CHAPTER 5 The Working Cell Modules 5.5 – 5.9

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

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

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

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

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

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

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

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