Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 8.4 Enzymes Speed Up Metabolic Reactions by Lowering Energy Barriers
Enzymes are biological catalysts that accelerate chemical reactions in the cell by lowering the activation energy required for the reactions to proceed.
1. The Activation Energy Barrier
- Activation Energy (Eₐ): The initial energy investment needed to start a chemical reaction; it determines the rate at which the reaction will occur.
- Transition State: A high-energy, unstable state that reactants must reach for the reaction to proceed; overcoming the activation energy barrier allows the reactants to transform into products.
2. How Enzymes Speed Up Reactions
- Catalysis: Enzymes lower the activation energy barrier, enabling reactants to reach the transition state more easily and increasing the reaction rate without being consumed in the process.
- Substrate Specificity: Each enzyme has a unique active site that binds specifically to its substrate(s), forming an enzyme-substrate complex that facilitates the conversion to product(s).
3. The Active Site and Catalytic Cycle of an Enzyme
- Active Site: The region on the enzyme where the substrate binds; its specific shape and chemical environment facilitate the reaction.
- Catalytic Cycle:
- Substrate enters the active site, forming an enzyme-substrate complex.
- Induced fit occurs, where the enzyme changes shape slightly to better accommodate the substrate.
- The enzyme catalyzes the conversion of substrate to product.
- The product is released, and the enzyme is free to bind to new substrate molecules.
4. Effects of Local Conditions on Enzyme Activity
- Temperature and pH: Each enzyme has an optimal temperature and pH at which it functions most effectively; deviations from these optimal conditions can reduce enzyme activity or denature the enzyme.
- Cofactors and Coenzymes: Non-protein molecules that assist enzymes in catalyzing reactions; cofactors are often metal ions, while coenzymes are organic molecules, such as vitamins.
Enzyme Inhibitors:
Competitive Inhibitors: Bind to the active site, blocking substrate access.
Noncompetitive Inhibitors: Bind to a different part of the enzyme, causing a conformational change that reduces the enzyme's activity.
In a Nutshell
- Enzymes are catalysts that speed up chemical reactions by lowering the activation energy barrier.
- They exhibit substrate specificity, binding to specific substrates at their active sites.
- Enzyme activity can be influenced by environmental factors, cofactors, and inhibitors.