Enzymes Practice Questions

Q: What is the cofactor for glutathione peroxidase?

Selenium. ***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.

Q: 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?

Trypsin. ***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.

Q: Which enzyme joins two substrates?

Ligase. ***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.

Q: Inactive precursors of enzymes are known as:

Proenzymes. ***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.

Q: Chymotrypsinogen is converted to chymotrypsin by:

Trypsin. ***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.

Q: Curdling of milk is caused by which factor?

Rennin. ***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**.

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

Requires biotin as a cofactor. ***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.

Q: Caspases are involved in:

Programmed cell death (apoptosis). ***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.

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

Presence of Ser-His-Asp catalytic triad at the active site.. ***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.

Question 1

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

Accepted Answer

Chymotrypsin retains binding ability but loses catalytic activity

Answer

Chymotrypsin retains catalytic activity but loses binding ability

Answer

Chymotrypsin retains both binding and catalytic activity

Answer

Chymotrypsin loses both binding and catalytic activity

Question 2

What is the cofactor for glutathione peroxidase?

Accepted Answer

Selenium

Answer

Mg2+

Answer

Mn2+

Answer

Ca2+

Question 3

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?

Accepted Answer

Trypsin

Answer

Amylase

Answer

Pepsin

Answer

Lactase

Question 4

Which enzyme joins two substrates?

Accepted Answer

Ligase

Answer

Synthase

Answer

Lyase

Answer

Isomerase

Question 5

Inactive precursors of enzymes are known as:

Accepted Answer

Proenzymes

Answer

Apoenzymes

Answer

Coenzymes

Answer

Holoenzymes

Question 6

Chymotrypsinogen is converted to chymotrypsin by:

Accepted Answer

Trypsin

Answer

Pepsin

Answer

Alkaline pH

Answer

Elastase

Question 7

Curdling of milk is caused by which factor?

Accepted Answer

Rennin

Answer

Lipase

Answer

Elastase

Answer

Amylase

Question 8

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

Accepted Answer

Requires biotin as a cofactor

Answer

Enzyme found in the cytosol

Answer

Enzyme found in mitochondria

Answer

Catalyzes a condensation reaction

Question 9

Caspases are involved in:

Accepted Answer

Programmed cell death (apoptosis)

Answer

Processes leading to cellular damage

Answer

Fluid absorption

Answer

Cell signaling

Question 10

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

Accepted Answer

Presence of Ser-His-Asp catalytic triad at the active site.

Answer

Tight binding of pancreatic trypsin inhibitor is characteristic of all serine proteases.

Answer

Cleavage of peptide bonds adjacent to serine residues.

Answer

Autocatalytic activation of zymogen precursors.

Enzymes — MCQs

Enzymes — MCQs

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