What are enzymes?
Enzymes are biological catalysts made of protein. They speed up the rate of chemical reactions in living organisms but are not used up, so a small amount can be used again and again.
Key idea
An enzyme lowers the activation energy needed for a reaction, so the reaction happens faster at body temperature. Enzyme + substrate → enzyme-substrate complex → enzyme + product(s).
The lock-and-key model
Each enzyme has a region called the active site with a specific shape. Only a substrate with a complementary shape can fit into it, like a key fitting a lock. This is why enzymes are specific — one enzyme usually works on only one substrate. The substrate binds to form an enzyme-substrate complex; the reaction takes place, and the products are released, leaving the enzyme unchanged.
Some named enzymes
- Amylase breaks down starch into maltose (sugar).
- Protease breaks down protein into amino acids.
- Lipase breaks down fats into fatty acids and glycerol.
- Catalase breaks down hydrogen peroxide into water and oxygen.
Enzymes control almost every reaction in a cell, including those that build large molecules as well as those that break them down. Because only a matching substrate fits the active site, a cell can control its many reactions separately by making different enzymes. A tiny amount of enzyme can catalyse a very large number of reactions, one after another, since the enzyme is released unchanged each time and is free to bind another substrate molecule.
Remember
- Enzymes are proteins and catalysts — not used up in the reaction.
- Only a matching substrate fits the active site (specificity).
- Enzymes lower activation energy; they do not raise it.