Enzymes Practice Questions

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: Following enzymes are found in lysosomes, except

Pepsin. ***Pepsin*** - **Pepsin** is a **proteolytic enzyme** that functions in the stomach's highly acidic environment to digest proteins. - It is secreted by the chief cells of the stomach as **pepsinogen** and activated by hydrochloric acid. *Arylsulfatases* - **Arylsulfatases** are a class of enzymes found in lysosomes that catalyze the hydrolysis of **sulfate esters** from arylsulfates. - Their deficiency can lead to lysosomal storage disorders like **metachromatic leukodystrophy**. *Ribonucleases* - **Ribonucleases** are enzymes present in lysosomes responsible for the breakdown of **RNA** into smaller oligonucleotides and nucleosides. - This process is crucial for the recycling of cellular components. *Glycosidases* - **Glycosidases** are a diverse group of enzymes found in lysosomes that hydrolyze **glycosidic bonds** in carbohydrates. - They are essential for breaking down complex carbohydrates and glycoconjugates.

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.

Q: Which of the following does NOT directly influence the activity of existing enzyme molecules?

Induction. ***Induction does NOT directly influence existing enzyme activity.*** - **Enzyme induction** refers to the process where the **synthesis rate** of an enzyme is increased, typically in response to specific substrates or substances. - This leads to a **higher concentration** of the enzyme, rather than directly modifying the catalytic activity of existing enzyme molecules. - Induction increases **enzyme quantity**, not the activity of pre-existing enzymes. *Incorrect: Acetylation directly influences enzyme activity.* - **Acetylation** is a reversible post-translational modification that involves the addition of an **acetyl group** (CH3CO) to existing enzyme molecules, typically at lysine residues. - This modification can alter the enzyme's **conformation**, substrate binding, and catalytic efficiency, thereby directly influencing its activity. *Incorrect: Phosphorylation directly influences enzyme activity.* - **Phosphorylation** is one of the most important regulatory mechanisms where a **phosphate group** is added to existing enzyme molecules, often by kinases. - This modification can **activate or inactivate** enzymes by changing their shape or charge, thus directly altering their catalytic activity. - Classic examples: glycogen phosphorylase, hormone-sensitive lipase. *Incorrect: Methylation directly influences enzyme activity.* - **Methylation** involves the addition of a **methyl group** to existing enzyme molecules, commonly at lysine or arginine residues. - This post-translational modification can directly impact enzyme function by altering conformation and substrate binding.

Q: In the context of clinical biochemistry, which isoenzyme of lactate dehydrogenase (LDH) has the maximum electrophoretic mobility?

LDH-1. ***LDH-1*** - **LDH-1** is composed of four **H subunits** (HHHH) and is the most **anionic** form of lactate dehydrogenase. - Due to its high negative charge, it migrates fastest towards the **anode** during electrophoresis. *LDH-5* - **LDH-5** is composed of four **M subunits** (MMMM) and is the least **anionic** form. - It has the **lowest electrophoretic mobility**, migrating slowest towards the anode or even towards the cathode. *LDH-2* - **LDH-2** consists of three **H subunits** and one **M subunit** (HHHM). - It has the second-highest electrophoretic mobility, positioned after LDH-1. *LDH-3* - **LDH-3** is composed of two **H subunits** and two **M subunits** (HHMM). - Its electrophoretic mobility is intermediate, falling between LDH-2 and LDH-4.

Q: Which of the following is the metal cofactor of the enzyme ALA dehydratase?

Zinc. ***Zinc*** - **Zinc** acts as a crucial metal ion cofactor for **ALA dehydratase**, also known as **porphobilinogen synthase**. - It plays a vital role in the enzyme's catalytic activity, facilitating the **condensation of two molecules of aminolevulinate (ALA)** to form porphobilinogen. *Copper* - **Copper** is a cofactor for several enzymes, including **cytochrome c oxidase** and **superoxide dismutase**, but it is not the prosthetic group for ALA dehydratase. - While essential for various biological processes, its role does not extend to the direct catalysis of **heme synthesis** at the ALA dehydratase step. *Lead* - **Lead** is a well-known inhibitor of **ALA dehydratase**, not a prosthetic group. - The binding of lead to the enzyme's active site displaces essential cofactors like zinc, leading to the accumulation of **ALA** and causing **lead poisoning**. *Magnesium* - **Magnesium** is an important cofactor for many enzymes involved in **ATP hydrolysis**, **DNA replication**, and **RNA synthesis**. - However, it does not function as the prosthetic group for **ALA dehydratase** in the heme biosynthetic pathway.

Q: Which enzyme has the highest turnover number in biochemical reactions?

Catalase. ***Catalase*** - **Catalase** exhibits an exceptionally high turnover number, converting millions of molecules of hydrogen peroxide to water and oxygen per second. - Its high catalytic efficiency is crucial for protecting cells from **oxidative damage**, as hydrogen peroxide is a toxic byproduct of metabolism. *LDH* - **Lactate dehydrogenase (LDH)** catalyzes the interconversion of pyruvate and lactate, an important step in anaerobic metabolism. - While an efficient enzyme, its turnover number is significantly lower than that of catalase due to different metabolic requirements and substrate specificities. *Trypsin* - **Trypsin** is a protease involved in protein digestion, cleaving peptide bonds at specific sites. - Its catalytic rate is high for its function as a digestive enzyme, but it does not reach the extraordinary turnover numbers of enzymes like catalase, which handle highly reactive and abundant substrates. *None of the options* - This option is incorrect because **catalase** is a known enzyme with one of the highest turnover numbers reported in biochemistry. - Identifying the enzyme with the highest turnover among the given choices is a direct knowledge recall question in enzymology.

Q: Which of the following is an example of allosteric inhibition?

Inhibition of PFK-1 by citrate. ***Inhibition of PFK-1 by citrate*** - **Citrate** acts as an **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, a key enzyme in glycolysis. - Citrate binds to a site distinct from the active site, inducing a conformational change that reduces PFK-1's affinity for **fructose-6-phosphate**, thus slowing glycolysis. *Inactivation of glycogen synthase by phosphorylation* - This is an example of **covalent modification** (phosphorylation), not allosteric regulation. - Phosphorylation alters the enzyme's activity by adding a phosphate group, changing its structure and function. *Decreased synthesis of glucokinase by glucagon* - This describes **transcriptional regulation** or **gene expression control**, where glucagon affects the amount of enzyme produced. - It is not an example of allosteric regulation, which involves direct binding of a molecule to an enzyme to alter its activity. *None of the options* - This option is incorrect because the inhibition of PFK-1 by citrate is a classic example of allosteric inhibition.

Question 1

Chymotrypsinogen is converted to chymotrypsin by:

Accepted Answer

Trypsin

Answer

Pepsin

Answer

Alkaline pH

Answer

Elastase

Question 2

Curdling of milk is caused by which factor?

Accepted Answer

Rennin

Answer

Lipase

Answer

Elastase

Answer

Amylase

Question 3

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 4

Following enzymes are found in lysosomes, except

Accepted Answer

Pepsin

Answer

Arylsulfatases

Answer

Glycosidases

Answer

Ribonucleases

Question 5

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.

Question 6

Which of the following does NOT directly influence the activity of existing enzyme molecules?

Accepted Answer

Induction

Answer

Acetylation

Answer

Phosphorylation

Answer

Methylation

Question 7

In the context of clinical biochemistry, which isoenzyme of lactate dehydrogenase (LDH) has the maximum electrophoretic mobility?

Accepted Answer

LDH-1

Answer

LDH-5

Answer

LDH-2

Answer

LDH-3

Question 8

Which of the following is the metal cofactor of the enzyme ALA dehydratase?

Accepted Answer

Zinc

Answer

Magnesium

Answer

Lead

Answer

Copper

Question 9

Which enzyme has the highest turnover number in biochemical reactions?

Accepted Answer

Catalase

Answer

LDH

Answer

Trypsin

Answer

None of the options

Question 10

Which of the following is an example of allosteric inhibition?

Accepted Answer

Inhibition of PFK-1 by citrate

Answer

Decreased synthesis of glucokinase by glucagon

Answer

Inactivation of glycogen synthase by phosphorylation

Answer

None of the options

Enzymes — MCQs

Enzymes — MCQs

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