1. CHAPTER 5 The Working Cell
Modules 5.5 – 5.9

2. For a chemical reaction to begin, reactants must absorb some energy
HOW ENZYMES WORK
5.5 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
For a chemical reaction to begin, reactants must absorb some energy
This energy is called the energy of activation (EA)
This represents the energy barrier that prevents molecules from breaking down spontaneously

3. A protein catalyst called an enzyme can decrease the energy barrier
EA barrier
Enzyme
Reactants 1
Products 2
Figure 5.5A

4. EA without enzyme EA with enzyme Reactants Net change in energy
Products
Figure 5.5B

5. 5.6 A specific enzyme catalyzes each cellular reaction
Enzymes are selective
This selectivity determines which chemical reactions occur in a cell

6. The enzyme is unchanged and can repeat the process
How an enzyme works
Active site
Enzyme (sucrase)
Substrate (sucrose)
Glucose
Fructose 1 4
Enzyme available with empty active site
Products are released 3 2
Substrate is converted to products
Substrate binds to enzyme with induced fit
Figure 5.6
The enzyme is unchanged and can repeat the process

7. 5.7 The cellular environment affects enzyme activity
Enzyme activity is influenced by
temperature
salt concentration pH
Some enzymes require nonprotein cofactors
Some cofactors are organic molecules called coenzymes

8. 5.8 Enzyme inhibitors block enzyme action
Inhibitors interfere with enzymes
A competitive inhibitor takes the place of a substrate in the active site
A noncompetitive inhibitor alters an enzyme’s function by changing its shape
Substrate
Active site
Enzyme
NORMAL BINDING OF SUBSTRATE
Competitive inhibitor
Noncompetitive inhibitor
ENZYME INHIBITION
Figure 5.8

9. 5.9 Connection: Some pesticides and antibiotics inhibit enzymes
Certain pesticides are toxic to insects because they inhibit key enzymes in the nervous system
Many antibiotics inhibit enzymes that are essential to the survival of disease-causing bacteria
Penicillin inhibits an enzyme that bacteria use in making cell walls