1. ENZYMES

2. 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)
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3. 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)
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4. 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

5. 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.
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6. 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

7. 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.
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8. Substrates enter active site.
Figure 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

9. Substrates enter active site.
Figure 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

10. Substrates enter active site.
Figure 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

11. 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
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12. 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
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13. 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

14. 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

15. 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