How enzymes work
Every chemical reaction has an activation energy — the minimum energy needed for reactants to form products. Enzymes work by binding reactants (called the substrate) at a specific region called the active site, lowering the activation energy and making the reaction proceed faster.
Two complementary models describe this binding:
- Lock-and-key model (Fischer, 1894) — the substrate fits precisely into a rigid active site, like a key in a lock.
- Induced-fit model (Koshland, 1958) — the active site flexes slightly when the substrate binds, like a hand closing around a ball. This is now the accepted model.
After the reaction, the product is released and the enzyme is free to catalyse another reaction. A single enzyme molecule can catalyse millions of reactions per second.
See our cells and organelles guide to understand where enzyme reactions happen inside the cell.
Factors affecting enzyme activity
Enzyme activity is sensitive to its environment. Four key factors:
Temperature
Activity rises to an optimum (~37 °C in humans) then falls as the enzyme denatures and the active site loses its shape.
pH
Each enzyme has an optimum pH. Pepsin works best at pH 2; most intracellular enzymes at pH 7–8. Extreme pH disrupts ionic bonds.
Substrate concentration
More substrate increases rate — up to the point where all active sites are occupied (Vmax). Adding more substrate after that has no effect.
Enzyme concentration
More enzyme molecules mean more active sites available, increasing rate — as long as substrate is not the limiting factor.
Enzyme inhibition
Inhibitors reduce enzyme activity. They have major roles in metabolism (feedback inhibition) and in medicine (many drugs are enzyme inhibitors).
- Competitive inhibition — the inhibitor has a similar shape to the substrate and competes for the active site. Its effect is reversible and can be overcome by increasing substrate concentration.
- Non-competitive inhibition — the inhibitor binds to an allosteric site (not the active site), changing the enzyme's overall shape and reducing activity. Adding more substrate does not help.
Example: aspirin irreversibly inhibits the enzyme COX-2, which produces prostaglandins involved in inflammation and pain.