3.2.2 Energy of Activation

Introduction

Molecules do not react with one another unless they are activated in some way. The energy that must be added to cause molecules to react with one another is called the energy of activation. Enzymes speed up chemical reactions by lowering this energy of activation. This is one of the most important concepts in enzyme biology.

Definition

Energy of Activation: The energy that must be added to cause molecules to react with one another is called the energy of activation.

In simple words: Activation energy is the minimum energy required to start a chemical reaction.

Alternative Definition: The energy of activation is "energy required to break the existing bonds and begin the reaction."

Core Concept

• Molecules do not react with one another unless they are activated in some way.
• The energy that must be added to cause molecules to react is called energy of activation.
• In non-living systems, we use heat as energy of activation to increase the number of effective collisions between molecules.
• In living systems, large amounts of heat cannot be used as energy of activation.
• Why? All living cells and organisms are mainly composed of temperature-sensitive protein molecules.
• About 1,000 chemical reactions are being carried out in a cell at any time.
• Energy of activation required for such a large number of reactions cannot be provided by living systems.
• The living system works in isothermal conditions.
• The excited state of molecules or reactants is achieved by biochemical processes.
• Enzymes lower the energy of activation by changing an energy-dependent process to an energy-independent process.

Detailed Explanation

Why Living Systems Cannot Use Heat

How Enzymes Lower Activation Energy

Enzyme-Substrate Interaction:

A + E → AE Complex + B → AB + E

• A = Reactant
• E = Enzyme
• AE = Transitional complex
• B = Second reactant
• AB = Product
• E = Enzyme (released unchanged)

Step-by-Step:

• Enzyme (E) reacts with reactant (A) to form an AE transitional complex.
• The energy level of the AE complex reaches the energy level of reactant B.
• AE complex then reacts with reactant B to form AB (product).
• Enzyme (E) is released and can catalyze another reaction.

Key Points:

• The enzyme does not change the overall energy change of the reaction.
• The enzyme lowers the activation energy required to start the reaction.
• The enzyme stabilizes the transition state.
• The enzyme increases the rate of the reaction.
• The enzyme does not affect the equilibrium of the reaction.

Energy Profile Diagram

The energy profile of a reaction shows:

Key Point: The enzyme lowers the activation energy but does not change the overall free energy change of the reaction.

Activation Energy in Detail

Definition: The energy of activation is the minimum amount of energy required to convert reactant molecules into products.

• To break the existing bonds in the reactants.
• To overcome the repulsion between molecules.
• To reach the transition state.

• The highest energy state in a chemical reaction.
• The state at which bonds are breaking and forming.
• The enzyme stabilizes the transition state.

• Temperature (higher temperature = more molecules have enough energy)
• Concentration of reactants
• Presence of a catalyst (enzyme)

How Enzymes Lower Activation Energy

Mechanisms:

• Orienting substrates: Enzymes bring substrates together in the correct orientation.
• Straining bonds: Enzymes put stress on the bonds of the substrate, making them easier to break.
• Providing a favorable environment: The active site provides a specific environment (e.g., hydrophobic, charged) that favors the reaction.
• Participating in the reaction: Some enzymes temporarily form covalent bonds with the substrate.
• Lowering the transition state energy: Enzymes stabilize the transition state.

Summary of Enzyme Action and Activation Energy

Intermediate Level

Energy of Activation in Detail

Enzyme-Substrate Interaction

Comparison: Non-Living vs Living Systems

How Enzymes Lower Activation Energy

Energy Profile of a Reaction

Important Points

• 1. Molecules do not react with one another unless they are activated in some way.
• 2. The energy that must be added to cause molecules to react with one another is called the energy of activation.
• 3. In non-living systems, we use heat as energy of activation to increase the number of effective collisions between molecules.
• 4. In living systems, large amount of heat cannot be used as energy of activation.
• 5. Why? All living cells and organisms are mainly composed of temperature-sensitive protein molecules.
• 6. About 1,000 chemical reactions are being carried out in a cell at any time.
• 7. Energy of activation required for such a large number of reactions cannot be provided by living systems.
• 8. The living system works in isothermal condition.
• 9. The excited state of molecules or reactants is achieved by biochemical process.
• 10. Enzyme (E) reacts with reactant (A) to form an AE transitional complex.
• 11. The energy level of AE complex reaches to the energy level of reactant B.
• 12. AE complex then reacts with reactant B to form AB and enzyme (E) is released.
• A + E → AE Complex + B → AB + E
• 13. Enzyme does decrease the energy of activation by changing energy dependent process to energy independent process.
• 14. Thus the energy of activation is "energy required to break the existing bonds and begin the reaction".
• 15. An enzyme greatly reduces the activation energy necessary to initiate a chemical reaction.

Remember

• - Energy of activation = energy required to start a reaction
• - Also called = activation energy
• - **Symbol** = Ea
• - Enzymes lower Ea
• - Living systems cannot use heat for activation energy
• - Living systems work in isothermal conditions
• - Enzymes work by lowering Ea
• - Enzymes do not change ΔG
• - Enzymes do not change equilibrium
• - Enzymes increase the rate of reaction
• - Enzyme action: E + S → ES → EP → E + P
• - Enzymes are not consumed

Common Mistakes

• 1. Thinking enzymes provide energy – Enzymes lower activation energy; they do not provide energy.
• 2. Thinking enzymes change the equilibrium – Enzymes do not change the equilibrium of a reaction.
• 3. Thinking enzymes change ΔG – Enzymes do not change the overall free energy change.
• 4. Thinking heat can be used in living systems – Heat denatures proteins; living systems use enzymes instead.
• 5. Confusing activation energy with free energy – Activation energy = energy to start reaction; Free energy = overall energy change.
• 6. Thinking enzymes are consumed – Enzymes are not consumed; they are reused.
• 7. Forgetting that enzymes work in isothermal conditions – Living systems work at constant temperature.
• 8. Thinking all reactions need the same activation energy – Different reactions have different activation energies.
• 9. Confusing the transition state with the product – Transition state = highest energy state; Product = final state.
• 10. Thinking enzymes increase the energy of reactants – Enzymes lower the activation energy, not increase reactant energy.
• 11. Forgetting the role of the AE complex – The AE complex is a transitional complex that forms during the reaction.
• 12. Thinking enzymes work by increasing temperature – Enzymes work by lowering activation energy, not by increasing temperature.

Quick Revision

• Molecules do not react with one another unless they are activated in some way.
• The energy that must be added to cause molecules to react with one another is called the energy of activation.
• In non-living system, we use heat as energy of activation to increase the number of effective collision between molecules.
• In living systems large amount of heat cannot be used as energy of activation.
• Why? All living cells and organisms are mainly composed of temperature sensitive protein molecules.
• About 1,000 chemical reactions are being carried out in a cell at any time.
• Energy of activation required for such a large number of reactions cannot be provided by living system.
• The living system works in isothermal condition.
• The excited state of molecules or reactants is achieved by biochemical process.
• Enzyme (E) reacts with reactant (A) to form an AE transitional complex.
• The energy level of AE complex reaches to the energy level of reactant B.
• AE complex then reacts with reactant B to form AB and enzyme (E) is released.
• A + E → AE Complex + B → AB + E
• Enzyme does decrease the energy of activation by changing energy dependent process to energy independent process.
• Thus the energy of activation is "energy required to break the existing bonds and begin the reaction".
• An enzyme greatly reduces the activation energy necessary to initiate a chemical reaction.

60-Second Revision

• Energy of activation = energy required to start a reaction
• Also called = activation energy
• **Symbol** = Ea
• Enzymes lower Ea
• Living systems cannot use heat for activation energy
• Living systems work in isothermal conditions
• Enzymes work by lowering Ea
• Enzymes do not change ΔG
• Enzymes do not change equilibrium
• Enzymes increase the rate of reaction
• Enzyme action: E + S → ES → EP → E + P
• Enzymes are not consumed
• Warning: Enzymes lower activation energy, they do not provide energy. Enzymes do not change ΔG or equilibrium. Living systems cannot use heat for activation energy. Enzymes are not consumed. The transition state is the highest energy state. Enzymes stabilize the transition state.