Enzymes

8 MCQs1 revision card9-step worked example

Source: NCERT Unit 19PYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

Enzymes are biological catalysts — proteins (with rare RNA exceptions called ribozymes) that accelerate biochemical reactions without being consumed. NCERT Class 12 Chemistry Chapter 6 (Part 2), page 18 defines enzymes as thermolabile proteins that catalyse specific reactions under mild conditions.

The NEET-critical properties:

Specificity — Each enzyme acts on one substrate or a narrow class. Lock-and-key model (Fischer, 1894) vs. induced-fit model (Koshland, 1958). NEET questions test whether you can distinguish these two models.
Nomenclature — Enzymes named by substrate + suffix "-ase" (maltase hydrolyses maltose; lipase hydrolyses lipids). Six IUBMB classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Optimum conditions — Each enzyme has a pH and temperature optimum. Beyond these, activity drops due to denaturation of the protein's tertiary structure. Pepsin (stomach, pH ~2) vs. trypsin (intestine, pH ~8) is a high-frequency recall pair.
Cofactors and coenzymes — Many enzymes need non-protein helpers. Metal ions (Zn²⁺, Mg²⁺) = cofactors. Organic molecules (NAD⁺, FAD) = coenzymes. The protein part alone = apoenzyme; apoenzyme + cofactor = holoenzyme.
Enzyme inhibition — Competitive inhibitors resemble substrate and occupy the active site. Non-competitive inhibitors bind elsewhere and alter enzyme shape. NEET tests whether a given inhibitor is reversible or irreversible.

Watch-out: Students confuse "enzyme denaturation" with "enzyme inhibition." Denaturation is irreversible structural unfolding (temperature/pH extreme). Competitive inhibition is reversible and overcome by excess substrate.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which of the following correctly defines a holoenzyme?

MCQ 2Easy RecallPractice

Enzymes that catalyse the transfer of a functional group from one substrate to another belong to which IUBMB class?

MCQ 3Easy RecallPractice

Which enzyme is active at approximately pH 2 in the human digestive system?

MCQ 4Direct ApplicationPractice

A competitive inhibitor of an enzyme works by:

MCQ 5Direct ApplicationPractice

An enzyme requires Zn²⁺ ions for catalytic activity. The Zn²⁺ ion is best described as:

MCQ 6Direct ApplicationPractice

The enzyme sucrase (invertase) hydrolyses sucrose but does not act on maltose or lactose. This property of enzymes is called:

MCQ 7Concept TrapPractice

When an enzyme is heated to 80°C, it loses catalytic activity irreversibly. Which level(s) of protein structure are disrupted?

MCQ 8CalculationPractice

A linear polypeptide chain contains 150 amino acid residues. During its synthesis, how many water molecules were released in forming all the peptide bonds?

Quick recall before you leave

Worked Example

1
Given
An enzyme catalyses the reaction: Maltose + H₂O → 2 Glucose. You are asked to classify this enzyme by IUBMB class and name it.
2
Required
(a) IUBMB class of this enzyme. (b) Systematic naming principle applied.
3
Concept
Enzyme classification depends on the type of reaction catalysed. Hydrolases catalyse hydrolysis (bond cleavage using water). Naming convention: substrate name + "-ase" suffix.
4
Formula/Rule
IUBMB Class 3 = Hydrolases (catalyse hydrolytic cleavage of C–O, C–N, C–C, and other bonds with addition of water). Name = maltase (acts on maltose).
5
Substitution
Reaction involves: substrate (maltose) + water → products (glucose units). Bond broken: glycosidic bond in maltose. Method: hydrolysis.
6
Calculation
Classification: reaction uses water to cleave a bond → hydrolase (Class 3). Systematic name: maltose glycohydrolase. Common name: maltase.
7
Final answer
The enzyme belongs to **IUBMB Class 3 (Hydrolases)** and is named **maltase**. *Note: This is a classification question — no numerical calculation is involved. The integers (Class 3, "2 Glucose") are exact counting numbers.*
8
Common trap
Students confuse hydrolases with lyases. Key distinction: hydrolases use water to break bonds; lyases remove groups to form double bonds (or add groups to double bonds) WITHOUT hydrolysis or oxidation.
9
Similar NEET-style question
"The enzyme that catalyses the hydrolysis of a peptide bond in a protein is classified as: (a) Oxidoreductase (b) Transferase (c) Hydrolase (d) Isomerase." Answer: (c) Hydrolase — peptidases/proteases cleave peptide bonds using water. ---

Before solving, remember these

Definition

Enzymes

Biological catalysts (mostly proteins) that lower activation energy of biochemical reactions. Specific (lock-and-key model). Substrate binds at active site. Examples: pepsin, amylase, urease.

-- NCERT, p. 18

Formulas

DNA hydrogen bonds per base pair

A - T : 2; G - C : 3

Used to compute total H-bonds in a duplex of given GC%/AT% composition.

Symbol	Quantity	SI Unit
%GC	GC content	-

Valid when

Watson–Crick double helix

General formula of monosaccharides

C_{n} (H_{2} O)_{n}

Empirical formula of simple monosaccharides; glucose/fructose are C6H12O6.

Symbol	Quantity	SI Unit
n	carbon count	-

Valid when

Open-chain or cyclic forms of aldoses/ketoses

Peptide bonds in a polypeptide of N residues

# bonds = N - 1

Number of amide (peptide) bonds in a linear polypeptide of N amino acids.

Symbol	Quantity	SI Unit
N	residue count	-

Valid when

Linear (non-cyclic) polypeptide

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

DNA vs RNA base pairing

Category: Similar Terms

Student writes A=U for DNA or A=T for RNA. DNA: A=T, G≡C. RNA: A=U (no T), G≡C.

When it triggers

Question on base pairing or sugar identity.

How to avoid

DNA: deoxyribose, A-T-G-C bases. RNA: ribose, A-U-G-C bases (uracil instead of thymine). H-bond pairs: A=T (DNA) or A=U (RNA), G≡C (3 H-bonds, both).

Trap

Denaturation does NOT break primary structure

Category: Similar Terms

Student claims denaturation breaks peptide bonds. Denaturation only breaks H-bonds, ionic, hydrophobic interactions; primary structure (peptide bonds) intact.

When it triggers

Question about protein denaturation effects.

How to avoid

Denaturation: heat/pH/organic solvents disrupt secondary, tertiary, quaternary structure. Primary structure (covalent peptide bonds) requires hydrolysis to break.

Mistake

Claims denaturation breaks peptide bonds (primary structure).

Root cause: concept gap

Correction

Denaturation breaks H-bonds, ionic, hydrophobic; preserves peptide bonds. Hydrolysis (acid/base/enzyme) breaks primary structure.

Mistake

Writes A=U for DNA or A=T for RNA.

Root cause: concept gap

Correction

DNA: deoxyribose, A=T, G≡C. RNA: ribose, A=U (no T), G≡C.

Past Year Questions

6 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Match List-I with List-II. List-I List-II (Name of Vitamin) (Deficiency disease) A. Vitamin B I. Cheilosis 12 B. Vitamin D II. Convulsions C. Vitamin B III. Rickets 2 D. Vitamin B IV. Pernicious anaemia 6 Choose the correct answer from the options given below:

1A-IV, B-III, C-II, D-I

2A-I, B-III, C-II, D-IV

3A-IV, B-III, C-I, D-II

4A-II, B-III, C-I, D-IV

NTA Answer: Option 3(final)

NEET 2025

Given below are two statements : Statement-I : Benzenediazonium salt is prepared by the reaction of aniline with nitrous acid at 273 – 278 K. It decomposes easily in the dry state. Statement-II : Insertion of iodine into the benzene ring is difficult and hence iodobenzene is prepared through the reaction of benzenediazonium salt with KI. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but Statement II is correct

2Both Statement I and Statement II are correct

3Both Statement I and Statement II are incorrect

4Statement I is correct but Statement II is incorrect

NTA Answer: Option 2(final)

NEET 2024Revised key

Match List I with List II. List I List II (Reaction) (Reagents/Condition) A. I. B. II. CrO 3 C. III. KMnO /KOH,  4 D. IV. (i) O 3 (ii) Zn-H O 2 Choose the correct answer from the options given below:

1A-IV, B-I, C-III, D-II

2A-III, B-I, C-II, D-IV

3A-IV, B-I, C-II, D-III

4A-I, B-IV, C-II, D-III

NTA Answer: Option 3(revised_final)

NEET 2023

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R : Assertion A : A reaction can have zero activation energy. Reasons R : The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to threshold value, is called activation energy. In the light of the above statements, choose the correct answer from the options given below :

1Both A and R are true and R is NOT the correct explanation of A

2A is true but R is false

3A is false but R is true

4Both A and R are true and R is the correct explanation of A

NTA Answer: Option 3(final)

NEET 2022

The incorrect statement regarding enzymes is

1Enzymes are very specific for a particular reaction and substrate.

2Enzymes are biocatalysts.

3Like chemical catalysts enzymes reduce the activation energy of bio processes.

4Enzymes are polysaccharides.

NTA Answer: Option 4(final)

NEET 2021

An organic compound contains 78% (by wt.) carbon and remaining percentage of hydrogen. The right option for the empirical formula of this compound is : [Atomic wt. of C is 12, H is 1]

1CH

2CH 4

3CH

4CH 2 3

NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Biomolecules — recall and classify carbohydrates, proteins, nucleic acids, vitamins; identify base pairs and peptide bonds.

RecallMedium

Common distractors

swapped classes

Tempts surface-level recall.

Test yourself on this topic with real past-paper questions:

Practice this topic →

Free NEET study resources

Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.

Enzymes

Lesson

Practice MCQs

Quick recall before you leave

Peptide bonds = N − 1 (linear polypeptide of N residues)

Worked Example

Before solving, remember these

Formulas

Valid when

Valid when

Valid when

Exam Traps & Common Mistakes

When it triggers

How to avoid

When it triggers

How to avoid

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Free NEET study resources