Enzymes
NCERT Class 12 Chemistry Chapter 10 Section 10.3
Quick answer
Enzymes are biocatalysts, almost all globular proteins; they are very specific for a reaction and a substrate, are needed only in small amounts and lower the activation energy, and their names usually end in –ase, as maltase hydrolyses maltose. In enzyme action the substrate binds at the active site to form an enzyme–substrate complex, which gives the product and releases the unchanged enzyme: E + S ⇌ ES → EP → E + P.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 10 “Biomolecules”, Section 10.3 “Enzymes”, page 295.
Open the official NCERT chapter (PDF, 3.1 MB, ncert.nic.in) (opens in a new tab)
Checked against the 2026-27 reprint on .
What are enzymes?
Enzymes
Enzymes are biocatalysts. Digesting food, absorbing the right molecules and producing energy all take a sequence of reactions, and in the body these happen under very mild conditions because enzymes speed them up (NCERT Section 10.3, page 295). On their chemical nature NCERT says:
Almost all the enzymes are globular proteins.
NCERT Chemistry Part II (Class 12), Section 10.3, page 295
In plain words: an enzyme is a folded protein with a pocket that fits one kind of molecule, and it helps that molecule react far faster than it would alone, without being used up.
NCERT says “almost all” because a few enzymes are not proteins: some nucleic acids, called ribozymes, act as enzymes (NCERT Class 11 Biology, Chapter 9, page 112).
Properties and naming
- Very specific: each enzyme works on a particular reaction and a particular substrate (NCERT page 295).
- Naming: an enzyme is usually named after the compound it acts on, as maltase hydrolyses maltose to glucose. Sometimes it is named after the reaction: oxidoreductases oxidise one substrate while reducing another. Enzyme names end in –ase (NCERT page 295), with a few older exceptions such as trypsin, which NCERT’s Class 11 Biology book lists as an enzyme (Chapter 9, Table 9.5, page 109).
- Small amounts: only small quantities are needed (NCERT page 295).
- Other enzymes in NCERT: invertase turns sucrose into glucose and fructose, and zymase, from yeast, ferments them to ethanol (NCERT Unit 7, page 214; see ethanol by fermentation). Carbonic anhydrase and carboxypeptidase A contain coordinated metal ions (NCERT Unit 5, page 137).
- Lower activation energy: like chemical catalysts, enzymes reduce the activation energy. NCERT’s example is the hydrolysis of sucrose: 6.22 kJ mol−1 with acid, but only 2.15 kJ mol−1 with the enzyme sucrase (NCERT page 295). A catalyst works this way by giving an alternative pathway with a lower barrier (NCERT Unit 3, page 82; see catalysts and activation energy).
- Note on NCERT’s numbers: 6.22 and 2.15 kJ mol−1 are much smaller than the activation energies in NCERT’s own Unit 3 problems, such as 60 kJ mol−1 (page 88) and 209.5 kJ mol−1 (page 84). Taken at face value, they would make the enzyme only about five times faster (Example 1), while NCERT’s Class 11 Biology book reports that another enzyme, carbonic anhydrase, speeds up its reaction about 10 million times (page 114). They would also make the acid hydrolysis only about 8 per cent faster for a 10 K rise, where NCERT’s rule of thumb is nearly double (Example 2). In an exam, quote NCERT’s values if asked, as given.
Maltase
C12H22O11 (maltose) + H2O → 2C6H12O6 (glucose)
NCERT draws this as a scheme, with maltase over the arrow and no water (page 295); the water is added here so that the equation balances.
Mechanism of enzyme action
- NCERT slip (page 295): the Chemistry book says that the mechanism of enzyme action “has been discussed”, but the 2026–27 Class 12 Chemistry books do not discuss it anywhere else. NCERT’s Class 11 Biology book does (Chapter 9, Section 9.8.3, page 115), so the mechanism below follows that book. For an exam answer on the mechanism, use the account below.
- Each enzyme, E, has a binding site, the active site: a crevice or pocket in the folded protein into which the substrate, S, fits (NCERT Class 11 Biology, page 113).
- The enzyme and substrate form a short-lived, highly reactive enzyme–substrate complex, ES. This passes through an enzyme–product complex, EP, which releases the product, P, and the unchanged enzyme. The ES complex is essential for catalysis (NCERT Class 11 Biology, page 115).
- The cycle in four steps (NCERT Class 11 Biology, pages 115–116):
- the substrate binds to the active site, fitting into it;
- the binding makes the enzyme change shape and fit more tightly round the substrate;
- the active site breaks bonds in the substrate, and the enzyme–product complex forms;
- the enzyme releases the product and is free to bind another substrate molecule.
- The substrate must pass through a high-energy transition state, and the enzyme lowers this energy barrier, the activation energy (NCERT Class 11 Biology, pages 114–115). See the Class 11 Biology page on how enzymes speed up reactions for the energy diagram.
Enzyme action
E + S ⇌ ES → EP → E + P
Solved examples
Example 1: How much faster, by NCERT’s numbers?
Take NCERT’s activation energies for the hydrolysis of sucrose, 6.22 kJ mol−1 with acid and 2.15 kJ mol−1 with sucrase, and the same Arrhenius factor A for both. By what factor does the enzyme raise the rate constant at 298 K? (R = 8.314 J K−1 mol−1.)
Answer: From k = A e−Ea/RT (NCERT Unit 3, page 79), kenzyme/kacid = e(6220 − 2150)/(8.314 × 298) = e4070/2478 = e1.643 ≈ 5.2 (reasoned; the equal A is an assumption).
Example 2: A 10 K rise with NCERT’s value
Suppose the acid hydrolysis of sucrose had Ea = 6.22 kJ mol−1, as NCERT gives. By what factor would its rate constant rise from 298 K to 308 K? NCERT’s rule of thumb is that the rate constant nearly doubles for a 10° rise (Unit 3, page 78). (R = 8.314 J K−1 mol−1.)
Answer: ln(k308/k298) = (Ea/R)(1/298 − 1/308) = (6220/8.314) × 1.090 × 10−4 = 748.1 × 1.090 × 10−4 = 0.0815, so the factor is e0.0815 ≈ 1.08: only about 8 per cent faster, not double. Doubling from 298 K to 308 K needs Ea of about 53 kJ mol−1 (NCERT’s answer to Intext Question 3.8, page 88), so with 6.22 kJ mol−1 warming would barely speed up the acid hydrolysis (reasoned from NCERT Unit 3, pages 78–79).
Common mistakes to avoid
Common mistake
Saying an enzyme is used up
The enzyme comes out of the cycle unchanged and binds the next substrate molecule, so a small amount does a lot of work.
Common mistake
Naming every enzyme after its substrate
Many are (maltase, sucrase), but some are named after the reaction they catalyse, such as the oxidoreductases.
Quick revision summary
- Enzymes are biocatalysts and almost all are globular proteins. They are highly specific, needed in small amounts, and lower the activation energy; their names usually end in –ase (trypsin does not).
- Naming: by substrate (maltase hydrolyses maltose to glucose) or by reaction (oxidoreductase).
- NCERT’s figures for sucrose hydrolysis: 6.22 kJ mol−1 (acid) and 2.15 kJ mol−1 (sucrase).
- Mechanism (from NCERT Class 11 Biology): the substrate binds at the active site, ES → EP → E + P, and the enzyme is regenerated.
Formula sheet
- E + S ⇌ ES → EP → E + P
- CHO + HO (maltase) → 2CHO
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Why is the enzyme that hydrolyses maltose called maltase?
Show answerHide answer
Enzymes are often named after the compound they act on, with the ending –ase; maltase acts on maltose, hydrolysing it to glucose (NCERT page 295).
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Q2. In E + S ⇌ ES → EP → E + P, what are ES and EP, and at which step of the catalytic cycle does EP form?
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ES is the enzyme–substrate complex and EP the enzyme–product complex. EP forms at the third step, when the active site breaks the chemical bonds of the bound substrate; at the fourth, the enzyme releases the product and is free again (NCERT Class 11 Biology, pages 115–116).