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NCERT describes the six enzyme classes in a few lines each, with almost no named enzymes. That is why a question like "carbonic anhydrase belongs to which class?" catches so many students. This post gives each class a one-line test you can apply to any reaction, real enzymes with their official numbers, and six reactions to classify yourself.
Where are enzyme classes in the syllabus?
The NEET (UG) 2026 syllabus lists "Enzymes-types, properties, enzyme action, classification and nomenclature of enzymes". The text is section 9.8.5 of the NCERT Class 11 chapter Biomolecules (2026-27 reprint). The CBSE Class XI curriculum for 2026-27 lists enzyme types, properties and action for the same chapter. Check the syllabus for your exam year.
NCERT says enzymes are divided into 6 classes, each with 4 to 13 subclasses, and are "named accordingly by a four-digit number". In practice this is the EC number: four numbers separated by dots, such as EC 3.4.17.1. The first number is the class.
What does each class do, in one test?
Oxidoreductases (EC 1). Is one substrate oxidised while another is reduced? Look for NAD⁺ becoming NADH. NCERT also calls them dehydrogenases.
Transferases (EC 2). Does a group other than hydrogen move from one molecule to another?
Hydrolases (EC 3). Is water used to split a bond into two products? NCERT lists ester, ether, peptide, glycosidic, C–C, C–halide and P–N bonds.
Lyases (EC 4). Is a group removed without hydrolysis, leaving a double bond?
Isomerases (EC 5). One substrate in, one isomer out, with the same formula.
Ligases (EC 6). Are two molecules joined while ATP is broken down?
The ATP part of test 6 is not in NCERT's sentence, but it is in the official definition. The IUBMB nomenclature committee's introduction to EC 6 says ligases join two molecules "with concomitant hydrolysis of the diphosphate bond in ATP or a similar triphosphate". Its introduction to EC 4 says lyases cleave C–C, C–O, C–N and other bonds "by other means than by hydrolysis or oxidation".
Which real enzymes belong to each class?
These numbers come from ExplorEnz, the IUBMB enzyme database. Several are enzymes NCERT uses elsewhere.
EC 1: alcohol dehydrogenase (EC 1.1.1.1); succinate dehydrogenase (EC 1.3.5.1), NCERT's competitive inhibition example; catalase (EC 1.11.1.6), which turns hydrogen peroxide into oxygen and water.
EC 2: hexokinase (EC 2.7.1.1), moving phosphate from ATP to a hexose; DNA polymerase (EC 2.7.7.7), adding nucleotides to DNA.
EC 3: α-amylase (EC 3.2.1.1), splitting glucosidic links in starch; carboxypeptidase A (EC 3.4.17.1), the zinc enzyme NCERT gives as its metal-ion cofactor example.
EC 4: carbonic anhydrase (EC 4.2.1.1), listed with the reaction H₂CO₃ = CO₂ + H₂O; RuBisCO, ribulose-bisphosphate carboxylase (EC 4.1.1.39).
EC 5: triose-phosphate isomerase (EC 5.3.1.1), interconverting the two three-carbon sugar phosphates of glycolysis.
EC 6: DNA ligase (EC 6.5.1.1), which seals DNA using ATP; pyruvate carboxylase (EC 6.4.1.1), which uses ATP to add bicarbonate to pyruvate.
Worked example: classify six reactions
Each reaction below is the listed reaction of a real enzyme. Apply the tests before reading the answer.
(a) Lactose + H₂O → galactose + glucose. Water splits one molecule into two: hydrolase (lactase, EC 3.2.1.108).
(b) Glucose 6-phosphate → fructose 6-phosphate. One substrate, one product, same formula: isomerase (EC 5.3.1.9).
(c) Malate + NAD⁺ → oxaloacetate + NADH + H⁺. Malate is oxidised and NAD⁺ reduced: oxidoreductase (EC 1.1.1.37).
(d) Malate → fumarate + H₂O. Water is removed and a double bond forms, with no second product: lyase (fumarate hydratase, EC 4.2.1.2).
(e) ATP + glutamate + NH₃ → ADP + phosphate + glutamine. Two molecules joined, ATP broken: ligase (glutamine synthetase, EC 6.3.1.2).
(f) Aspartate + 2-oxoglutarate → oxaloacetate + glutamate. An amino group moves between molecules: transferase (aspartate transaminase, EC 2.6.1.1).
Reaction (d) is the trap. Water appears, so many students say hydrolase. But a hydrolase uses water to break one molecule into two; fumarate hydratase removes or adds water across a double bond, which is lyase behaviour.
Which mistakes cost marks?
"Water means hydrolase." Check whether water splits a bond into two products or is added across a double bond.
Calling every enzyme that builds something a ligase. DNA polymerase builds DNA but is a transferase; it moves nucleotide units. Ligases need ATP or a similar triphosphate.
Trusting the name. Carbonic anhydrase sounds like a hydrolase and is a lyase.
Reading "four-digit number" literally. EC numbers have four parts, and parts can be more than one digit, as in EC 1.11.1.6.
Stopping at six outside the exam. ExplorEnz now lists a seventh class, translocases (EC 7), which catalyse the translocation of hydrons, ions and other molecules. NCERT and the exam still use six.
A practice routine
Classifying is category learning, and mixing examples helps it. In Kornell and Bjork (2008), people who studied paintings by several artists interleaved were better at naming the artist of new paintings than those who saw each artist's work in a block, even though they felt the blocked method worked better.
Write 18 cards, three enzymes per class, with the reaction on the front and the class on the back.
Shuffle all classes together every session.
For each card, say which test from the list above decided it.
Revisit NCERT's cofactor section and add each cofactor example to its class.
If a reaction type such as oxidation-reduction is the real gap, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can fix the chemistry first. For enzyme kinetics and inhibition, read our guide to enzymes for biology.
Tonight, classify reactions (a) to (f) again from memory, then check which test you used for each.
