Enzyme Classification and Nomenclature — MCQs
What is the primary mechanism of action of 5-α reductase?
Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
Apoenzyme is ?
Which enzyme requires zinc as a cofactor?
Enzyme causing covalent bond cleavage without hydrolysis ?
Which of the following enzymes is not classified as an oxidoreductase?
What type of enzyme is hexokinase?
How many stereoisomers are possible for an aldohexose?
Which of the following statements about isozymes is true?
What is the specific activity of an enzyme?
Enzyme Classification and Nomenclature Indian Medical PG Practice Questions and MCQs
Question 1: What is the primary mechanism of action of 5-α reductase?
- A. Reduction of C4-C5 double bond (Correct Answer)
- B. Breakage of amide bond
- C. Breakage of C-N bond
- D. Breakage of N-N bond
Explanation: ***Reduction of C4-C5 double bond*** - 5-α reductase is a **NADPH-dependent reductase enzyme** that catalyzes the **reduction (saturation) of the C4-C5 double bond** in the A-ring of testosterone to form **dihydrotestosterone (DHT)**. - This reduction involves **adding two hydrogen atoms** across the double bond, converting it to a single bond with **5-α stereochemistry**. - DHT is a more potent androgen crucial for **prostate development, external genitalia formation, and male pattern baldness**, making 5-α reductase inhibitors (like finasteride) clinically important for treating benign prostatic hyperplasia and androgenetic alopecia. *Breakage of amide bond* - Breaking **amide bonds (C-N bonds with a carbonyl)** is the function of **proteases and amidases**, not reductases. - This process involves **hydrolysis** and is fundamental to protein degradation and peptide metabolism. *Breakage of C-N bond* - **Carbon-nitrogen bond cleavage** occurs in reactions like **deamination** (catalyzed by deaminases) or metabolism of nitrogenous compounds. - Reductases perform **electron transfer reactions**, not bond cleavage reactions. *Breakage of N-N bond* - **Nitrogen-nitrogen bond** cleavage is rare in human biochemistry and may occur in hydrazine metabolism or by specialized enzymes. - Steroid hormones do not contain N-N bonds, making this mechanism irrelevant to 5-α reductase function.
Question 2: Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
- A. Dienoyl CoA isomerase and Enoyl CoA isomerase
- B. Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase
- C. Enoyl CoA isomerase and Enoyl CoA reductase
- D. Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase (Correct Answer)
Explanation: **Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase** - **Enoyl CoA isomerase** is necessary to convert *cis* double bonds to *trans* double bonds at the 3,4 position, which allows the beta-oxidation enzymes to continue processing the fatty acid. - **2,4 Dienoyl CoA reductase** is required to reduce *cis-2, cis-4* or *trans-2, cis-4* dienoyl intermediates into a single *trans-3* enoyl CoA, which can then be isomerized by enoyl CoA isomerase. *Dienoyl CoA isomerase and Enoyl CoA isomerase* - This option is incorrect because **Dienoyl CoA isomerase** is not a commonly recognized single enzyme directly involved in PUFA beta-oxidation in the way described. The key is to reduce a diene, which reductase does. - While **Enoyl CoA isomerase** is crucial, pairing it with another isomerase in this context does not fully address the reduction step needed for certain PUFAs. *Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase* - This option incorrectly names **Dienoyl CoA isomerase** as one of the two main required enzymes. A 2,4 Dienoyl CoA reductase does exist. - While **2,4 Dienoyl CoA reductase** is essential, the other enzyme should be Enoyl CoA isomerase to handle the initial *cis* to *trans* isomerizations. *Enoyl CoA isomerase and Enoyl CoA reductase* - This option is incorrect because **Enoyl CoA reductase** without the "2,4" prefix generally refers to the enzyme involved in fatty acid synthesis, not beta-oxidation of PUFAs. - **Enoyl CoA isomerase** is correctly identified, but the other enzyme specifically for PUFA oxidation is the **2,4 Dienoyl CoA reductase**.
Question 3: Apoenzyme is ?
- A. Protein moiety (Correct Answer)
- B. Organic cofactor
- C. Inactive enzyme component
- D. Non-protein component required for enzyme activity
Explanation: ***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.
Question 4: Which enzyme requires zinc as a cofactor?
- A. Lactate dehydrogenase
- B. Carbonic anhydrase (Correct Answer)
- C. Glutathione peroxidase
- D. Hexokinase
Explanation: ***Carbonic anhydrase*** - **Carbonic anhydrase** is a critical enzyme that rapidly interconverts carbon dioxide and water into carbonic acid, which then dissociates into a proton and a bicarbonate ion. - This enzyme contains a **zinc ion** in its active site, which is essential for its catalytic activity, particularly in binding and activating water for the hydration of CO2. *Lactate dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of pyruvate to lactate, a key step in anaerobic glycolysis. - LDH primarily relies on **NAD+ or NADH** as cofactors and does not require zinc. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** is an antioxidant enzyme that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides to water using glutathione. - Most mammalian glutathione peroxidases are **selenium-dependent enzymes**, incorporating selenocysteine at their active site, rather than zinc. *Hexokinase* - **Hexokinase** is an enzyme that phosphorylates hexoses, most notably glucose, to glucose-6-phosphate, the first step in glycolysis. - This enzyme requires **magnesium (Mg2+)** as a cofactor for its activity, as it forms a complex with ATP, facilitating the transfer of the phosphate group.
Question 5: Enzyme causing covalent bond cleavage without hydrolysis ?
- A. Lyase (Correct Answer)
- B. Ligase
- C. Hydrolase
- D. Transferase
Explanation: ***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.
Question 6: Which of the following enzymes is not classified as an oxidoreductase?
- A. Alcohol dehydrogenase
- B. Catalase
- C. Peroxidase
- D. Glucokinase (Correct Answer)
Explanation: ***Glucokinase*** - **Glucokinase** is a **transferase** enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. - Its function is primarily in **glucose metabolism** and **insulin secretion**, not in oxidation or reduction reactions. *Catalase* - **Catalase** is an **oxidoreductase** that catalyzes the decomposition of **hydrogen peroxide** into water and oxygen. - This reaction involves the **oxidation and reduction** of substrates, fitting the definition of an oxidoreductase. *Alcohol dehydrogenase* - **Alcohol dehydrogenase** is an **oxidoreductase** that catalyzes the interconversion between alcohols and aldehydes or ketones with the concomitant reduction and oxidation of **NAD+** to **NADH**. - This enzyme is crucial in **detoxifying alcohol** by oxidizing it and is a classic example of an oxidoreductase. *Peroxidase* - **Peroxidase** is an **oxidoreductase** that catalyzes the oxidation of a substrate by **hydrogen peroxide**. - Peroxidases work by using hydrogen peroxide to accept electrons from another molecule, thereby **oxidizing** that molecule.
Question 7: What type of enzyme is hexokinase?
- A. Ligase
- B. Transferase (Correct Answer)
- C. Oxidoreductase
- D. Reductase
Explanation: ***Transferase*** - Hexokinase catalyzes the transfer of a **phosphate group** from **ATP** to glucose, forming glucose-6-phosphate. - Enzymes that catalyze the transfer of functional groups from one molecule to another are classified as **transferases**. *Ligase* - **Ligases** are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, usually accompanied by the hydrolysis of a small pendant chemical group on one of the larger molecules or the less-stable of the two products. - This activity usually involves reactions like **DNA ligation**, not phosphate group transfer to a sugar. *Oxidoreductase* - **Oxidoreductases** catalyze **oxidation-reduction reactions**, involving the transfer of electrons from one molecule to another. - Hexokinase does not perform redox reactions; it transfers a phosphate group. *Reductase* - **Reductases** are a specific type of **oxidoreductase** that catalyze reactions where a molecule is reduced (gains electrons). - This is a subset of oxidation-reduction chemistry and is not the function of hexokinase.
Question 8: How many stereoisomers are possible for an aldohexose?
- A. 32
- B. 64
- C. 16 (Correct Answer)
- D. 8
Explanation: ***16*** - An aldohexose (like glucose) has **four chiral centers** (C2, C3, C4, and C5 in the open-chain form). - The number of possible stereoisomers for a molecule with 'n' chiral centers is given by the formula **2^n**. Therefore, 2^4 = **16 stereoisomers**. - These 16 stereoisomers include D-glucose, D-mannose, D-galactose, D-allose, and their corresponding L-forms. *32* - This number would be true if an aldohexose had **five chiral centers** (2^5 = 32), which it does not. - Aldohexoses are six-carbon sugars, but C1 (aldehyde carbon) and C6 (primary alcohol) are not chiral centers. *64* - This number would imply **six chiral centers** (2^6 = 64), which is incorrect for aldohexoses. - This would require all six carbons to be chiral centers, which is structurally impossible in an aldohexose. *8* - This number suggests **three chiral centers** (2^3 = 8), which is an underestimation. - Aldohexoses have **four chiral centers**, not three, resulting in 16 possible stereoisomers.
Question 9: 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 10: What is the specific activity of an enzyme?
- A. Enzyme units per mg of protein (Correct Answer)
- B. Concentration of substrate transformed per minute
- C. Enzyme units per mg of substrate
- D. Limit of enzyme per gram of substrate
Explanation: ***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.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free