Question 1

   Question: The following reaction depicts the activity of a particular class of enzymes 

Identify the enzymes class 'E' from the following options:

   Options:

A) Transferases

B) Isomerases

C) Lyases

D) Ligases

   Correct Answer: C

   Year: 2026

   Solution: Lyases are the enzymes that catalyse removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds. Transferases are the enzymes that catalyse a transfer of a group between a pair of substrates. Isomerases catalyse inter-conversion of optical, geometric or positional isomers. Ligases catalyse the linking together of 2 compounds.

   Step Solution:

    1.  Examine the reaction diagram for the specific chemical change.

    2.  Note that a group is being removed from the substrate without using water (non-hydrolytic).

    3.  Observe the formation of a double bond in the product.

    4.  Compare this mechanism against standard enzyme classes: Transferases (group transfer), Isomerases (rearrangement), Ligases (joining).

    5.  Confirm that non-hydrolytic group removal leaving double bonds is the defining characteristic of Lyases.

   The difficulty level: Easy

   The Concept Name: Classification of Enzymes

   Short cut solution: Look for the creation of a double bond; if it's there and not a result of hydrolysis, it is a Lyase.

Question 1

   Question: Name the class of enzyme that usually catalyze the following reaction:

    $S - G + S ^ {\#} \rightarrow S + S ^ {\#} - G$

    Where,  $\mathbf G \to \mathbf a$  group other than hydrogen, $S $ a substrate, S# → another substrate.

   Options: A. Transferase, B. Ligase, C. Hydrolase, D. Lyase

   Correct Answer: A

   Year: NEET 2025

   Solution: Enzymes that catalyze the transfer of a group $G$ (other than hydrogen) between a pair of substrates, $S$ and $S^{\#}$, are known as transferases. 

   Step Solution: 

    1. Identify the reaction type: A functional group ($G$) is moving from one substrate to another.

    2. Recognize that $G$ is explicitly stated as "a group other than hydrogen."

    3. Recall the classification of enzymes: enzymes that move functional groups are "Transferases".

    4. Match the definition to Option A.

   Difficulty Level: Easy

   Concept Name: Enzyme Classification (Transferases)

   Short cut solution: The reaction shows a transfer of group $G$; therefore, the enzyme is a Transferase.

Question 8

   Question: Ligases is a class of enzymes responsible for catalysing the linking together of two compounds. Which of the following bonds is not catalysed by it?

   Options: A. C − C, B. P − O, C. C − O, D. C − N

   Correct Answer: A

   Year: NEET 2024 Re

   Solution: Ligases are enzymes that catalyse the linking together of 2 compounds, e.g., enzymes which catalyse joining of C − O, C − S, C − N, P − O etc. bonds.

   Step Solution: 

    1. Define Ligases: Enzymes that link two compounds together.

    2. List the bonds typically formed by ligases: C−O, C−S, C−N, and P−O.

    3. Compare the list with the options provided.

    4. Observe that C−C is not included in the standard examples of bonds joined by ligases in this context.

   Difficulty Level: Medium

   Concept Name: Enzyme Classification (Ligases)

   Short cut solution: Ligases typically join carbon to a heteroatom (O, N, S) or Phosphorus to Oxygen; C–C bonds are usually handled by other classes like Lyases or Transferases.

Question 11

   Question: Enzymes that catalyse the removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds, are known as :

   Options: A. Transferases, B. Oxidoreductases, C. Dehydrogenases, D. Lyases

   Correct Answer: D

   Year: NEET 2024 Re

   Solution: Lyases are a group of enzymes that catalyse the removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds.

   Step Solution: 

    1. Identify the key mechanism: "removal of groups."

    2. Identify the condition: "other than hydrolysis."

    3. Identify the structural result: "leaving double bonds."

    4. Recall the specific enzyme class definition: Lyases perform this exact function.

   Difficulty Level: Easy

   Concept Name: Enzyme Classification (Lyases)

   Short cut solution: Double bonds formed without water = Lyases.

Question 18

   Question: Regarding catalytic cycle of an enzyme action, select the correct sequential steps : 

    A. Substrate enzyme complex formation. 

    B. Free enzyme ready to bind with another substrate. 

    C. Release of products. 

    D. Chemical bonds of the substrate broken. 

    E. Substrate binding to active site.

   Options: A. E, A, D, C, B; B. A, E, B, D, C; C. B, A, C, D, E; D. E, D, C, B, A

   Correct Answer: A

   Year: NEET 2024

   Solution: The catalytic cycle involves: (1) Substrate binds to the active site (E). (2) Binding induces shape change for tighter fit (A). (3) Active site breaks bonds and forms product complex (D). (4) Enzyme releases products (C) and returns to free state (B).

   Step Solution: 

    1. Start with the substrate approaching: E (Substrate binding).

    2. Formation of the intermediate: A (Complex formation).

    3. The chemical reaction occurs: D (Bonds broken).

    4. The result is discharged: C (Release of products).

    5. The enzyme resets: B (Free enzyme).

   Difficulty Level: Medium

   Concept Name: Mechanism of Enzyme Action

   Short cut solution: Order of life: Bind (E) $\rightarrow$ React (D) $\rightarrow$ Release (C). Only Option A starts with E and ends with B.

Question 45

   Question: A non-proteinaceous enzyme is

   Options: A. lysozyme, B. ribozyme, C. ligase, D. deoxyribonuclease.

   Correct Answer: B

   Year: NEET II 2016

   Solution: A ribozyme is a ribonucleic acid (RNA) enzyme that catalyses a chemical reaction in a similar way to that of a protein enzyme.

   Step Solution: 

    1. Recall the general rule: Almost all enzymes are proteins.

    2. Recall the exception: Some RNA molecules act as catalysts.

    3. Identify the term for catalytic RNA: Ribozyme.

    4. Select Option B.

   Difficulty Level: Easy

   Concept Name: Ribozymes (Catalytic RNA)

   Short cut solution: Ribozyme is made of Ribonucleic acid, not protein.

Question 53

   Question: Select the option which is not correct with respect to enzyme action.

   Options: A. Substrate binds with enzyme at its active site. B. Addition of lot of succinate does not reverse the inhibition of succinic dehydrogenase by malonate. C. A non-competitive inhibitor binds the enzyme at a site distinct from that which binds the substrate. D. Malonate is a competitive inhibitor of succinic dehydrogenase.

   Correct Answer: B

   Year: (Source year not specified for this question)

   Solution: The reduction of activity of succinate dehydrogenase by malonate is an example of competitive inhibition, which is reversible. Addition of a lot of succinate can reverse the inhibition.

   Step Solution: 

    1. Identify Malonate as a competitive inhibitor of succinate dehydrogenase.

    2. Recall that competitive inhibition is reversible by increasing substrate concentration.

    3. Analyze Option B: It claims adding succinate (substrate) does not reverse inhibition.

    4. Conclude that Option B is the incorrect statement.

   Difficulty Level: Medium

   Concept Name: Enzyme Inhibition (Competitive vs. Non-competitive)

   Short cut solution: Competitive inhibition is always reversible by adding more substrate. Statement B says it isn't, so B is wrong.

Question 57

   Question: Transition state structure of the substrate formed during an enzymatic reaction is

   Options: A. transient and unstable, B. permanent and stable, C. transient but stable, D. permanent but unstable.

   Correct Answer: A

   Year: NEET 2013

   Solution: Transition state is the formation of an unstable intermediate structural state. This state is transient and highly unstable.

   Step Solution: 

    1. Define Transition State: The point where bonds are being broken and made.

    2. Determine stability: Because it is the highest energy state, it is unstable.

    3. Determine duration: It exists only for a moment, meaning it is transient.

   Difficulty Level: Easy

   Concept Name: Transition State Theory

   Short cut solution: Transition states are always high-energy (unstable) and short-lived (transient).

Question 59

   Question: Which of the following statements about enzymes is wrong?

   Options: A. Enzymes are denatured at high temperatures. B. Enzymes are mostly proteins but some are lipids also. C. Enzymes are highly specific. D. Enzymes require optimum pH and temperature for maximum activity.

   Correct Answer: B

   Year: KN NEET 2013

   Solution: Enzymes are mostly proteins but some are RNA (ribozymes). No lipid working as enzymes are known.

   Step Solution: 

    1. Evaluate A: True, proteins denature with heat.

    2. Evaluate B: False, enzymes are proteins or RNA, never lipids.

    3. Evaluate C & D: True, these are fundamental properties of enzymes.

   Difficulty Level: Easy

   Concept Name: General Properties of Enzymes

   Short cut solution: Lipid enzymes do not exist.

Question 69

   Question: Three of the following statements about enzymes are correct and one is wrong. Which one is wrong?

   Options: A. Enzymes require optimum pH for maximal activity. B. Enzymes are denatured at high temperature but in certain exceptional organisms they are effective even at temperatures 80 - 90 C. C. Enzymes are highly specific. D. Most enzymes are proteins but some are lipids.

   Correct Answer: D

   Year: Mains 2010

   Solution: Enzymes are mostly proteins but some are RNA (ribozymes). No lipid working as enzymes are known.

   Step Solution: 

    1. Follow the same logic as Question 59.

    2. Identify that Option D incorrectly suggests some enzymes are lipids.

   Difficulty Level: Easy

   Concept Name: Enzyme Composition

   Short cut solution: Enzymes are proteins or RNA; D is wrong because it mentions lipids.

Question 70

   Question: The figure given below shows the conversion of a substrate into product by an enzyme. In which one of the four options (a-d) the components of reaction labelled as A, B, C and D are identified correctly? 

   Options: A. (a), B. (b), C. (c), D. (d).

   Correct Answer: B

   Year: Mains 2010

   Solution: The figure depicts activation energy. Enzymes act by lowering the activation energy required for a reaction.

   Step Solution: 

    1. Analyze the typical enzyme energy graph: The peak represents the Transition State.

    2. The energy gap without enzyme is the higher activation energy.

    3. The energy gap with enzyme is the lower activation energy.

    4. Match the labels to Option B based on the standard diagrammatic representation.

   Difficulty Level: Medium

   Concept Name: Activation Energy and Catalysis

   Short cut solution: Enzymes lower the "hump" (activation energy) on the graph.

Question 74

   Question: Modern detergents contain enzyme preparations of

   Options: A. thermoacidophiles, B. thermophiles, C. acidophiles, D. alkaliphiles.

   Correct Answer: D

   Year: 2008

   Solution: Modern detergents contain enzyme preparations of alkaline protease which are called alkaliphiles, for removing protein stains.

   Step Solution: 

    1. Consider the environment of a detergent: Laundry water is typically alkaline (high pH).

    2. Identify the need: Enzymes must be stable and active in alkaline conditions.

    3. Recall the term for alkali-loving organisms: Alkaliphiles.

   Difficulty Level: Medium

   Concept Name: Industrial Applications of Enzymes

   Short cut solution: Detergents = Alkaline pH $\rightarrow$ Alkaliphiles.

Question 87

   Question: Enzyme first used for nitrogen fixation

   Options: A. nitrogenase, B. nitroreductase, C. transferase, D. transaminase.

   Correct Answer: A

   Year: 2001

   Solution: Nitrogen fixation involves conversion of atmospheric nitrogen to ammonia, facilitated by the nitrogenase enzyme.

   Step Solution: 

    1. Define the process: Nitrogen fixation.

    2. Identify the biological catalyst responsible for breaking $N_2$ bonds: Nitrogenase.

    3. Match with Option A.

   Difficulty Level: Easy

   Concept Name: Biological Nitrogen Fixation

   Short cut solution: Process = Nitrogen fixation; Enzyme = Nitrogenase.

Question 88

   Question: Role of enzyme in reactions is to/as

   Options: A. decrease activation energy, B. increase activation energy, C. inorganic catalyst, D. none of the above.

   Correct Answer: A

   Year: 2000

   Solution: Enzymes speed up a reaction by lowering the activation energy. 

   Step Solution: 

    1. Recall the "energy barrier" theory.

    2. Understand that higher activation energy means a slower reaction.

    3. Recognize that enzymes provide an alternative pathway to decrease this energy.

   Difficulty Level: Easy

   Concept Name: Activation Energy

   Short cut solution: Enzyme = Catalyst = Lower Activation Energy ($E_a$).

Question 109

   Question: Enzymes having slightly different molecular structure but performing identical activity are

   Options: A. holoenzymes, B. isoenzymes, C. apoenzymes, D. coenzymes

   Correct Answer: B

   Year: 1991

   Solution: There are certain enzymes which have slightly different molecular structure but have similar catalytic function; such enzymes are known as isoenzymes.

   Step Solution: 

    1. Identify the definition: "different structure, same function."

    2. Distinguish from other terms: Holoenzyme/Apoenzyme relate to protein-cofactor complexes.

    3. Recall the specific term for variant forms of the same enzyme: Isoenzymes.

   Difficulty Level: Easy

   Concept Name: Isoenzymes

   Short cut solution: Iso (same) + Enzyme (activity) = Same activity, different structure.