Enzymes — MCQs
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Following enzymes are found in lysosomes, except
Enzyme replacement therapy is available for which of the following lysosomal storage disorders?
Which of the following does NOT directly influence the activity of existing enzyme molecules?
Which enzyme has the highest turnover number in biochemical reactions?
Which of the following is the metal cofactor of the enzyme ALA dehydratase?
Which of the following is an example of allosteric inhibition?
Which enzyme is competitively inhibited by malonate in the Krebs cycle?
Which of the following statements about isozymes is true?
Maltase hydrolyzes maltose to form:
Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?
Enzymes Indian Medical PG Practice Questions and MCQs
Question 551: Following enzymes are found in lysosomes, except
- A. Arylsulfatases
- B. Glycosidases
- C. Pepsin (Correct Answer)
- D. Ribonucleases
Explanation: ***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.
Question 552: Enzyme replacement therapy is available for which of the following lysosomal storage disorders?
- A. Gaucher's disease (Correct Answer)
- B. Tay-Sachs disease
- C. None of the above
- D. Niemann-Pick disease Type C
Explanation: ***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
Question 553: Which of the following does NOT directly influence the activity of existing enzyme molecules?
- A. Acetylation
- B. Phosphorylation
- C. Induction (Correct Answer)
- D. Methylation
Explanation: ***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.
Question 554: Which enzyme has the highest turnover number in biochemical reactions?
- A. LDH
- B. Trypsin
- C. Catalase (Correct Answer)
- D. None of the options
Explanation: ***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.
Question 555: Which of the following is the metal cofactor of the enzyme ALA dehydratase?
- A. Magnesium
- B. Zinc (Correct Answer)
- C. Lead
- D. Copper
Explanation: ***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.
Question 556: Which of the following is an example of allosteric inhibition?
- A. Decreased synthesis of glucokinase by glucagon
- B. Inactivation of glycogen synthase by phosphorylation
- C. Inhibition of PFK-1 by citrate (Correct Answer)
- D. None of the options
Explanation: ***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 557: Which enzyme is competitively inhibited by malonate in the Krebs cycle?
- A. Fumarate dehydrogenase
- B. Succinate thiokinase
- C. Aconitase
- D. Succinate dehydrogenase (Correct Answer)
Explanation: ***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**.
Question 558: Which of the following statements about isozymes is true?
- A. They catalyze the same reaction but may differ in structure. (Correct Answer)
- B. They have the same quaternary structure.
- C. They have the same enzyme classification but differ in number and name.
- D. They are distributed uniformly across different organs.
Explanation: ***They catalyze the same reaction but may differ in structure.*** - Isozymes are **different forms of an enzyme** that catalyze the **same biochemical reaction** but have distinct amino acid sequences. - Due to their different amino acid sequences, isozymes can exhibit variations in their **molecular structure**, kinetic properties, and regulatory mechanisms. *They have the same quaternary structure.* - While some isozymes might have a similar quaternary structure (e.g., both being tetramers), it is not a defining characteristic; they often have **different subunit compositions** or arrangements. - Their structural differences, including quaternary structure, contribute to their distinct properties and often reflect their expression in **different tissues or developmental stages**. *They have the same enzyme classification but differ in number and name.* - Isozymes belong to the **same enzyme classification** (e.g., EC number) because they catalyze the identical reaction, but they are **not necessarily numbered differently** as distinct enzymes. - Their differing names typically reflect the tissue they are found in or their specific subunits (e.g., lactate dehydrogenase isozymes **LDH-1 to LDH-5**). *They are distributed uniformly across different organs.* - Isozymes typically exhibit a **tissue-specific distribution**, meaning their presence and relative abundance vary significantly between different organs and tissues. - This differential distribution allows for **fine-tuning metabolic pathways** to meet the specific physiological demands of each tissue.
Question 559: Maltase hydrolyzes maltose to form:
- A. Glucose and fructose
- B. Galactose and fructose
- C. Two glucose molecules (Correct Answer)
- D. Glucose and galactose
Explanation: ***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.
Question 560: Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?
- A. Gamma-glutamyl transferase (GGT) and Alkaline phosphatase (ALP)
- B. Creatine kinase (CK) and Lactate dehydrogenase (LDH) (Correct Answer)
- C. Deaminase and Peroxidase
- D. Alkaline phosphatase (ALP) and Creatine kinase (CK-MB)
Explanation: ***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.
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
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