Lesson 5 of 7 / Biological molecules
How enzymes speed up reactions
Why does one enzyme act on a particular substrate?
In this lesson: Explain specificity and enzyme action using the active site, enzyme-substrate complex and activation energy.
About 5 min
The key ideaA suitable substrate binds to an enzyme's active site. The enzyme lowers the activation energy for the reaction and is available to act again after the products leave.
A substrate is the substance an enzyme acts on. Activation energy is the energy barrier that reactants must overcome for a reaction to occur.
Explore the idea
Fit, react, release
The substrate has a complementary shape. It can bind at this active site.
Orange curve: without enzyme. Green curve: with enzyme. Original lock-and-key and energy models; neither diagram is to scale. Lower activation energy increases rate without the enzyme supplying extra energy.
Explanation
An enzyme is a biological catalyst, usually a protein. It speeds up a reaction without being used up in the reaction. An enzyme has an active site: a region with a shape that can bind its substrate.
In the lock-and-key model, the substrate has a shape complementary to the active site. A suitable substrate fits and forms an enzyme-substrate complex. A different molecule that does not fit cannot form the same productive complex, explaining enzyme specificity.
The enzyme provides a route for the reaction with a lower activation energy. More substrate molecules can therefore react in a given time at the same temperature. The enzyme does not supply the reaction with extra energy or need to be consumed to make this happen.
The reaction changes the substrate into products. These leave the active site, and the enzyme becomes available for another substrate. Some enzymes break molecules down; others help join molecules. The same principles of binding, catalysis and release apply.
The lock-and-key picture is a useful model for this syllabus. It emphasises matching and specificity, rather than describing every movement of a real enzyme molecule. The energy diagram is also a comparison of reaction pathways, not a graph of reaction speed against time.
Step by step
- 1
Bind the substrate
A complementary substrate fits the active site and forms an enzyme-substrate complex.
- 2
Lower the energy barrier
The enzyme allows the reaction to occur with lower activation energy, increasing its rate.
- 3
Release the products
Products leave. The enzyme is not used up and can bind another substrate.
Worked example
Why maltase does not digest starch
Maltase catalyses the breakdown of maltose. Explain why the same active site does not digest a starch molecule.
One way to explain it
The active site of maltase is complementary to maltose. Maltose can bind and form an enzyme-substrate complex. Starch does not have the appropriate fit at this active site, so maltase does not catalyse its digestion.
Why this answer works
- Name the specific substrate.
- Explain complementarity and complex formation.
- Connect a poor fit to the absence of catalysis by this enzyme.
Is this true? "Enzymes speed reactions by giving energy to the substrate."
Enzymes lower the activation energy of the reaction. They do not supply extra energy to the substrate or get used up as a fuel.