Enzymes Practice Questions

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: Enzyme replacement therapy is available for which of the following lysosomal storage disorders?

Gaucher's disease. ***Gaucher's disease*** - Enzyme replacement therapy (ERT) with **imiglucerase**, **velaglucerase alfa**, or **taliglucerase alfa** is a highly effective treatment for Type 1 and Type 3 Gaucher's disease - It provides the deficient enzyme **glucocerebrosidase** (β-glucosidase), which breaks down glucocerebroside, preventing its accumulation in macrophages - ERT significantly reduces hepatosplenomegaly, improves bone disease, and corrects cytopenias *Niemann-Pick disease Type C* - **No enzyme replacement therapy** is currently available for Niemann-Pick disease Type C - This disorder involves defective cholesterol trafficking, not a single enzyme deficiency amenable to replacement - Treatment is primarily **supportive** with miglustat for neurological symptoms in some cases *Tay-Sachs disease* - There is currently **no effective enzyme replacement therapy** for Tay-Sachs disease - The deficient enzyme **hexosaminidase A** cannot effectively cross the **blood-brain barrier** to reach affected neurons in the CNS - Treatment is purely **supportive and palliative** *None of the above* - This option is incorrect because **Gaucher's disease** has well-established and FDA-approved enzyme replacement therapy

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: 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.

Q: Which enzyme is competitively inhibited by malonate in the Krebs cycle?

Succinate dehydrogenase. ***Succinate dehydrogenase*** - **Malonate** is structurally similar to **succinate**, allowing it to bind to the active site of **succinate dehydrogenase**, thus competitively inhibiting the enzyme. - This enzyme is crucial for the conversion of **succinate to fumarate** in the **Krebs cycle** (or tricarboxylic acid cycle). *Fumarate dehydrogenase* - This enzyme is not a standard component of the Krebs cycle; instead, a **fumarase** enzyme catalyzes the hydration of **fumarate to malate**. - Malonate does not directly inhibit **fumarase**. *Succinate thiokinase* - This enzyme, also known as **succinyl-CoA synthetase**, catalyzes the conversion of **succinyl-CoA to succinate**. - It is not competitively inhibited by malonate during this step. *Aconitase* - **Aconitase** is an enzyme that catalyzes the isomerization of **citrate to isocitrate** via _cis_-aconitate in the Krebs cycle. - Its activity is not affected by malonate; it is instead inhibited by **fluoroacetate**, which is metabolized to **fluorocitrate**.

Q: Maltase hydrolyzes maltose to form:

Two glucose molecules. ***Two glucose molecules*** - Maltase is an enzyme that specifically breaks down **maltose**. - Maltose, a disaccharide, is composed of **two glucose units** linked by an α-1,4-glycosidic bond. *Glucose and fructose* - This is the hydrolysis product of **sucrose**, a disaccharide broken down by the enzyme **sucrase**. - Sucrose consists of one **glucose** and one **fructose** molecule. *Galactose and fructose* - This combination does not represent a common disaccharide hydrolysis product. - While galactose and fructose are monosaccharides, they do not form a common dietary disaccharide linked together. *Glucose and galactose* - This is the hydrolysis product of **lactose**, a disaccharide broken down by the enzyme **lactase**. - Lactose is composed of one **glucose** and one **galactose** molecule.

Q: Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?

Creatine kinase (CK) and Lactate dehydrogenase (LDH). ***Creatine kinase (CK) and Lactate dehydrogenase (LDH)*** - **CK and LDH** are the most commonly measured enzyme markers in CSF for detecting **neuronal and glial cell damage**. - Elevated **LDH** in CSF indicates cellular injury and is seen in conditions like **meningitis, stroke, CNS malignancies, and traumatic brain injury**. - Elevated **CK** (particularly CK-BB isoform) in CSF indicates **brain tissue damage** and is elevated in conditions like **stroke, seizures, and CNS trauma**. - These enzymes are **not normally present in significant concentrations** in CSF but become elevated with cellular damage. *Gamma-glutamyl transferase (GGT) and Alkaline phosphatase (ALP)* - **GGT** is primarily a marker of **hepatobiliary disease** and is not routinely measured in CSF for neurological assessment. - **ALP** is found mainly in **liver, bone, and placenta**; it is not a standard CSF marker for neurological conditions. *Deaminase and Peroxidase* - **Adenosine deaminase (ADA)** can be elevated in CSF in **tuberculous meningitis**, but it is not a routine marker for general neurological damage. - **Peroxidase** is not a standard CSF enzyme marker for neurological disease assessment. *Alkaline phosphatase (ALP) and Creatine kinase (CK-MB)* - **CK-MB** is the cardiac-specific isoform used for **myocardial damage** detection, not for CNS assessment. - The brain-specific isoform is **CK-BB**, not CK-MB. - **ALP** is not a relevant CSF marker for neurological conditions.

Question 1

Following enzymes are found in lysosomes, except

Accepted Answer

Pepsin

Answer

Arylsulfatases

Answer

Glycosidases

Answer

Ribonucleases

Question 2

Enzyme replacement therapy is available for which of the following lysosomal storage disorders?

Accepted Answer

Gaucher's disease

Answer

Tay-Sachs disease

Answer

None of the above

Answer

Niemann-Pick disease Type C

Question 3

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

Accepted Answer

Induction

Answer

Acetylation

Answer

Phosphorylation

Answer

Methylation

Question 4

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

Accepted Answer

Zinc

Answer

Magnesium

Answer

Lead

Answer

Copper

Question 5

Which enzyme has the highest turnover number in biochemical reactions?

Accepted Answer

Catalase

Answer

LDH

Answer

Trypsin

Answer

None of the options

Question 6

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

Question 7

Which of the following statements about isozymes is true?

Accepted Answer

They catalyze the same reaction but may differ in structure.

Answer

They have the same quaternary structure.

Answer

They have the same enzyme classification but differ in number and name.

Answer

They are distributed uniformly across different organs.

Question 8

Which enzyme is competitively inhibited by malonate in the Krebs cycle?

Accepted Answer

Succinate dehydrogenase

Answer

Fumarate dehydrogenase

Answer

Succinate thiokinase

Answer

Aconitase

Question 9

Maltase hydrolyzes maltose to form:

Accepted Answer

Two glucose molecules

Answer

Glucose and fructose

Answer

Galactose and fructose

Answer

Glucose and galactose

Question 10

Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?

Accepted Answer

Creatine kinase (CK) and Lactate dehydrogenase (LDH)

Answer

Gamma-glutamyl transferase (GGT) and Alkaline phosphatase (ALP)

Answer

Deaminase and Peroxidase

Answer

Alkaline phosphatase (ALP) and Creatine kinase (CK-MB)

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?