The Six Enzyme Classes (EC 1 to EC 6)
Mnemonic: "Over The Hill Lies Ice Lake" → O-T-H-L-I-L
| EC # | Class | Reaction Type | Example Enzyme | Example Reaction |
|---|---|---|---|---|
| EC 1 | Oxidoreductases | Oxidation-reduction (redox) | Alcohol dehydrogenase | Ethanol → Acetaldehyde (N reduced to NADH) |
| EC 2 | Transferases | Group transfer between molecules | Transaminase | Amino group transfer between amino acids |
| EC 3 | Hydrolases | Hydrolysis (cleavage using water) | Lipase | Triglycerides → Glycerol + Fatty acids |
| EC 4 | Lyases | Non-hydrolytic, non-redox bond cleavage/addition | Aldolase | Fructose-1,6-bisphosphate → G3P + DHAP |
| EC 5 | Isomerases | Isomerization (molecular rearrangement) | Phosphoglucose isomerase | Glucose-6-P ↔ Fructose-6-P |
| EC 6 | Ligases | Bond formation using ATP | DNA ligase | Joins DNA fragments using ATP |
Key Points for NEET
- DNA ligase → Ligase (EC 6) → Uses ATP → joins Okazaki fragments
- Lipase → Hydrolase (EC 3) → uses → digests fats
- Aldolase → Lyase (EC 4) → no water, no ATP → cleaves C-C bond
- Transaminase → Transferase (EC 2) → transfers amino groups
- Dehydrogenase → Oxidoreductase (EC 1) → redox (gains/loses H or electrons)
- Phosphoglucose isomerase → Isomerase (EC 5) → rearranges without adding/removing atoms
Quick Identification Rule
- Water used to BREAK bond → Hydrolase
- ATP used to FORM bond → Ligase
- Electron transfer → Oxidoreductase
- Group moves from one molecule to another → Transferase
- Bond broken WITHOUT water or ATP → Lyase
- Same atoms, different arrangement → Isomerase