1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 5.5 Enzymes speed up the cell’s chemical reactions by lowering energy barriers HOW ENZYMES WORK

2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A protein catalyst called an enzyme can decrease the energy barrier E A barrier Reactants 1Products2 Enzyme Figure 5.5A

3. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Enzymes are selective –This selectivity determines which chemical reactions occur in a cell 5.6 A specific enzyme catalyzes each cellular reaction

4. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme (sucrase) Active site 1 2 3 Substrate (sucrose) Enzyme available with empty active site Substrate binds to enzyme with induced fit Substrate is converted to products 4 Products are released GlucoseFructose How an enzyme works The enzyme is unchanged and can repeat the process Figure 5.6

5. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme activity is influenced by –temperature –salt concentration –pH Some enzymes require nonprotein cofactors –Some cofactors are organic molecules called coenzymes 5.7 The cellular environment affects enzyme activity

6. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 5.8 Enzyme inhibitors block enzyme action Substrate Enzyme Active site NORMAL BINDING OF SUBSTRATE Competitive inhibitor Noncompetitive inhibitor ENZYME INHIBITION Figure 5.8

7. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 5.9 Connection: Some pesticides and antibiotics inhibit enzymes