Enzymes Practice Questions

Q: In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:

Coenzyme-cosubstrate. **Explanation:** The correct answer is **B. Coenzyme-cosubstrate.** **1. Why it is correct:** Enzymes often require non-protein components called **cofactors** for activity. Cofactors are divided into inorganic ions and organic molecules called **coenzymes**. Coenzymes are further classified based on their binding affinity: * **Cosubstrates:** These are loosely and transiently bound to the enzyme. They bind, undergo a chemical change (like NAD+ being reduced to NADH), and then **dissociate** from the enzyme to be regenerated by a different reaction. * In this case, NAD+ acts as a cosubstrate because it shuttles electrons from the alcohol and leaves the alcohol dehydrogenase (ADH) complex as NADH. **2. Why other options are incorrect:** * **A. Apoenzyme:** This refers to the protein portion of the enzyme that is catalytically inactive without its cofactor. * **C. Coenzyme-prosthetic group:** A prosthetic group is a coenzyme that is **tightly or covalently bound** to the enzyme and does not dissociate during the reaction (e.g., FAD in Succinate Dehydrogenase or Heme in Cytochrome c). * **D. Cofactor:** While NAD+ is technically a cofactor, "Coenzyme-cosubstrate" is the more specific and accurate description required for NEET-PG level biochemistry. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alcohol Metabolism:** ADH is the rate-limiting enzyme in alcohol metabolism (Zero-order kinetics). It is inhibited by **Fomepizole** (used in methanol/ethylene glycol poisoning). * **Niacin (Vitamin B3):** NAD+ and NADP+ are derived from Niacin. Deficiency leads to **Pellagra** (4 Ds: Dermatitis, Diarrhea, Dementia, Death). * **Key Cosubstrates:** NAD+, NADP+, and Coenzyme A. * **Key Prosthetic Groups:** FAD, FMN, Biotin, and Pyridoxal Phosphate (PLP).

Q: Which of the following enzymes is activated in its dephosphorylated state?

HMG Co A reductase. ### Explanation In metabolic regulation, many key enzymes are controlled via **reversible covalent modification**, specifically phosphorylation and dephosphorylation. **1. Why HMG-CoA Reductase is Correct:** HMG-CoA reductase is the rate-limiting enzyme of cholesterol synthesis. It follows the general rule for **anabolic (synthetic) pathways**: they are typically **active in the dephosphorylated state**. * **Mechanism:** Insulin promotes the dephosphorylation of HMG-CoA reductase via protein phosphatase, thereby activating it. Conversely, Glucagon and AMPK trigger phosphorylation, which inactivates the enzyme to conserve energy. **2. Analysis of Incorrect Options:** * **B & C (Glycogen phosphorylase and Phosphorylase kinase):** These are key enzymes in **glycogenolysis** (a catabolic pathway). Catabolic enzymes are generally **active in their phosphorylated state**. This ensures that during fasting (high glucagon) or stress (high epinephrine), glycogen is broken down to release glucose. * **D (Citrate lyase):** ATP-citrate lyase, which provides acetyl-CoA for fatty acid synthesis, is actually **activated by phosphorylation** (specifically by Akt/PKB in response to insulin), making it an exception to the general rule that anabolic enzymes are dephosphorylated. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Thumb":** Most rate-limiting enzymes of **Anabolic** pathways (e.g., Glycogen synthase, HMG-CoA reductase, Acetyl-CoA carboxylase) are **Active when Dephosphorylated**. * Most **Catabolic** enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase) are **Active when Phosphorylated**. * **Statins:** These drugs are competitive inhibitors of HMG-CoA reductase, mimicking the structure of the HMG-CoA substrate. * **AMPK:** This "energy sensor" inhibits HMG-CoA reductase by phosphorylating it when cellular ATP levels are low.

Q: All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?

Coenzyme A. The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that converts pyruvate to Acetyl-CoA. It requires five distinct cofactors. The distinction between "tightly bound" (prosthetic groups) and "mobile" (transient) cofactors is a high-yield NEET-PG concept. ### 1. Why Coenzyme A is the Correct Answer **Coenzyme A (CoA)** and **NAD+** are considered **mobile carriers** or "dissociable" cofactors. They are not permanently attached to the enzyme complex. Instead, they enter the reaction, pick up the products (the Acetyl group and electrons, respectively), and then diffuse away to participate in other metabolic pathways (like the TCA cycle or Electron Transport Chain). ### 2. Why the Other Options are Incorrect The remaining three cofactors are **prosthetic groups**, meaning they are covalently or very tightly bound to their respective sub-enzymes: * **Thiamine Pyrophosphate (TPP):** Tightly bound to **E1** (Pyruvate decarboxylase). It is essential for the decarboxylation step. * **Lipoic Acid (Lipoamide):** Tightly bound to **E2** (Dihydrolipoyl transacetylase) via a lysine residue. It swings between active sites to transfer the acetyl group. * **Flavin Adenine Dinucleotide (FAD):** Tightly bound to **E3** (Dihydrolipoyl dehydrogenase). It accepts electrons from lipoamide to become FADH₂. ### 3. Clinical Pearls for NEET-PG * **The "Tender Loving Care For No One" Mnemonic:** TPP, Lipoic Acid, CoA, FAD, NAD. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the **SH groups of Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * **Location:** The PDH complex is located in the **mitochondrial matrix**.

Q: What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?

Reduces NADP+. **Explanation:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the Pentose Phosphate Pathway (Hexose Monophosphate Shunt). Its primary function is to catalyze the oxidation of Glucose-6-phosphate to 6-phosphogluconolactone. During this reaction, electrons are transferred to the coenzyme **NADP+**, thereby **reducing it to NADPH**. **Why Option B is correct:** The reaction catalyzed by G6PD is: *Glucose-6-P + NADP+ → 6-phosphogluconolactone + NADPH + H+* By reducing NADP+, G6PD ensures a steady supply of NADPH, which is essential for reductive biosynthesis (e.g., fatty acids) and for maintaining the pool of **reduced glutathione** to protect cells against oxidative stress. **Why other options are incorrect:** * **Option A:** G6PD does not oxidize NADPH; it produces it. NADPH is oxidized back to NADP+ by enzymes like Glutathione Reductase or in biosynthetic pathways. * **Options C & D:** G6PD is highly specific for the NADP+/NADPH cofactor system. It does not utilize the NAD+/NADH system, which is primarily involved in ATP-generating catabolic pathways (like Glycolysis or the TCA cycle). **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **Non-immune Hemolytic Anemia** because RBCs lack mitochondria and rely solely on G6PD for NADPH to neutralize reactive oxygen species (ROS). * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency. * **Bite Cells:** Result from splenic macrophages removing Heinz bodies. * **Triggers:** Hemolysis is typically triggered by infections, Fava beans, or drugs (e.g., Primaquine, Sulphonamides, Dapsone).

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: Peptidyl transferase is a/an

Ribozyme. **Explanation:** **1. Why Ribozyme is the correct answer:** Peptidyl transferase is the primary enzyme responsible for peptide bond formation during protein synthesis (translation). Unlike most enzymes which are proteins, peptidyl transferase is a **Ribozyme**—an RNA molecule with catalytic activity. Specifically, in the large ribosomal subunit (60S in eukaryotes, 50S in prokaryotes), it is the **23S rRNA** (prokaryotes) or **28S rRNA** (eukaryotes) that catalyzes the transfer of the amino acid from the tRNA in the P-site to the aminoacyl-tRNA in the A-site. **2. Why other options are incorrect:** * **Enzyme:** While peptidyl transferase functions as an enzyme, "Ribozyme" is the more specific and accurate biochemical classification required for NEET-PG. Most enzymes are proteins; ribozymes are the notable exception. * **Catalyst:** This is a broad term. While all ribozymes are biological catalysts, the question tests the specific structural nature of this molecule. * **Elongation factor:** Elongation factors (like EF-Tu or EF-G) are proteins that facilitate the translation process (e.g., bringing tRNA to the ribosome or translocation), but they do not possess the catalytic activity to form peptide bonds. **3. Clinical Pearls & High-Yield Facts:** * **Mechanism of Action:** Peptidyl transferase catalyzes the nucleophilic attack of the A-site amino group on the P-site ester linkage. * **Antibiotic Link:** **Chloramphenicol** is a high-yield antibiotic that acts by inhibiting the peptidyl transferase activity of the bacterial 50S ribosomal subunit. * **Other Ribozymes:** Apart from the ribosome, other examples include **SnRNAs** (involved in splicing) and **Ribonuclease P** (involved in tRNA processing).

Question 1

In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:

Accepted Answer

Coenzyme-cosubstrate

Answer

Apoenzyme

Answer

Coenzyme-prosthetic group

Answer

Cofactor

Question 2

Which of the following enzymes is activated in its dephosphorylated state?

Accepted Answer

HMG Co A reductase

Answer

Glycogen phosphorylase

Answer

Glycogen phosphorylase kinase

Answer

Citrate lyase

Question 3

All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?

Accepted Answer

Coenzyme A

Answer

Thiamine pyrophosphate

Answer

Lipoic acid

Answer

Flavin adenine dinucleotide

Question 4

What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?

Accepted Answer

Reduces NADP+

Answer

Oxidizes NADPH

Answer

Oxidizes NAD

Answer

Reduces NAD

Question 5

Which class of enzymes catalyzes the transfer of electrons?

Accepted Answer

Oxidoreductases

Answer

Transferases

Answer

Lyases

Answer

Ligases

Question 6

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 7

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 8

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 9

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 10

Peptidyl transferase is a/an

Accepted Answer

Ribozyme

Answer

Enzyme

Answer

Catalyst

Answer

Elongation factor

Enzymes — MCQs

Enzymes — MCQs

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