase, the first enzyme of glycolysis, requires **Magnesium ($Mg^{2+}$)** as a cofactor, not Zinc. Magnesium binds to ATP to form a Mg-ATP complex, which is the actual substrate for the reaction. This is a high-yield distinction: most kinases involved in phosphate transfer require $Mg^{2+}$ or $Mn^{2+}$. **2. Analysis of other options:** * **Carbonic Anhydrase:** This is the classic example of a Zinc-containing metalloenzyme. Zinc is essential for the hydration of $CO_2$ to bicarbonate. * **Insulin:** While not an enzyme (it is a hormone), Zinc is crucial for its structural integrity. In the pancreas, insulin is stored as a **zinc-insulin hexamer**. * **Carboxypeptidase:** This proteolytic digestive enzyme requires Zinc for its catalytic mechanism to cleave peptide bonds at the carboxyl terminal. **Clinical Pearls for NEET-PG:** * **Zinc-containing enzymes (Mnemonic: "C-ALCO"):** **C**arbonic anhydrase, **A**lcohol dehydrogenase, **L**actate dehydrogenase, **C**arboxypeptidase, and Superoxide dismutase (cytosolic). * **DNA/RNA Polymerases:** These are also Zinc-dependent. * **Clinical Correlation:** Zinc deficiency leads to **Acrodermatitis enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Key Distinction:** If a question asks for the most common metal cofactor in the body, it is often $Mg^{2+}$ (for kinases) or $Zn^{2+}$ (for a wide variety of enzyme classes).

Q: All of the following enzymes are involved in oxidation-reduction reactions, except?

Hydrolases. **Explanation:** The classification of enzymes is a high-yield topic for NEET-PG. Enzymes are categorized into six major classes based on the type of reaction they catalyze (EC classification). **1. Why Hydrolases is the Correct Answer:** **Hydrolases (Class 3)** catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They do not involve the transfer of electrons or changes in oxidation states. Common examples include digestive enzymes like pepsin, trypsin, and alkaline phosphatase. Since they do not facilitate redox reactions, they are the exception in this list. **2. Analysis of Incorrect Options (Oxidoreductases):** The other three options belong to **Class 1: Oxidoreductases**, which catalyze the transfer of electrons ($H^+$ or $e^-$) from a reductant to an oxidant. * **Dehydrogenases:** These transfer hydrogen from a substrate to an electron acceptor like $NAD^+$ or $FAD$ (e.g., Lactate Dehydrogenase). * **Oxygenases:** These catalyze the direct incorporation of oxygen into a substrate (e.g., Cytochrome P450). * **Peroxidases:** These use hydrogen peroxide ($H_2O_2$) as an electron acceptor to oxidize substrates (e.g., Glutathione peroxidase). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Lyases vs. Hydrolases:** Lyases (Class 4) also break bonds but do so without water or oxidation, often forming double bonds (e.g., Carbonic anhydrase). * **Ligases (Class 6):** These join two molecules together and **require ATP** (e.g., Pyruvate carboxylase).

Q: Which of the following enzymes does not utilize copper?

Prolyl hydroxylase. **Explanation:** The correct answer is **Prolyl hydroxylase** because it is a **Vitamin C and Iron (Fe²⁺)** dependent enzyme, not a copper-dependent one. **1. Why Prolyl hydroxylase is correct:** Prolyl hydroxylase is essential for the post-translational modification of collagen. It hydroxylates proline residues to hydroxyproline, which stabilizes the collagen triple helix. This reaction requires **Ferrous iron (Fe²⁺)**, molecular oxygen, alpha-ketoglutarate, and **Ascorbic acid (Vitamin C)**. Vitamin C maintains iron in its reduced ferrous state; its deficiency leads to Scurvy. **2. Why the other options are incorrect:** * **Tyrosinase:** This is a copper-containing enzyme responsible for converting Tyrosine to DOPA and then to Dopachrome in the synthesis of **melanin**. Deficiency leads to Albinism. * **Ceruloplasmin:** This is the primary copper-carrying protein in the blood. It also functions as a **ferroxidase** (converting Fe²⁺ to Fe³⁺ for binding to transferrin), utilizing its internal copper atoms for the redox reaction. * **Superoxide dismutase (SOD):** The cytosolic form of SOD (SOD1) requires both **Copper and Zinc** (Cu-Zn SOD) to scavenge free radicals. (Note: The mitochondrial form, SOD2, requires Manganese). **High-Yield Clinical Pearls for NEET-PG:** * **Copper-containing enzymes (Mnemonic: "C-Cyto-T-S-L"):** **C**eruloplasmin, **Cyto**chrome c oxidase (Complex IV), **T**yrosinase, **S**uperoxide dismutase, **L**ysyl oxidase (essential for collagen cross-linking). * **Menkes Disease:** ATP7A mutation leading to copper deficiency; symptoms include "kinky" hair due to defective lysyl oxidase. * **Wilson Disease:** ATP7B mutation leading to copper toxicity and low serum ceruloplasmin.

Q: An increase in LDH-5 enzyme is seen in the following conditions, except:

Pulmonary embolism. **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme with five isoenzymes (LDH 1-5). **LDH-5 ($M_4$)** is primarily found in the liver and skeletal muscle. **Why Pulmonary Embolism is the Correct Answer:** In **Pulmonary Embolism (PE)**, the characteristic finding is an increase in **LDH-3**. LDH-3 is the predominant isoenzyme in lung tissue. While a "triad" of increased LDH, increased bilirubin, and normal AST is classically associated with PE, the specific isoenzyme elevation involves LDH-3, not LDH-5. **Analysis of Other Options:** * **Malignancies of the CNS:** While LDH-1 and 2 are usually associated with the brain, certain CNS tumors and metastatic deposits can cause a shift toward anaerobic metabolism, leading to a significant rise in **LDH-5**. * **Muscular Dystrophies:** Since LDH-5 is the primary isoenzyme in skeletal muscle, any condition causing muscle necrosis or dystrophy (like Duchenne Muscular Dystrophy) will result in a marked elevation of LDH-5. * **Breast Carcinoma:** Many solid tumors, including breast and hepatic carcinomas, show an increase in LDH-5. This is often used as a marker for tumor burden and metabolic shift (Warburg effect) in malignant tissues. **High-Yield NEET-PG Pearls:** * **LDH-1 ($H_4$):** Heart and RBCs. Elevated in Myocardial Infarction (shows "flipped pattern" where LDH-1 > LDH-2). * **LDH-2 ($H_3M_1$):** Reticuloendothelial system. Normally the most abundant fraction in serum. * **LDH-3 ($H_2M_2$):** Lungs, Spleen, and Pancreas. * **LDH-4 ($H_1M_3$):** Kidney and Placenta. * **LDH-5 ($M_4$):** Liver and Skeletal Muscle. * **LDH-X (LDH-6):** Found in post-pubertal testes.

Q: Malonate is a competitive inhibitor of which of the following?

Complex II. **Explanation:** **1. Why Complex II is the Correct Answer:** Malonate is a classic example of a **competitive inhibitor**. It is a structural analog of **succinate**, the natural substrate for the enzyme **Succinate Dehydrogenase (SDH)**. In the Citric Acid Cycle (TCA) and the Electron Transport Chain (ETC), SDH is known as **Complex II**. Because malonate mimics the structure of succinate, it competes for the same active site on the enzyme. By binding to Complex II, malonate prevents the oxidation of succinate to fumarate, thereby halting the flow of electrons from FADH₂ into the ETC. **2. Why Other Options are Incorrect:** * **Complex I (NADH Dehydrogenase):** Inhibited by substances like **Rotenone**, Amobarbital (Amytal), and Piericidin A. It transfers electrons from NADH to Coenzyme Q. * **Complex III (Cytochrome bc1 complex):** Inhibited by **Antimycin A** and British Anti-Lewisite (BAL). It transfers electrons from Coenzyme Q to Cytochrome c. * **Complex IV (Cytochrome c Oxidase):** Inhibited by **Cyanide (CN⁻)**, **Carbon Monoxide (CO)**, Hydrogen Sulfide (H₂S), and Azide (N₃⁻). These bind to the iron in heme, preventing the final transfer of electrons to oxygen. **3. High-Yield Facts for NEET-PG:** * **Complex II Unique Feature:** It is the only enzyme that participates in both the TCA cycle and the ETC. It is also the only complex that does **not** pump protons across the inner mitochondrial membrane. * **Competitive Inhibition Rule:** The inhibitory effect of malonate can be overcome by increasing the concentration of the substrate (succinate). * **Kinetics:** In competitive inhibition, the **Km increases** (affinity decreases) while the **Vmax remains unchanged**.

Question 1

This is an example of:

Accepted Answer

Competitive inhibition

Answer

Non-competitive inhibition

Answer

Both

Answer

None

Question 2

Which of the following enzymes does not belong to the isomerase class?

Accepted Answer

Transaminase

Answer

Mutase

Answer

Racemase

Answer

Phosphohexoisomerase

Question 3

Which of the following is NOT a marker of mitochondria?

Accepted Answer

5-nucleotidase

Answer

ATP synthase

Answer

Glutamate dehydrogenase

Answer

Creatine kinase

Question 4

Dehydrogenases use all of the following as coenzymes, except?

Accepted Answer

Ferriprotoporphyrin

Answer

FMN

Answer

FAD

Answer

NADOP+

Question 5

Zinc is a component of which of the following enzymes, except?

Accepted Answer

Hexokinase

Answer

Carbonic anhydrase

Answer

Insulin

Answer

Carboxypeptidase

Question 6

All of the following enzymes are involved in oxidation-reduction reactions, except?

Accepted Answer

Hydrolases

Answer

Dehydrogenases

Answer

Oxygenases

Answer

Peroxidases

Question 7

If Km remains the same and Vmax is reduced, what type of enzyme inhibition is this?

Accepted Answer

Noncompetitive Inhibition

Answer

Competitive Inhibition

Answer

Uncompetitive inhibition

Answer

Suicide Inhibition

Question 8

Which of the following enzymes does not utilize copper?

Accepted Answer

Prolyl hydroxylase

Answer

Tyrosinase

Answer

Ceruloplasmin

Answer

Superoxide dismutase

Question 9

An increase in LDH-5 enzyme is seen in the following conditions, except:

Accepted Answer

Pulmonary embolism

Answer

Malignancies of the central nervous system

Answer

Muscular dystrophies

Answer

Breast carcinoma

Question 10

Malonate is a competitive inhibitor of which of the following?

Accepted Answer

Complex II

Answer

Complex I

Answer

Complex III

Answer

Complex IV

Enzymes — MCQs

Enzymes — MCQs

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