Enzymes Practice Questions

Q: What are isoenzymes?

Physically distinct forms of the same enzyme. ***Physically distinct forms of the same enzyme*** - Isoenzymes are **multiple forms of an enzyme** that catalyze the **same reaction** but differ in their **physical or biochemical properties**, such as electrophoretic mobility, optimal pH, or kinetic parameters. - These differences usually arise from **genetic variations** (different genes encoding isoforms) or **post-translational modifications** (e.g., phosphorylation, glycosylation). *Physically same forms of different enzymes* - This statement is incorrect as isoenzymes are forms of the **same enzyme**, not different enzymes. - While different enzymes can catalyze similar reactions in certain pathways, they are not referred to as isoenzymes if they are structurally identical. *Forms of same enzyme that catalyze different reactions* - This describes enzymes with **broad substrate specificity** or those that act on different substrates but are not necessarily isoenzymes. - Isoenzymes specifically catalyze the **same chemical reaction**, but they may do so with different efficiencies or under different regulatory controls. *Forms of different enzyme that catalyze same reactions* - This describes a scenario where different enzymes might exhibit **catalytic promiscuity** or broad specificity, but not isoenzymes. - Isoenzymes are always derived from the **same parent enzyme** and catalyze the identical reaction.

Q: Selenocysteine is associated with ?

Deiodinase. ***Deiodinase*** - Selenocysteine is a critical component of **iodothyronine deiodinases**, a family of enzymes that regulate **thyroid hormone metabolism**. - These enzymes catalyze the removal of iodine from thyroid hormones, converting **thyroxine (T4)** into the more active **triiodothyronine (T3)** or inactive forms. *Carbonic anhydrase* - This enzyme contains **zinc** as its essential metal cofactor and is involved in the interconversion of **carbon dioxide** and **bicarbonate**. - Its primary role is in pH regulation and CO2 transport, without any direct association with selenocysteine. *Catalase* - Catalase is an enzyme primarily found in **peroxisomes** and contains **iron-porphyrin** groups as its prosthetic group. - Its function is to convert **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. *Transferase* - Transferases are a broad class of enzymes that catalyze the transfer of **functional groups** (e.g., methyl, glucose) from one molecule to another. - While essential for many metabolic processes, there is no inherent association of the general class of transferases with selenocysteine.

Q: Km value is defined as:

Substrate concentration at Vmax/2. ***Substrate concentration at Vmax/2*** - The **Michaelis constant (Km)** is defined as the **substrate concentration** at which the reaction velocity is **half of the maximum velocity (Vmax/2)**. - It reflects the **affinity of an enzyme for its substrate**; a lower Km indicates higher affinity. *Substrate concentration at which reaction rate is maximum* - The **maximum reaction rate (Vmax)** is achieved when the enzyme is **saturated with substrate**, meaning all active sites are occupied. - Km specifically refers to the substrate concentration needed to reach **half of this maximum rate**, not the maximum rate itself. *Substrate concentration at Vmax* - At **Vmax**, the enzyme is fully saturated with substrate, and the reaction rate cannot increase further by adding more substrate. - The **Km value** is a measure related to the **efficiency of substrate binding** at conditions below saturation, specifically at half Vmax. *Substrate concentration at which enzyme activity is optimal* - **Optimal enzyme activity** is generally influenced by factors such as **pH and temperature**, which affect the enzyme's structure and catalytic efficiency. - Km is specifically related to the **substrate concentration** required to achieve a specific reaction rate, not the overall optimal environmental conditions for the enzyme.

Q: Fluoroacetate inhibits?

Aconitase. ***Aconitase*** - **Fluoroacetate** is metabolically converted to **fluorocitrate**, which is a potent competitive inhibitor of **aconitase**. - **Aconitase** is the enzyme responsible for converting **citrate to isocitrate** in the **Krebs cycle**, and its inhibition blocks the cycle. *Citrate synthase* - This enzyme is responsible for the formation of **citrate** from **acetyl-CoA** and **oxaloacetate**. - While fluoroacetate indirectly affects the cycle, it does not directly inhibit **citrate synthase**. *Succinate dehydrogenase* - This enzyme is part of the **Krebs cycle** and the **electron transport chain**, converting **succinate to fumarate**. - **Malonate** is a competitive inhibitor of succinate dehydrogenase, not **fluoroacetate**. *Alpha-ketoglutarate dehydrogenase* - This enzyme catalyzes the conversion of **alpha-ketoglutarate to succinyl-CoA** in the **Krebs cycle**. - Specific inhibitors of this enzyme include **arsenite** and **mercury compounds**, but not fluoroacetate.

Q: Enzymes that move a molecular group from one molecule to another are known as -

Transferases. ***Transferases*** - **Transferases** are a class of enzymes that catalyze the transfer of a specific functional group (e.g., methyl, acetyl, phosphate) from one molecule (the donor) to another (the acceptor). - This broad category includes enzymes vital for many metabolic pathways, such as **kinases** (transferring phosphate groups) and **transaminases** (transferring amino groups). *Ligases* - **Ligases** are enzymes responsible for joining two large molecules together, typically by forming a new chemical bond. - This process usually involves the concomitant hydrolysis of a small, energy-rich molecule such as **ATP**, to provide the necessary energy for bond formation. *Dipeptidases* - **Dipeptidases** are a type of hydrolase enzyme that specifically cleaves the peptide bond within a **dipeptide**, releasing two free amino acids. - They are crucial for the final stages of protein digestion, breaking down small peptides into absorbable **amino acid units**. *Oxido-reductases* - **Oxido-reductases** are enzymes that catalyze **oxidation-reduction reactions** (redox reactions), where electrons are transferred from one molecule to another. - This class includes enzymes like **dehydrogenases** and **oxidases**, which play critical roles in cellular respiration and energy production.

Q: What is the cofactor required for the enzyme xanthine oxidase?

Molybdenum. ***Molybdenum*** - **Xanthine oxidase** is a key enzyme in **purine metabolism**, responsible for the oxidation of **hypoxanthine to xanthine** and further to **uric acid**. - **Molybdenum** is an essential trace element that serves as a **cofactor** for several enzymes, including xanthine oxidase, where it helps facilitate electron transfer reactions. *Selenium* - **Selenium** is a cofactor for **glutathione peroxidase**, an enzyme involved in antioxidant defense. - It is not directly involved in the function of **xanthine oxidase**. *Zinc* - **Zinc** is a cofactor for a wide range of enzymes, including **carbonic anhydrase** and **alcohol dehydrogenase**. - It does not serve as a cofactor for **xanthine oxidase**. *Magnesium* - **Magnesium** is a critical cofactor for many enzymes, particularly those involved in **ATP hydrolysis and synthesis** and **DNA/RNA synthesis**. - It is not a cofactor for **xanthine oxidase**.

Q: How do enzymes function in biochemical reactions?

Decrease in activation energy. ***Decrease in activation energy*** - Enzymes act as **biological catalysts** by providing an alternative reaction pathway with a lower **transition state energy**. - This reduction in the **activation energy** allows a higher proportion of reactant molecules to overcome the energy barrier and react, thereby increasing the reaction rate. *Increase in activation energy* - This statement is incorrect as increasing activation energy would slow down the reaction rate, which is contrary to the function of enzymes. - Enzymes are designed to accelerate reactions, not inhibit them, by making them energetically more favorable to proceed. *Shift equilibrium constant* - Enzymes catalyze both the forward and reverse reactions equally, meaning they accelerate the rate at which equilibrium is reached but **do not alter the equilibrium constant (Keq)** of a reaction. - The equilibrium constant is determined by the difference in free energy between reactants and products, which enzymes do not change. *Provide energy to the reaction* - This statement is incorrect because enzymes do **not provide energy** to reactions; they only lower the activation energy barrier. - Enzymes facilitate reactions by stabilizing the transition state, not by adding energy to the system, which would violate thermodynamic principles.

Q: What is the specific activity of an enzyme?

Enzyme units per mg of protein. ***Enzyme units per mg of protein*** - **Specific activity** is defined as the number of **enzyme units** (representing catalytic activity) per milligram of total protein in the sample. - It is a measure of **purity**, indicating the amount of active enzyme relative to other proteins in a preparation. - Formula: Specific activity = Units of enzyme activity / mg of total protein - Used to track enzyme purification progress during isolation procedures. *Concentration of substrate transformed per minute* - This describes the **reaction velocity** or rate of catalysis, but not the specific activity of the enzyme. - While related to enzyme activity, it does not normalize the activity to the amount of **total protein**. - This would be expressed as reaction rate or velocity (V), not specific activity. *Enzyme units per mg of substrate* - This is an incorrect formulation that confuses substrate with protein. - **Specific activity** is normalized to the amount of **protein** in the enzyme preparation, not the substrate. - This option represents a common misconception in enzyme kinetics terminology. *Limit of enzyme per gram of substrate* - This phrase does not correspond to any standard biochemical measure of enzyme activity or concentration. - It does not provide information about the **catalytic efficiency** or **purity** of the enzyme preparation. - The term "limit" is not used in the context of specific activity measurements.

Q: Enzyme causing covalent bond cleavage without hydrolysis ?

Lyase. ***Lyase*** - **Lyases** are enzymes that catalyze the cleavage of **covalent bonds** (C-C, C-O, C-N, and others) by means other than hydrolysis or oxidation, often creating a new double bond or a ring structure. - They remove groups from substrates to form double bonds, or conversely, add groups to double bonds. - **Examples:** Aldolase (cleaves C-C bonds in glycolysis), carbonic anhydrase (reversible cleavage of C-O bond), fumarase (C-C bond cleavage in TCA cycle). *Ligase* - **Ligases** are enzymes that join two large molecules by forming a new chemical bond, usually accompanied by the **hydrolysis of ATP**. - They are involved in synthesis reactions, not the cleavage of bonds. *Hydrolase* - **Hydrolases** specifically catalyze the hydrolysis of a chemical bond, involving the **addition of water** across the bond. - They break down large molecules into smaller ones using water - this is the key difference from lyases. *Transferase* - **Transferases** catalyze the transfer of a **functional group** from one molecule (the donor) to another (the acceptor). - They do not cause covalent bond cleavage without hydrolysis but rather move existing groups between molecules.

Q: Glucose oxidase converts glucose to?

Gluconic acid. ***Gluconic acid*** - **Glucose oxidase** specifically catalyzes the oxidation of glucose, producing **gluconic acid** and hydrogen peroxide. - This reaction forms the basis for many common **glucose diagnostic tests**, such as those used in blood glucose monitors. *Glucuronic acid* - **Glucuronic acid** is formed from the oxidation of glucose at carbon 6, typically through the **uronic acid pathway**. - It is known for its role in **detoxification** and conjugation reactions in the liver, not as a direct product of glucose oxidase. *Galactonic acid* - **Galactonic acid** is an oxidized form of galactose, a different monosaccharide from glucose. - Its formation is not associated with the action of **glucose oxidase**, an enzyme specific to glucose. *Iduronic acid* - **Iduronic acid** is a C5 epimer of glucuronic acid and is a common component of various **glycosaminoglycans** like dermatan sulfate and heparan sulfate. - It is not produced by the action of **glucose oxidase** on glucose.

Question 1

What are isoenzymes?

Accepted Answer

Physically distinct forms of the same enzyme

Answer

Physically same forms of different enzymes

Answer

Forms of same enzyme that catalyze different reactions

Answer

Forms of different enzyme that catalyze same reactions

Question 2

Selenocysteine is associated with ?

Accepted Answer

Deiodinase

Answer

Carbonic anhydrase

Answer

Catalase

Answer

Transferase

Question 3

Km value is defined as:

Accepted Answer

Substrate concentration at Vmax/2

Answer

Substrate concentration at which reaction rate is maximum

Answer

Substrate concentration at Vmax

Answer

Substrate concentration at which enzyme activity is optimal

Question 4

Fluoroacetate inhibits?

Accepted Answer

Aconitase

Answer

Citrate synthase

Answer

Succinate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Question 5

Enzymes that move a molecular group from one molecule to another are known as -

Accepted Answer

Transferases

Answer

Ligases

Answer

Dipeptidases

Answer

Oxido-reductases

Question 6

What is the cofactor required for the enzyme xanthine oxidase?

Accepted Answer

Molybdenum

Answer

Selenium

Answer

Zinc

Answer

Magnesium

Question 7

How do enzymes function in biochemical reactions?

Accepted Answer

Decrease in activation energy

Answer

Increase in activation energy

Answer

Shift equilibrium constant

Answer

Provide energy to the reaction

Question 8

What is the specific activity of an enzyme?

Accepted Answer

Enzyme units per mg of protein

Answer

Concentration of substrate transformed per minute

Answer

Enzyme units per mg of substrate

Answer

Limit of enzyme per gram of substrate

Question 9

Enzyme causing covalent bond cleavage without hydrolysis ?

Accepted Answer

Lyase

Answer

Ligase

Answer

Hydrolase

Answer

Transferase

Question 10

Glucose oxidase converts glucose to?

Accepted Answer

Gluconic acid

Answer

Glucuronic acid

Answer

Galactonic acid

Answer

Iduronic acid

Enzymes — MCQs

Enzymes — MCQs

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