Lesson 1 of 4 / Enzymes and investigations
How an enzyme lowers the barrier
Why does an enzyme speed a reaction without changing its overall energy change?
I can explain specificity, enzyme-substrate complexes and activation energy.
I can explain specificity, enzyme-substrate complexes, activation energy and enzyme models.Syllabus
Syllabus 9477, Core 1(p). Explain specificity, enzyme-substrate complexes, activation energy and enzyme models.
About 6 min
Make a guess. You are not marked.
A reaction releases 30 kJ of energy without an enzyme. How much does it release with an enzyme?
Show the answer
Still 30 kJ. Only the activation energy falls.
An enzyme provides a lower-energy route to the same products. The overall energy change stays the same.
The key idea
An enzyme provides a lower-activation-energy route; it does not change the reactants, products or overall energy difference.
Explore the idea
A flexible active site and a lower barrier
The rigid lock-and-key model emphasises complementarity. This flexible model also allows the site to adjust during binding.
Orange: uncatalysed. Green: catalysed. Original qualitative energy profiles; reaction progress is not time. The catalyst changes the barrier, not the overall reactant-product energy difference.
Explanation
Reactants must pass through an unstable transition state before products form. Activation energy is the energy barrier to reaching this state. At a given temperature, lowering the barrier increases the proportion of collisions that lead to reaction, so product forms faster.
A substrate binds at an active site to form an enzyme-substrate complex. The site has a particular shape and chemical environment. Binding can orient reactants, strain bonds and stabilise the transition state. Products leave, making the enzyme available for another catalytic cycle.
The lock-and-key hypothesis emphasises complementary fit between substrate and active site. The induced-fit hypothesis adds that binding can change the enzyme's conformation to improve catalytic alignment. A functioning enzyme is flexible rather than a rigid mould.
Enzymes do not supply a net energy source, change the overall free-energy difference or make an unfavourable reaction favourable merely by catalysis. They accelerate approach to equilibrium in both directions. Coupling to another reaction is a different mechanism from lowering activation energy.
Step by step
- 1
Mark reactants and products
Keep their relative energy levels unchanged.
- 2
Compare the peaks
The catalysed route has a smaller barrier.
- 3
Explain molecular action
Connect binding and transition-state stabilisation to faster reaction.
Worked example
Work through the evidence
A catalyst lowers a barrier from 80 to 35 arbitrary energy units. Reactants remain at 20 and products at 10. What changed?
One way to explain it
The activation barrier decreased, but the products remain 10 units below the reactants. The reaction pathway and rate changed, not the overall energy difference.
Why this answer works
- Read the barrier relative to reactants.
- Do not infer that enzyme action creates energy.
Watch out for this
A student says: "An enzyme makes a reaction faster by increasing the energy of all reactants." What is wrong with this?
Show the answer
It lowers the activation barrier through a different reaction pathway.
Check your understanding
Which statement describes induced fit?
Show the answer
Substrate binding changes the active-site conformation.
This can improve alignment and stabilise the transition state.
Quick recall
Why does an enzyme speed a reaction without changing its overall energy change?