1. Enzymes 5

2. Objectives
Explain how enzymes affect activation energy.
Describe how an enzyme's shape is important to its function

3. Each of your cells is like a miniature chemical factory capable of performing thousands of different reactions. As a cell's needs change, some of these reactions speed up, while others slow down.

4. Enzymes and Activation Energy To start a chemical reaction, it is first necessary to weaken chemical bonds in the reactant molecules. This activation process requires that the molecules absorb energy. For example, to burn a candle you need to provide the initial energy with a match.
This "start-up" energy is called activation energy because it activates the reactants and triggers a chemical reaction.

5. cellular reactions depend on the assistance of catalysts, compounds that speed up chemical reactions. The main catalysts of chemical reactions in organisms are specialized proteins called enzymes.

6. Enzymes provide a way for reactions to occur at the cell's normal temperature.
An enzyme doesn't supply activation energy to the reacting molecules, but instead lowers the energy requirement barrier so that the reaction can proceed at normal cell temperatures.

7.                                                                                                                                   
The activation energy barrier is like a wall between two parts of a pond. If an enzyme lowers the wall, more frogs have enough energy to reach the other side

8. Each enzyme catalyzes a specific kind of chemical reaction
Each enzyme catalyzes a specific kind of chemical reaction. At any moment in the cell's life, the specific enzymes that are present and active determine which reactions occur.

9. How Enzymes Work Just how does an enzyme catalyze only one type of reaction? The reason is that the shape of each enzyme fits the shape of only particular reactant molecules. A specific reactant acted upon by an enzyme is called the enzyme's substrate. The substrate fits into a particular region of the enzyme, called the active site

10. Figure 5-16 follows the action of the enzyme sucrase, which catalyzes the hydrolysis of sucrose (the substrate).
(Most enzymes have names that end in -ase.) Sucrose is slightly distorted as it enters the active site.

11. The weakened bond reacts with water
The weakened bond reacts with water. The result is two products: a glucose molecule and a fructose molecule. Once these products are released, the enzyme's active site is ready to accept another molecule of sucrose.
This recycling ability is a key characteristic of enzymes.