# Activation Energy of Enzymes: Definition, Calculation & Example

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• 0:00 Reactions & Activation Energy
• 1:33 Enzymes & Catalysis
• 2:07 Induced-Fit Theory
• 2:36 Arrhenius Law
• 4:55 Lesson Summary

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Lesson Transcript
Instructor: Alfonso Reina
With this lesson you will understand what the activation energy of a chemical reaction is. You will also learn what enzymes are and how they affect chemical reactions.

## Reactions and Activation Energy

Let's imagine that you are hiking, and you need to go up a hill to reach the other side. You need to spend some energy going up the hill. The higher the hill, the more energy you need to use to go to the other side.

In biochemical processes, molecules similarly require energy in order to start a reaction. For example, molecules need to have some kinetic energy, or velocity, to collide with other molecules to initiate a reaction. If the collisions don't happen often or don't have enough kinetic energy, no reaction will take place. The energy required to start a reaction is called the activation energy. The lower the activation energy, the faster a reaction happens. The lower the hill you are hiking, the faster you go over to the other side of the hill.

Reactants and products have specific energies. In order to transform the reactants into products, the reactants would have to go through a transition state which is usually higher in energy. To get to this transition state, the system requires the activation energy. Finally, the products reduce their energy to arrive to the final product state.

In this graph we see the plot of energy versus the progress of a reaction. Reactants have higher energy than products. The energy of the reactants increase and then decrease to the final product energy. The highest point in the curve represents the energy of the intermediate state in the reaction. The energy required to achieve the intermediate state is the activation energy of the reaction. Enzymes lower the activation energy of a given reaction, shown by the green curve.

## Enzymes and Catalysis

Enzymes are proteins that reduce the energy required to achieve the transition state. Enzymes reduce the activation energy through a process called catalysis. A biochemical reaction when an enzyme is present is called a catalyzed reaction. Catalysis can happen in different ways. Enzymes can use the transfer of protons or electrons to the reactants to modify the state of the reactants. Enzymes also use electric charge to stabilize the state of the reactants. Enzymes, however, do not modify the final products of the reaction.

## Induced-Fit Theory

Each enzyme has an active site where reactant molecules bind. The molecule that binds to the active site is called a substrate. The enzyme induces a change in the molecule which lowers the activation energy of the reaction. For example, in reactions involving the breaking of bonds, the enzyme may put stress on the molecule to make it easier to break those bonds. After the induced change occurs, the molecule is released and the enzyme comes back to its original state.

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