Classification
The International Union of Biochemistry and Molecular Biology sorts every enzyme into one of six classes based on the reaction it catalyzes. The MCAT will not ask for EC numbers, but you absolutely must recognize the six classes and what each one does.
The Six Classes
| Class | What it does | Common suffix / name clue | Example |
|---|---|---|---|
| 1. Oxidoreductases | Transfer electrons (redox) | “dehydrogenase”, “oxidase”, “reductase” | Lactate dehydrogenase |
| 2. Transferases | Move a functional group from one molecule to another | ”kinase” (moves phosphate), “transaminase” | Hexokinase |
| 3. Hydrolases | Break bonds using water | ”ase” on a substrate name (lipase, protease, amylase) | Trypsin |
| 4. Lyases | Break bonds without water, often leaving a double bond | ”synthase”, “decarboxylase”, “aldolase” | Pyruvate decarboxylase |
| 5. Isomerases | Rearrange atoms within a molecule (no net change in formula) | “isomerase”, “mutase” | Phosphoglucose isomerase |
| 6. Ligases | Join two molecules using ATP hydrolysis | ”synthetase”, “ligase” | DNA ligase, pyruvate carboxylase |
How to Tell Which Class from the Name
Enzyme names are often transparent once you know the suffix rules.
- -dehydrogenase removes H atoms (electrons + protons). Oxidoreductase. Example: alcohol dehydrogenase, lactate dehydrogenase.
- -kinase transfers a phosphate (usually from ATP). Transferase. Example: hexokinase, protein kinase A.
- -ase on a substrate name (lipase = breaks lipids, lactase = breaks lactose) is usually a hydrolase. It adds water across a bond.
- -decarboxylase removes CO2 without adding water. Lyase. Example: pyruvate decarboxylase.
- -synthase (no ATP) is a lyase. -synthetase (uses ATP) is a ligase. This one trips students up - the single extra letter changes the class.
- -isomerase or -mutase rearranges atoms. Isomerase. Example: triose phosphate isomerase, phosphoglycerate mutase.
- -ligase or -carboxylase (with ATP) joins things. Ligase. Example: pyruvate carboxylase, DNA ligase.
One-Sentence Examples per Class
- Oxidoreductase: lactate dehydrogenase moves H (and electrons) between lactate and pyruvate, paired with NAD+/NADH.
- Transferase: hexokinase transfers a phosphate from ATP onto glucose, producing glucose-6-phosphate.
- Hydrolase: trypsin adds water across a peptide bond, cleaving a protein into fragments.
- Lyase: fructose-1,6-bisphosphate aldolase cleaves a six-carbon sugar into two three-carbon pieces without water.
- Isomerase: triose phosphate isomerase interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
- Ligase: DNA ligase seals a nick in the DNA backbone, using ATP to drive the joining.
Which enzyme class uses water to break a bond, and which uses ATP to form a bond?
Click to reveal answer
Hydrolases (class 3) use water to cleave bonds. Ligases (class 6) use ATP to form bonds between two substrates. The two are, in effect, thermodynamic opposites.
An enzyme rearranges glucose-6-phosphate into fructose-6-phosphate without changing the atomic formula. What class is it?
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Isomerase (class 5). This particular enzyme is phosphoglucose isomerase. No atoms are added or removed - the molecule is simply rearranged.
Is glycogen synthase a lyase or a ligase?
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Neither exactly as named - the "synthase" suffix suggests a lyase, but glycogen synthase actually uses UDP-glucose (an activated sugar) and is classified as a glycosyltransferase. The rule "synthase = lyase, synthetase = ligase" is a good starting point, but not every enzyme obeys it perfectly.