Lesson 5 of 7 / Biological molecules
How enzymes speed up reactions
Why does one enzyme act on a particular substrate?
In this lesson: Explain enzyme specificity using the active site, enzyme-substrate complex and lock-and-key model.
About 5 min
The key ideaA complementary substrate fits an enzyme active site, forms a temporary complex and is changed into products. The enzyme can then work again.
A substrate is the substance an enzyme acts on. An active site is the region of an enzyme where a suitable substrate binds.
Explore the idea
Fit, react, release
The substrate has a complementary shape. It can bind at this active site.
Original lock-and-key model, not to scale. Complementary shape explains specificity. The substrate changes into products while the enzyme remains available to work again.
Explanation
An enzyme is a biological catalyst, usually a protein. It increases the rate of a reaction without being used up by that reaction. Its active site has a particular shape that can bind a suitable substrate.
The lock-and-key model explains specificity using complementary shapes. A suitable substrate fits the active site and forms an enzyme-substrate complex. Another molecule with an unsuitable shape cannot form the same productive complex.
While the substrate is bound, the enzyme catalyses its conversion into products. In a digestion example, a larger molecule is broken down into smaller products. Other enzymes help join molecules, so enzyme action does not always mean breaking substances down.
The products leave the active site. The enzyme remains available for another substrate molecule, so one enzyme can catalyse many successive reactions. The lock-and-key picture is a simplified model of binding and specificity, not the exact shape or motion of every enzyme.
Step by step
- 1
Bind
A complementary substrate fits the active site.
- 2
Form a complex
The temporary enzyme-substrate complex allows the reaction to be catalysed.
- 3
Release and reuse
Products leave and the enzyme is available again.
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? "The enzyme is used up when a substrate is digested."
The substrate changes into products, while the enzyme remains available to catalyse another reaction.