Enzyme classification — oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases

Question

Classify enzymes into six major classes based on the type of reaction they catalyse. Give one example of each class with the specific reaction it catalyses. How does the enzyme nomenclature system work?

(NEET + CBSE Board pattern)

Solution — Step by Step

Class	Reaction Type	What It Does	Example
1. Oxidoreductases	Oxidation-reduction	Transfer electrons (H atoms) between molecules	Alcohol dehydrogenase (ethanol → acetaldehyde)
2. Transferases	Group transfer	Transfer a functional group from one molecule to another	Transaminase (transfers amino group between amino acids)
3. Hydrolases	Hydrolysis	Break bonds using water	Lipase (triglyceride + H₂O → glycerol + fatty acids)
4. Lyases	Non-hydrolytic bond cleavage	Remove groups to form double bonds (or add groups to double bonds)	Aldolase (fructose-1,6-bisphosphate → two 3-C fragments)
5. Isomerases	Isomerization	Rearrange atoms within a molecule (same formula, different structure)	Phosphoglucose isomerase (glucose-6-P → fructose-6-P)
6. Ligases	Bond formation using ATP	Join two molecules with energy from ATP hydrolysis	DNA ligase (joins DNA fragments)

Each enzyme gets a 4-digit EC number (Enzyme Commission number):

First digit = class (1-6)
Remaining digits specify the sub-class, sub-sub-class, and serial number

For example: EC 1.1.1.1 = Alcohol dehydrogenase

1 = Oxidoreductase
1.1 = acts on CH-OH group
1.1.1 = uses NAD⁺ as acceptor
1.1.1.1 = specific for alcohol

For NEET, you need to know the six classes and their reaction types — the EC numbers are not tested directly.

The name of the enzyme class tells you the reaction:

Oxido-reductase = oxidation + reduction
Trans-ferase = transfer
Hydro-lase = hydrolysis (water-mediated breaking)
Ly-ase = lysis (breaking without water)
Iso-merase = isomer formation
Lig-ase = ligation (joining)

graph TD
    A[Enzyme Classes] --> B["1. Oxidoreductases"]
    A --> C["2. Transferases"]
    A --> D["3. Hydrolases"]
    A --> E["4. Lyases"]
    A --> F["5. Isomerases"]
    A --> G["6. Ligases"]
    B --> B1["Redox reactions"]
    C --> C1["Group transfer"]
    D --> D1["Break bonds with H2O"]
    E --> E1["Break bonds without H2O"]
    F --> F1["Rearrange structure"]
    G --> G1["Join molecules using ATP"]
    style A fill:#fbbf24,stroke:#000,stroke-width:2px

Why This Works

The six-class system is based on the chemical reaction catalysed, not the enzyme’s structure or source. This makes it universally applicable — any newly discovered enzyme can be classified by asking: “What type of reaction does it catalyse?”

The most commonly encountered class in biology is hydrolases (digestive enzymes like pepsin, trypsin, and lipase all hydrolyse their substrates). Ligases are critical in molecular biology (DNA ligase joins Okazaki fragments during replication).

Common Mistake

Students confuse lyases with hydrolases. Both break bonds, but the mechanism differs. Hydrolases use WATER to break bonds (hydrolysis). Lyases break bonds WITHOUT water — they remove groups to form double bonds or add groups to double bonds. The classic NEET trap: “Which class of enzyme breaks a C-C bond without using water?” — the answer is lyase, not hydrolase.

Mnemonic for the six enzyme classes in order: “Over The Hill Lies Ice and Lakes” — Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases. Alternatively: OTHLIL — just the first letters. This order matches the EC numbering (1-6).