Step by Step
1
Oxidoreductases — electron transfer
Catalyze oxidation-reduction reactions, transferring electrons (often as hydride ions or hydrogen atoms) from one molecule to another. Dehydrogenases and oxidases both fall in this class.
2
Transferases — moving functional groups
Transfer a functional group (an amino group, phosphate group, or methyl group, for example) from one molecule to another. Kinases, which transfer a phosphate group from ATP, are one of the most clinically important examples.
3
Hydrolases — cleaving bonds with water
Break a bond by adding a water molecule across it (hydrolysis). Proteases, lipases, and amylases are all hydrolases — most digestive enzymes belong to this class.
Trypsin, a digestive protease, hydrolyzes peptide bonds in dietary protein.
4
Lyases — cleaving bonds without water
Break a bond by a mechanism other than hydrolysis or oxidation, often forming a double bond or ring in the process, without requiring a water molecule. Decarboxylases, which remove a carboxyl group as CO₂, are a common example.
5
Isomerases — rearranging within one molecule
Convert a molecule into one of its isomers — same atoms, rearranged structure — without adding or removing anything. Triose phosphate isomerase, a key glycolysis enzyme, interconverts two three-carbon sugar isomers.
6
Ligases — joining two molecules using ATP
Join two separate molecules together into one, always using energy from ATP hydrolysis to drive the reaction. DNA ligase, which seals breaks in the DNA backbone, is the classic example.
Applied Walkthrough
1
You're given an unfamiliar enzyme name and asked to predict its class from its function alone: "an enzyme that adds a phosphate group from ATP to a protein."
2
The key clue is "transfers a functional group" — specifically a phosphate group, moved from one molecule (ATP) to another (the protein).
3
That function — moving a group from one molecule to another — is the defining feature of a transferase, not a hydrolase (no water is added across a bond here) or a ligase (this isn't joining two large molecules into one).
4
This enzyme would correctly be classified as a kinase, a specific type of transferase — which is exactly why most enzyme names ending in "-kinase" signal the transferase class.
Exam Application
Exams commonly test whether you can name all six classes in order, match a given enzyme name or reaction description to its correct class, and correctly distinguish hydrolases (water added) from lyases (no water involved) — the pair students confuse most often.
⚠ Common Trap
Don't confuse hydrolases and lyases just because both "break bonds." Hydrolases specifically use a water molecule to break the bond (hydrolysis); lyases break bonds without water, typically forming a double bond or ring instead. If the mechanism involves adding H₂O across the bond, it's a hydrolase — if not, it's a lyase.
✓ Quick Self-Check
1. What does OT HaLIL stand for?
Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
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2. Which class catalyzes electron-transfer (oxidation-reduction) reactions?
Oxidoreductases.
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3. What's the key difference between a hydrolase and a lyase?
Hydrolases break bonds by adding water (hydrolysis); lyases break bonds without water, often forming a double bond or ring.
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4. What class do enzymes ending in "-kinase" belong to, and why?
Transferases — kinases transfer a phosphate group from ATP to another molecule.
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5. What energy source do ligases always require to join two molecules?
ATP (ATP hydrolysis drives the joining reaction).
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