Chapter 5 The Working Cell

HOW ENZYMES FUNCTION 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, called the energy of activation EA barrier Reactants Products 1 2 Enzyme Figure 5.5A

Progress of the reaction A protein catalyst called an enzyme Can decrease the energy of activation needed to begin a reaction Reactants EA without enzyme EA with enzyme Net change in energy Products Energy Progress of the reaction Figure 5.5B

5.6 A specific enzyme catalyzes each cellular reaction Enzymes have unique three-dimensional shapes That determine which chemical reactions occur in a cell

The catalytic cycle of an enzyme 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O 4 Products are released 3 Substrate is converted to products Figure 5.6

5.7 The cellular environment affects enzyme activity Temperature, salt concentration, and pH influence enzyme activity Some enzymes require nonprotein cofactors Such as metal ions or organic molecules called coenzymes

5.8 Enzyme inhibitors block enzyme action Inhibitors interfere with an enzyme’s activity

Normal binding of substrate 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 Enzyme Active site Normal binding of substrate Enzyme inhibition Noncompetitive inhibitor Competitive inhibitor Figure 5.8

CONNECTION 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors