ENZYMES

Enzymes speed up metabolic reactions by lowering energy barriers A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction An enzyme is a catalytic protein Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Sucrase Sucrose (C12H22O11) Fructose (C6H12O6) Glucose (C6H12O6) © 2011 Pearson Education, Inc. 2

The Activation Energy Barrier Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) © 2011 Pearson Education, Inc. 3

Course of reaction without enzyme EA without enzyme Figure 8.13 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy Course of reaction with enzyme G is unaffected by enzyme Figure 8.13 The effect of an enzyme on activation energy. Products Progress of the reaction 4

Substrate Specificity of Enzymes The reactant that an enzyme acts on is called the enzyme’s substrate The enzyme binds to its substrate, forming an enzyme-substrate complex The active site is the region on the enzyme where the substrate binds Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction For the Cell Biology Video Closure of Hexokinase via Induced Fit, go to Animation and Video Files. © 2011 Pearson Education, Inc. 5

Enzyme-substrate complex Figure 8.14 Substrate Active site Figure 8.14 Induced fit between an enzyme and its substrate. Enzyme Enzyme-substrate complex (a) (b) 6

The Enzyme’s Active Site In an enzymatic reaction, the substrate binds to the active site of the enzyme via weak interactions, such as hydrogen bonds and ionic bonds. The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate A singles enzyme can catalyze thousands of reactions in a second. Enzymes are unaffected by the reaction and are reusable. Most metabolic enzymes can catalyze a reaction in both the forward and reverse direction. The actual direction depends on the relative concentrations of products and reactants. Enzymes catalyze reactions in the direction of equilibrium. © 2011 Pearson Education, Inc. 7

Substrates enter active site. Figure 8.15-1 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme 8

Substrates enter active site. Figure 8.15-2 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site can lower EA and speed up a reaction. 3 Active site Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme Substrates are converted to products. 4 9

Substrates enter active site. Figure 8.15-3 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site can lower EA and speed up a reaction. 3 Active site is available for two new substrate molecules. 6 Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme 5 Products are released. Substrates are converted to products. 4 Products 10

Effects of Local Conditions on Enzyme Activity The three-dimensional structure of enzymes are influenced by environmental conditions. An enzyme’s activity can be affected by General environmental factors, such as temperature and pH Chemicals that specifically influence the enzyme © 2011 Pearson Education, Inc. 11

Effects of Temperature and pH Each enzyme has an optimal temperature in which it can function As temperature increases, collisions between substrates and active sites occur more frequently as molecules move faster. TOO much of an increase in temperature will disrupt the weak bonds that stabilize the protein’s active conformation and the protein will denature. Each enzyme has an optimal pH in which it can function Most enzymes have an optimal pH between 6-8. Some digestive enzymes in the stomach work best at lower pH values (pH=2), while those in the intestine work best at a pH of 8. The working environments influence the optimal pH of these enzymes. Optimal conditions favor the most active shape for the enzyme molecule © 2011 Pearson Education, Inc. 12

Figure 8.16 Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Rate of reaction 20 40 60 80 100 120 Temperature (°C) (a) Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Figure 8.16 Environmental factors affecting enzyme activity. Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH (b) Optimal pH for two enzymes 13

Optimal temperature for typical human enzyme (37°C) Figure 8.16a Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Rate of reaction Figure 8.16 Environmental factors affecting enzyme activity. 20 40 60 80 100 120 Temperature (°C) (a) Optimal temperature for two enzymes 14

Optimal pH for pepsin (stomach enzyme) Figure 8.16b Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction Figure 8.16 Environmental factors affecting enzyme activity. 1 2 3 4 5 6 7 8 9 10 pH (b) Optimal pH for two enzymes 15