Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 541: The catalytic serine (Ser195) of chymotrypsin is replaced with proline. Which of the following will occur?

A. Chymotrypsin retains catalytic activity but loses binding ability
B. Chymotrypsin retains binding ability but loses catalytic activity (Correct Answer)
C. Chymotrypsin retains both binding and catalytic activity
D. Chymotrypsin loses both binding and catalytic activity

Explanation: ***Chymotrypsin retains binding ability but loses catalytic activity*** - The **serine-195 residue** at the active site of chymotrypsin is crucial for its **catalytic mechanism, specifically for nucleophilic attack** on the peptide bond. - The **substrate binding pocket**, formed by other amino acid residues, would likely remain intact, allowing the enzyme to still recognize and bind its substrate even though it cannot catalyze the reaction. *Chymotrypsin retains catalytic activity but loses binding ability* - This is incorrect because **serine-195 at the active site is vital for catalysis**, not primarily for substrate binding. - Replacing this **catalytic serine** with proline eliminates the **hydroxyl group** essential for the nucleophilic attack mechanism. *Chymotrypsin retains both binding and catalytic activity* - This is incorrect because **proline substitution at serine-195** completely abolishes catalytic activity due to loss of the nucleophilic hydroxyl group. - The **catalytic triad** (Ser195, His57, Asp102) would be disrupted, making the **covalent intermediate formation** impossible. *Chymotrypsin loses both binding and catalytic activity* - While **catalytic activity is completely lost**, substrate binding sites are typically **separate from the catalytic site** and would remain functional. - The **overall protein structure** and **substrate recognition domains** may remain largely intact despite the single amino acid substitution.

Question 542: What is the cofactor for glutathione peroxidase?

A. Mg2+
B. Mn2+
C. Ca2+
D. Selenium (Correct Answer)

Explanation: ***Selenium*** - **Glutathione peroxidase** is a family of enzymes that protect the body from oxidative damage by reducing hydrogen peroxide and organic hydroperoxides. - **Selenium** is an essential trace element that is incorporated into these enzymes as **selenocysteine**, making it a crucial cofactor for their catalytic activity. *Mg2+* - **Magnesium** is a cofactor for many enzymes, particularly those involved in **ATP hydrolysis and synthesis**, DNA, and RNA metabolism. - It is not directly involved in the catalytic mechanism of glutathione peroxidase. *Mn2+* - **Manganese** is a cofactor for several enzymes, including **superoxide dismutase (SOD2)**, which is involved in antioxidant defense. - However, manganese does not serve as a cofactor for glutathione peroxidase. *Ca2+* - **Calcium** acts as a cofactor for many enzymes, often regulating their activity as a **second messenger** or through direct binding. - It is essential for processes like muscle contraction and neurotransmitter release but is not a cofactor for glutathione peroxidase.

Question 543: A young boy presents with failure to thrive. Biochemical analysis of a duodenal aspirate after a meal reveals a deficiency of enteropeptidase (enterokinase). Which one of the following digestive enzymes would be affected by this deficiency?

A. Amylase
B. Pepsin
C. Trypsin (Correct Answer)
D. Lactase

Explanation: ***Trypsin*** - Enteropeptidase (enterokinase) is crucial for activating **trypsinogen** into its active form, **trypsin**. Without active trypsin, the entire cascade of pancreatic protease activation is disrupted. - Trypsin then activates other pancreatic proteases like chymotrypsin, elastase, and carboxypeptidases, all of which are essential for **protein digestion** in the small intestine. *Amylase* - **Amylase** is a carbohydrate-digesting enzyme, primarily involved in breaking down starch. Its activity is independent of enteropeptidase. - **Pancreatic amylase** is secreted in its active form and does not require proteolytic cleavage by trypsin for activation. *Pepsin* - **Pepsin** is an enzyme found in the stomach that initiates protein digestion. It is activated by **hydrochloric acid** from its inactive precursor, pepsinogen. - Its activity is entirely independent of enteropeptidase, which functions in the duodenum. *Lactase* - **Lactase** is a brush border enzyme located in the small intestine that digests the disaccharide **lactose** into glucose and galactose. - Its production and activity are genetically regulated and not dependent on the protein-digesting enzymes or enteropeptidase.

Question 544: Which enzyme joins two substrates?

A. Synthase
B. Lyase
C. Ligase (Correct Answer)
D. Isomerase

Explanation: ***Ligase*** - **Ligases** are a class of enzymes that **catalyze the joining of two large molecules** by forming a new chemical bond, typically with the concomitant hydrolysis of a small pendant chemical group on one of the larger molecules or the coupling of a reaction to the cleavage of pyrophosphate on ATP or similar. - This process often involves the use of **ATP or other energy sources** to form a covalent bond. *Lyase* - **Lyases** are enzymes that **catalyze the breaking of chemical bonds** by means other than hydrolysis (e.g., elimination reactions). - They typically form a new double bond or a ring structure during the bond cleavage. *Synthase* - **Synthases** are a type of **lyase enzyme** that **catalyzes synthesis reactions** without the direct involvement of ATP or other nucleoside triphosphates for energy. - While they synthesize molecules, they don't necessarily "join two substrates" in the same way a ligase does, especially without consuming a high-energy phosphate. *Isomerase* - **Isomerases** catalyze the **rearrangement of atoms within a molecule**, converting a compound into one of its isomers. - They do not join two separate substrates; rather, they alter the structure of a single substrate.

Question 545: Inactive precursors of enzymes are known as:

A. Apoenzymes
B. Coenzymes
C. Proenzymes (Correct Answer)
D. Holoenzymes

Explanation: ***Proenzymes*** - **Proenzymes**, also known as **zymogens**, are inactive precursor forms of enzymes that require a biochemical change (e.g., proteolytic cleavage) to become active. - This mechanism allows for the **controlled activation** of enzymes, preventing premature or inappropriate enzymatic activity. *Apoenzymes* - An **apoenzyme** is the protein component of an enzyme that requires a **non-protein cofactor** (like a metal ion or coenzyme) to become active. - It describes the enzyme without its essential cofactor, making it inactive until the cofactor binds. *Coenzymes* - **Coenzymes** are small, non-protein organic molecules that bind to apoenzymes to assist in catalysis. - They often function as **carriers of electrons, atoms, or functional groups** during enzymatic reactions. *Holoenzymes* - A **holoenzyme** is the catalytically active form of an enzyme, consisting of an **apoenzyme** (protein part) combined with its essential **cofactor** (e.g., coenzyme or metal ion). - It represents the complete and functional enzyme complex.

Question 546: Chymotrypsinogen is converted to chymotrypsin by:

A. Pepsin
B. Alkaline pH
C. Trypsin (Correct Answer)
D. Elastase

Explanation: ***Trypsin*** - **Trypsin** cleaves the **inactive zymogen chymotrypsinogen** at specific peptide bonds, leading to a conformational change. - This cleavage activates chymotrypsinogen into its active form, **chymotrypsin**, an enzyme critical for **protein digestion** in the small intestine. *Pepsin* - **Pepsin** is an enzyme found in the stomach and is responsible for initiating protein digestion in an **acidic environment**. - It primarily cleaves proteins, but it does **not** activate other zymogens like chymotrypsinogen. *Alkaline pH* - While chymotrypsin functions optimally in an **alkaline environment** in the small intestine, mild alkaline pH alone does **not** directly convert chymotrypsinogen to chymotrypsin. - Activation requires specific **proteolytic cleavage** by another enzyme. *Elastase* - **Elastase** is another **pancreatic protease** that cleaves proteins, particularly those containing **elastin**. - However, it does not play a role in the **initial activation** of chymotrypsinogen into chymotrypsin; that role is specifically held by trypsin.

Question 547: Curdling of milk is caused by which factor?

A. Lipase
B. Elastase
C. Amylase
D. Rennin (Correct Answer)

Explanation: ***Rennin*** - **Rennin** (also known as chymosin) is an enzyme primarily found in the stomachs of mammalian infants, including humans, that specifically **coagulates milk protein** (casein). - It cleaves **kappa-casein**, destabilizing casein micelles and causing them to precipitate, forming a curd, which slows milk passage through the gut for better digestion. *Lipase* - **Lipase** is an enzyme responsible for breaking down **lipids** (fats) into fatty acids and glycerol. - While it plays a role in fat digestion, it does not directly cause the **curdling of milk proteins**. *Amylase* - **Amylase** is an enzyme that catalyzes the hydrolysis of **starch** into sugars, primarily in the mouth and pancreas. - Its function is in carbohydrate digestion and it has no role in **milk curdling**. *Elastase* - **Elastase** is a protease enzyme that breaks down **elastin**, a fibrous protein found in connective tissues. - It is involved in protein digestion but does not specifically target **casein for curdling**.

Question 548: Which of the following statements about carbamoyl phosphate synthase is incorrect?

A. Requires biotin as a cofactor (Correct Answer)
B. Enzyme found in the cytosol
C. Enzyme found in mitochondria
D. Catalyzes a condensation reaction

Explanation: ***Requires biotin as a cofactor*** - This is the **incorrect** statement and therefore the correct answer to this question. - Carbamoyl phosphate synthase (both CPS I and CPS II) does **NOT require biotin** as a cofactor. - Biotin is a cofactor for **carboxylase enzymes** such as pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA carboxylase. - Carbamoyl phosphate synthase requires **ATP** and **Mg²⁺** but not biotin. *Enzyme found in mitochondria* - This statement is **correct**. - **Carbamoyl phosphate synthase I (CPS I)** is located in the **mitochondrial matrix** and catalyzes the first step of the urea cycle. - CPS I uses free ammonia (NH₃) as the nitrogen source and is activated by N-acetylglutamate. *Enzyme found in the cytosol* - This statement is **correct**. - **Carbamoyl phosphate synthase II (CPS II)** is located in the **cytosol** and is involved in de novo pyrimidine biosynthesis. - CPS II uses the amide nitrogen of glutamine (not free ammonia) as the nitrogen source. *Catalyzes a condensation reaction* - This statement is **correct**. - Both CPS I and CPS II catalyze the condensation of CO₂ (as bicarbonate), ammonia/glutamine, and two molecules of ATP to form carbamoyl phosphate, 2 ADP, and inorganic phosphate. - This is a complex reaction involving phosphorylation and condensation steps.

Question 549: Caspases are involved in:

A. Programmed cell death (apoptosis) (Correct Answer)
B. Processes leading to cellular damage
C. Fluid absorption
D. Cell signaling

Explanation: ***Apoptosis*** - Caspases are **proteolytic enzymes** that play a crucial role in executing apoptosis, which is a programmed cell death process [1]. - They are activated in a cascade leading to the **cleavage of various cellular substrates**, facilitating the orderly dismantling of the cell [1]. *Cell signaling* - While caspases can influence **cell signaling pathways**, their primary function is not in this signaling but rather in executing cell death. - Cell signaling involves various other molecules like **kinases and phosphatases** that mediate communication in cells. *Pinocytosis* - Pinocytosis is a **form of endocytosis**, primarily involving the ingestion of liquids and small molecules by cells, which does not involve caspases. - This process is more aligned with membrane dynamics and is distinct from the programmed cell death features of caspases. *Cell injury* - Although cell injury can trigger apoptotic pathways [2], caspases are specifically involved in the **execution of apoptosis**, not in the injury itself. - Cell injury encompasses a broader range of processes including necrosis, which is very different from apoptosis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 63-67. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 80-81.

Question 550: Which of the following is a defining characteristic of all serine proteases?

A. Tight binding of pancreatic trypsin inhibitor is characteristic of all serine proteases.
B. Cleavage of peptide bonds adjacent to serine residues.
C. Presence of Ser-His-Asp catalytic triad at the active site. (Correct Answer)
D. Autocatalytic activation of zymogen precursors.

Explanation: ***Presence of Ser-His-Asp catalytic triad at the active site.*** - All **serine proteases** utilize a unique **catalytic triad** composed of **serine**, **histidine**, and **aspartate** residues in their active site to perform catalysis. - This specific arrangement of amino acids enables the serine residue to act as a **nucleophile**, facilitating the cleavage of peptide bonds. *Tight binding of pancreatic trypsin inhibitor is characteristic of all serine proteases.* - The **pancreatic trypsin inhibitor (BPTI)** specifically inhibits **trypsin**, a particular serine protease, but not all members of the serine protease family. - While it's an important regulatory mechanism for trypsin, it is not a universal characteristic that defines *all* serine proteases. *Cleavage of peptide bonds adjacent to serine residues.* - Serine proteases cleave peptide bonds, but the cleavage occurs adjacent to specific amino acid residues determined by the **specificity pocket** of each individual protease, not necessarily adjacent to serine residues. - For example, trypsin cleaves after **lysine** or **arginine**, while chymotrypsin cleaves after **aromatic residues**. *Autocatalytic activation of zymogen precursors is a common feature.* - Many serine proteases are synthesized as inactive **zymogens** and activated by **proteolytic cleavage**, which can sometimes be autocatalytic. - However, not all serine proteases are produced as zymogens, and their activation mechanisms can vary, making this a common feature but not a defining characteristic of *all* of them.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

Practice Questions

Enzyme Inhibition: Competitive and Non-competitive

Practice Questions

Allosteric Regulation

Practice Questions

Coenzymes and Cofactors

Practice Questions

Isoenzymes and Clinical Significance

Practice Questions

Enzyme Regulation: Covalent Modification

Practice Questions

Enzyme Regulation: Zymogen Activation

Practice Questions

Enzyme Induction and Repression

Practice Questions

Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

Practice Questions

Enzyme Therapy and Inhibitors as Drugs

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?