Effect of enzymes on activation energy

Molecules (Interaction and interdependence) · Enzymes and metabolism · note 4 of 4

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Effect of enzymes on activation energySpec C1.1.10

In short

Activation energy is the energy needed to break bonds within the substrate so that a reaction can start. Enzymes lower the activation energy, so more substrate molecules can react at a given temperature and the rate increases. Energy is released when new bonds form in the products. The overall energy change of the reaction stays the same.

Before a reaction can happen, bonds within the substrate must be broken or weakened. This needs an input of energy called the activation energy. When new bonds form to make the products, energy is released (the energy yield). In many metabolic reactions the products have less energy than the substrates, so there is an overall release of energy.

Enzymes lower the activation energy. Binding to the active site stresses and weakens bonds in the substrate, so less energy is needed to reach the transition state. A larger proportion of substrate molecules then have enough energy to react, and the reaction is faster at the same temperature. The enzyme does not change the energy of the substrates or the products, so the overall energy change is unchanged.

Enzymes lower the activation energy, measured from the substrate level to the peak; the overall energy change is unchanged.

Exam tip:

On an energy graph, the activation energy is measured from the substrate level to the peak, not from the products or from zero.

Exam tip:

Linking question: enzymes are a clear structure–function relationship in a macromolecule. The folded polypeptide creates an active site that lowers activation energy for one specific reaction.

Quick check

What is metabolism?
Show answer
The complex network of interdependent and interacting chemical reactions occurring in living organisms.
Give one example of an anabolic and one of a catabolic reaction.
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Anabolic: protein synthesis, glycogen formation or photosynthesis. Catabolic: hydrolysis in digestion or oxidation of glucose in respiration.
In the induced-fit model, what changes shape when the substrate binds?
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Both the substrate and the enzyme's active site.
What happens to the activation energy and the overall energy change when an enzyme catalyses a reaction?
Show answer
The activation energy is lowered; the overall energy change is unchanged.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because extra vibration breaks the bonds holding the protein's three-dimensional structure. The active site changes shape and chemical properties, so the substrate can no longer bind and no enzyme–substrate complexes form. This is why the rate falls steeply above the optimum temperature, and the change is usually permanent.

What is the induced-fit model of enzyme action?

The induced-fit model says that both the substrate and the enzyme change shape when the substrate binds to the active site. The active site moulds more tightly around the substrate, which stresses bonds in the substrate and lowers the activation energy. After the products leave, the active site returns to its original shape.

Why does the rate of an enzyme reaction level off at high substrate concentration?

The rate levels off because nearly all the active sites are occupied at any moment. Adding more substrate cannot increase the number of successful substrate–active site collisions, so enzyme concentration becomes the limiting factor. At low substrate concentrations, by contrast, more substrate means more frequent collisions and a faster rate.

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