Key ideas
- Catalyst. An enzyme speeds up a reaction and comes out unchanged, so one enzyme can work on many substrate molecules.
- Activation energy. Enzymes lower it. They do not change how much energy the reaction releases or absorbs overall.
- Active site and induced fit. The substrate binds in the active site, and the enzyme shifts its shape slightly to grip it more tightly. OpenStax calls this induced fit, a refinement of the older lock-and-key model.
- Helpers. Some enzymes need cofactors (inorganic ions such as or ) or coenzymes (organic molecules, often made from vitamins).
| Factor | Effect |
|---|---|
| Temperature | Warmer speeds reactions up to the optimum. Above it, the enzyme's shape breaks down (denatures) and activity falls fast. |
| pH | Each enzyme has an optimum pH. Far from it, the active site's charges change and the substrate no longer fits well. |
| Substrate concentration | More substrate means more collisions, until every enzyme is busy (saturated). Then the rate levels off. |
| Competitive inhibitor | Looks like the substrate and blocks the active site. Adding lots of substrate can outcompete it. |
| Noncompetitive inhibitor | Binds somewhere else (an allosteric site) and changes the enzyme's shape. More substrate does not help. |
Worked examples
Example 1: find the optimum temperature from data
Problem A student measures how fast an enzyme makes product at different temperatures: 10 °C: 2.1, 20 °C: 4.0, 30 °C: 7.2, 37 °C: 9.5, 45 °C: 6.1, 55 °C: 0.4 (all in µmol per minute). What is the optimum temperature, and what happened at 55 °C?
- The optimum is where the rate is highest: 9.5 µmol/min at 37 °C.
- Below 37 °C, rising temperature means faster molecules and more collisions, so the rate climbs.
- Above 37 °C the rate drops sharply. At 55 °C it is almost zero because heat has denatured the enzyme and changed its active site.
Answer 37 °C is the optimum; at 55 °C the enzyme is denatured
Example 2: calculate a reaction rate
Problem In the first 8.0 minutes of an experiment, an enzyme produces 24 µmol of product. What is the average rate?
- Rate is the amount of product divided by the time.
Answer 3.0 µmol/min
Example 3: which kind of inhibitor?
Problem An inhibitor slows an enzyme. When the student adds a lot more substrate, the rate climbs back almost to normal. What kind of inhibitor is it?
- A competitive inhibitor competes for the active site. With much more substrate, substrate molecules win most of the time.
- A noncompetitive inhibitor changes the enzyme's shape from another site, so extra substrate would not restore the rate.
Answer A competitive inhibitor
Common mistakes and how to fix them
- Saying enzymes are used up. They are catalysts and are reused. Only the substrates are changed.
- Saying high temperature kills an enzyme. Enzymes are not alive. Heat denatures them: their 3D shape unfolds.
- Saying enzymes make reactions happen that otherwise could not. They speed up reactions that could happen anyway, by lowering the activation energy.
- Thinking all enzymes have the same optimum. Optimum temperature and pH depend on where the enzyme works. A stomach enzyme works best in strong acid, while most enzymes in your blood work best near neutral pH.
Practice questions
How do enzymes speed up reactions?
- They add energy to the reaction
- They lower the activation energy
- They raise the temperature
- They change the products
Show answer
Answer: They lower the activation energy
Enzymes make it easier to reach the transition state. They do not change the overall energy of the reaction or what it makes.
An enzyme makes 18 µmol of product in 4.0 minutes. What is the average rate?
Show answer
Answer: 4.5 µmol/min
Rate = µmol per minute.
A stomach enzyme's activity is measured at pH 1: 3.1, pH 2: 5.8, pH 3: 4.0, pH 4: 1.5 and pH 5: 0.3 (µmol/min). What is its optimum pH?
- pH 1
- pH 2
- pH 3
- pH 5
Show answer
Answer: pH 2
The rate is highest at pH 2 (5.8 µmol/min). It falls on both sides of the optimum.
What happens to an enzyme at a temperature far above its optimum?
- It works faster forever
- It denatures and its active site changes shape
- It turns into its substrate
- It becomes a competitive inhibitor
Show answer
Answer: It denatures and its active site changes shape
High heat breaks the weak bonds holding the protein's shape, so the substrate no longer fits.
An inhibitor binds to a site away from the active site and changes the enzyme's shape. What kind of inhibition is this?
- Competitive
- Noncompetitive
- Feedback activation
- Denaturation
Show answer
Answer: Noncompetitive
Noncompetitive inhibitors bind at an allosteric site. Adding more substrate does not undo them.
Frequently asked questions
Are all enzymes proteins?
Almost all are. A few RNA molecules, called ribozymes, also act as catalysts. The ribosome's key step of linking amino acids is done by RNA. Most high school courses treat enzymes as proteins, which is true for nearly every enzyme you will study.
What is feedback inhibition?
It is when the end product of a pathway slows down an enzyme early in that pathway. When the cell has plenty of the product, production slows. When the product runs low, the enzyme speeds back up. It keeps the cell from wasting energy making more than it needs.
Why do enzyme names end in -ase?
It is a naming convention: the name often tells you the substrate or the job. Lactase breaks down lactose, DNA polymerase builds DNA polymers, and helicase unwinds the DNA helix. Some older names, like pepsin, do not follow the rule.
Sources
- OpenStax Biology 2e, 6.5 Enzymes, accessed October 1, 2026
