Biology
Enzymes — quick study summary
Enzymes are biological catalysts (usually proteins) that lower activation energy and speed up chemical reactions without being consumed. Each enzyme has an active site shaped to fit a specific substrate. Activity depends on temperature, pH, substrate concentration and the presence of inhibitors or cofactors.
Key points
- Lower activation energy without changing reaction equilibrium
- Specificity comes from active-site shape (lock-and-key + induced fit)
- Each enzyme has an optimum temperature and pH
- Competitive inhibitors bind the active site; non-competitive bind elsewhere
- Denaturation = permanent loss of active-site shape (high temp / extreme pH)
In more depth
How enzymes speed up reactions
Enzymes are biological catalysts, usually proteins. They lower the activation energy a reaction needs, so it runs quickly at body temperature. The substrate binds to the active site, forming an enzyme-substrate complex. In the induced-fit model, the active site changes shape slightly as the substrate binds, straining its bonds and making the reaction easier. The enzyme is unchanged at the end and can be reused.
Factors that affect enzyme activity
The rate increases with temperature up to an optimum. Above it, the bonds holding the protein's shape break, the active site changes and the enzyme denatures. Each enzyme also has an optimum pH. Increasing substrate concentration raises the rate until every active site is occupied, after which the amount of enzyme becomes the limit.
Inhibitors
Competitive inhibitors resemble the substrate and compete for the active site, so adding more substrate can overcome them. Non-competitive inhibitors bind elsewhere on the enzyme and change the shape of the active site, so extra substrate does not restore the rate.
Common mistakes
- Saying enzymes are 'killed' by heat. Enzymes are not alive; they denature.
- Thinking enzymes are used up in reactions. They are catalysts and are reused.
- Assuming a higher temperature always means a faster reaction. Beyond the optimum, the rate falls sharply.
Practice quiz
Click each question to reveal the answer.
1. What does an enzyme lower to speed up a reaction?
Answer: Activation energy
2. Where does the substrate bind on an enzyme?
- Allosteric site
- Active site
- Membrane
- Nucleus
Answer: Active site
3. What permanently destroys an enzyme's function at high temperatures?
Answer: Denaturation
Heat disrupts hydrogen and ionic bonds, distorting the active site irreversibly.
Last reviewed: May 2026