Enzymes as Catalysts
- Enzymes are biological catalysts that increase the rate of chemical reactions without being consumed in the process.
- They achieve this by lowering the activation energy required for a reaction to occur.
Definition
Activation Energy
Activation energy is the minimum amount of energy required to start a chemical reaction.
Enzymes do not alter the overall energy change of a reaction, they only make it easier for the reaction to proceed.
How Enzymes Work
- Enzymes are globular proteins with a specific region called the active site, where the substrate binds.
- The interaction between the enzyme and substrate follows the induced-fit model:
- Substrate Approaches: The substrate moves close to the enzyme’s active site.
- Binding: The active site undergoes a slight conformational change to fit the substrate more snugly.
- Catalysis: The enzyme lowers the activation energy, facilitating the conversion of the substrate into products.
- Release: The products are released, and the enzyme returns to its original shape, ready to catalyse another reaction.
- Consider the enzyme catalase, which breaks down hydrogen peroxide ($H_2O_2$) into water and oxygen.
- Without catalase, this reaction would occur very slowly, but with the enzyme, it happens almost instantaneously, preventing the buildup of toxic hydrogen peroxide in cells.
Why Enzymes Are Essential in Cells
1. Speeding Up Metabolic Reactions
- Metabolism consists of thousands of interconnected chemical reactions, divided into two main types:
- Anabolic reactions: Build larger molecules from smaller ones (e.g., protein synthesis).
- Catabolic reactions: Break down larger molecules into smaller ones (e.g., digestion).
- Without enzymes, these reactions would occur too slowly to sustain life.
- Enzymes can increase reaction rates by factors of millions or even billions.
- For example, the enzyme carbonic anhydrase accelerates the conversion of carbon dioxide and water into carbonic acid by a factor of 10 million.
2. Lowering Activation Energy
- Enzymes reduce the energy barrier required for reactions to proceed.
- This allows reactions to occur at the moderate temperatures and pressures found in living organisms.
- Think of activation energy as a hill that reactants must climb to transform into products.
- Enzymes act like a tunnel through the hill, providing a shortcut that requires less energy.
