Lesson 1 of 4 / Enzymes and investigations
Bind, lower the barrier, release
How can an enzyme speed a reaction without being used up?
In this lesson: Explain active sites, enzyme-substrate complexes, activation energy and specificity using both binding hypotheses.
About 6 min
The key ideaAn enzyme binds suitable substrates at an active site and lowers activation energy. Products leave and the enzyme can catalyse another reaction.
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
An enzyme is a biological catalyst. Substrate binds at its active site to form an enzyme-substrate complex. The site's shape and chemical properties contribute to specificity: a molecule must interact appropriately, not merely have a roughly similar outline.
The lock-and-key hypothesis models an active site already complementary to the substrate. The induced-fit hypothesis allows binding to cause a conformational change that improves the interaction and positions groups for catalysis. Enzymes are flexible proteins, so the second model adds useful detail to the first.
Catalysis lowers the activation-energy barrier, allowing a greater proportion of molecular encounters to lead to reaction at the same temperature. The enzyme does not supply the reaction's energy or change the overall energy difference between reactants and products.
Products separate from the active site, leaving the enzyme available again. The enzyme is not consumed by the reaction, although real enzyme molecules can eventually be damaged or degraded. A changed active site can reduce productive binding and catalytic activity.
Step by step
- 1
Bind specifically
Suitable chemical interactions form an enzyme-substrate complex.
- 2
Lower the barrier
Binding and positioning support a lower-activation-energy pathway.
- 3
Release and reuse
Products leave and the site becomes available for another cycle.
Worked example
Same temperature, faster conversion
Why can adding an enzyme increase rate without heating the mixture?
One way to explain it
The enzyme provides a pathway with lower activation energy. At the unchanged temperature, a greater proportion of encounters can overcome that lower barrier, so productive reactions occur more frequently.
Why this answer works
- Keep temperature and average kinetic energy unchanged.
- Change the energy barrier, not the energy supplied by the enzyme.
Is this true? "An enzyme works by giving extra energy to every substrate molecule."
It lowers the activation-energy requirement. It does not act as the fuel that supplies the reaction's energy.