Enzymes Practice Questions

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.

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: Apoenzyme is ?

Protein moiety. ***Protein moiety*** - An **apoenzyme** is the **protein component of an enzyme** that is catalytically inactive by itself. - It requires a **non-protein cofactor** (either an inorganic ion or an organic molecule) to become active. *Organic cofactor* - An **organic cofactor** is also known as a **coenzyme**, which binds to the apoenzyme to form a functional holoenzyme. - While essential for enzyme activity, the apoenzyme itself is the protein part, not the organic cofactor. *Inactive enzyme component* - While an apoenzyme is **inactive on its own**, this description is too broad and doesn't specify its chemical nature. - It is specifically the **protein component** that is inactive until bound to its cofactor. *Non-protein component required for enzyme activity* - This describes a **cofactor** (either inorganic or organic), not the apoenzyme itself. - The apoenzyme is the **protein portion**, which *requires* the non-protein component for activity.

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: 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: Type of inhibition of aconitase by trans-aconitate is?

Competitive. ***Competitive*** - **Competitive inhibition** occurs when the inhibitor (trans-aconitate) structurally resembles the enzyme's natural substrate (cis-aconitate) and binds to the **active site**, preventing the substrate from binding. - This type of inhibition can be overcome by increasing the concentration of the **substrate**. *Non-competitive* - **Non-competitive inhibitors** bind to a site on the enzyme other than the active site, causing a conformational change that reduces the enzyme's efficiency, regardless of substrate concentration. - Trans-aconitate's structural similarity to aconitate's substrate points away from a non-competitive mechanism. *Allosteric* - **Allosteric inhibition** involves an inhibitor binding to a regulatory site (allosteric site) on the enzyme, which is distinct from the active site, to alter enzyme activity. - While allosteric regulation is a type of non-competitive inhibition, trans-aconitate's direct structural resemblance to the substrate makes competitive inhibition the more specific and accurate description. *None of the options* - This option is incorrect because **competitive inhibition** accurately describes the mechanism by which trans-aconitate inhibits aconitase, given its structural similarity to the natural substrate. - The other options are less fitting due to the specific characteristics of trans-aconitate's action.

Q: What are digestive enzymes classified as?

Hydrolases. ***Hydrolases*** - Digestive enzymes like **amylase**, **lipase**, and **proteases** break down complex food molecules by adding water, a process known as **hydrolysis**. - This class of enzymes catalyzes the cleavage of a chemical bond with the concurrent addition of a water molecule. - All major digestive enzymes belong to this class according to the **EC enzyme classification system**. *Oxidoreductases* - These enzymes catalyze **redox reactions**, involving the transfer of electrons from one molecule to another. - Examples include **dehydrogenases** and **oxidases**, which are not primarily involved in breaking down food molecules in digestion. *Transferases* - Transferases catalyze the transfer of functional groups (such as methyl, acyl, or phosphate groups) from one molecule to another. - Examples include **kinases** and **transaminases**, which are involved in metabolic pathways but not in the digestive breakdown of food. *Ligases* - Ligases are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, typically with the concomitant hydrolysis of ATP. - They are involved in **DNA repair** and **biosynthetic reactions**, not in the breakdown of food during digestion.

Q: Aldehyde dehydrogenase requires NAD as ?

Coenzyme. ***Coenzyme*** - **NAD** (nicotinamide adenine dinucleotide) acts as a **coenzyme** for aldehyde dehydrogenase, serving as the **most specific and accurate classification** for its role. - As a coenzyme, **NAD** is an **organic, non-protein molecule** that binds reversibly to the enzyme and acts as a **transient carrier of electrons** (hydride ions, H⁻) during aldehyde oxidation. - **NAD⁺** accepts electrons from the aldehyde substrate, becoming reduced to **NADH**, which then dissociates and transfers electrons elsewhere in metabolism. - This is the **preferred answer** because coenzyme precisely describes NAD's organic nature, vitamin origin (niacin/B3), and its role as a mobile electron carrier. *Cofactor* - While technically **NAD is a type of cofactor** (cofactors include coenzymes, prosthetic groups, and metal ions), this term is **too general** for this context. - In biochemistry nomenclature, when both a general and specific term apply, the **more specific term (coenzyme) is preferred** to demonstrate precise understanding. - Choosing "cofactor" would be like calling a "cardiologist" a "doctor" - true but less specific. *Apoenzyme* - An **apoenzyme** is the **protein component** of an enzyme without its cofactor - it refers to the enzyme itself, not to NAD. - In this case, **aldehyde dehydrogenase** (the protein) is the apoenzyme, and **NAD** is the coenzyme that binds to it. - Together they form the active **holoenzyme** (apoenzyme + coenzyme = holoenzyme). *None of the options* - Incorrect because **coenzyme** is the accurate and specific term for NAD's role in aldehyde dehydrogenase function.

Question 1

Enzyme causing covalent bond cleavage without hydrolysis ?

Accepted Answer

Lyase

Answer

Ligase

Answer

Hydrolase

Answer

Transferase

Question 2

Glucose oxidase converts glucose to?

Accepted Answer

Gluconic acid

Answer

Glucuronic acid

Answer

Galactonic acid

Answer

Iduronic acid

Question 3

What is the cofactor required for the enzyme xanthine oxidase?

Accepted Answer

Molybdenum

Answer

Selenium

Answer

Zinc

Answer

Magnesium

Question 4

Apoenzyme is ?

Accepted Answer

Protein moiety

Answer

Organic cofactor

Answer

Inactive enzyme component

Answer

Non-protein component required for enzyme activity

Question 5

Which of the following best describes the difference between glucokinase and hexokinase?

Accepted Answer

Glucokinase has higher Km for glucose compared to hexokinase.

Answer

Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.

Answer

Glucokinase has a low affinity for glucose.

Answer

Glucokinase activity increases with glucose concentration while hexokinase remains saturated.

Question 6

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 7

Selenocysteine is associated with ?

Accepted Answer

Deiodinase

Answer

Carbonic anhydrase

Answer

Catalase

Answer

Transferase

Question 8

Type of inhibition of aconitase by trans-aconitate is?

Accepted Answer

Competitive

Answer

Non-competitive

Answer

Allosteric

Answer

None of the options

Question 9

What are digestive enzymes classified as?

Accepted Answer

Hydrolases

Answer

Oxidoreductases

Answer

Transferases

Answer

Ligases

Question 10

Aldehyde dehydrogenase requires NAD as ?

Accepted Answer

Coenzyme

Answer

None of the options

Answer

Apoenzyme

Answer

Cofactor

Enzymes — MCQs

Enzymes — MCQs

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