Lesson 5 of 8 / Biological molecules
How enzymes speed up reactions
Why does one enzyme act on a particular substrate?
I can explain enzyme specificity using the active site, enzyme-substrate complex and lock-and-key model.
I can explain enzyme action using lock-and-key specificity and a temporary complex.Syllabus
Syllabus K327 / K328, Topic 3(d). Explain enzyme action using lock-and-key specificity and a temporary complex.
About 5 min
Make a guess. You are not marked.
An enzyme makes a reaction go much faster. How does it do it?
Show the answer
It lowers the activation energy the reaction needs.
With a lower energy barrier, more substrate molecules react each second at the same temperature.
The key idea
A 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.
Watch out for this
A student says: "The enzyme is used up when a substrate is digested." What is wrong with this?
Show the answer
The substrate changes into products, while the enzyme remains available to catalyse another reaction.
Check your understanding
Why does maltase act on maltose rather than starch?
Show the answer
Maltose has a shape complementary to the maltase active site.
This allows a productive enzyme-substrate complex to form. Specificity depends on a suitable fit.
Exam-style questions
Check your understanding
A diagram shows an enzyme with a notch-shaped active site. Molecule P fits the notch exactly; molecule Q is a different shape. Which statement is correct?
Show the answer
P can form an enzyme-substrate complex because its shape is complementary.
The 3D shape of the active site is complementary to P, so P binds and is changed into products. Q does not fit.
Quick recall
Why does one enzyme act on a particular substrate?