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Enzymes Chapter 30
Enzymes Chapter 30
Hein * Best * Pattison * Arena
Colleen KelleyChemistry DepartmentPima Community College
© John Wiley and Sons, Inc.
Version 1.0
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Chapter Outline
30.1 Molecular Accelerators
30.2 Rates of Chemical Reactions
30.3 Enzyme Kinetics
30. 4 Industrial Strength Enzymes
30.5 Enzyme Active Site
30.6 Temperature and pH Effects on Enzyme Catalysis
30.7 Enzyme Regulation
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Molecular AcceleratorsMolecular Accelerators
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• Enzymes are the catalysts of biochemical reactions.
• Enzymes catalyze nearly all the myriad reactions that occur in living cells.
• Uncatalyzed reactions that require hours of boiling in the presence of a strong acid or strong base can occur in a fraction of a second in the presence of the proper enzyme.
• The catalytic functions of enzymes are directly dependent on their three-dimensional structures.
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Figure 30.1 A typical reaction-energy profile: The lower activation energy in the cell is due to the catalytic effect of enzymes.
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•Each organism contains thousands of enzymes:
1.Some are simple proteins consisting of only amino acid units.
2.Others are conjugated and consist of a protein part, or apoenzyme, and a nonprotein part, or coenzyme.
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•A functioning enzyme that consists of both the protein and nonprotein parts is called a holoenzyme.
•Apoenzyme + Coenzyme = Holoenzyme
•Often the coenzyme is derived from a vitamin, and one coenzyme may be associated with different enzymes.
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•For some enzymes, an inorganic component such as a metal ion (e.g. Ca2+, Mg2+, or Zn2+) is required.
•This inorganic component is an activator.
•The activator is analogous to a coenzyme.
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•Another remarkable property of enzymes is their specificity of reaction – that is, a certain enzyme catalyzes the reaction of a specific type of substance.
• e.g. lactase
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•The substance acted on by an enzyme is called the substrate.
•e.g. Sucrose is the substrate of the enzyme sucrase.
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Classes of Enzymes
1. Oxidoreductases: Enzymes that catalyze the oxidation-reduction between two substrates.
2. Transferases: Enzymes that catalyze the transfer of a functional group between two substrates.
3. Hydrolases: Enzymes that catalyze the hydrolysis of esters, carbohydrates, and proteins (polypeptides).
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Classes of Enzymes
4. Lyases: Enzymes that catalyze the removal of groups from substrates by mechanisms other than hydrolysis.
5. Isomerases: Enzymes that catalyze the interconversion of stereoisomers and structural isomers.
6. Ligases: Enzymes that catalyze the linking of two compounds by breaking a phosphate anhydride bond in ATP.
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Rates of Rates of Chemical ReactionsChemical Reactions
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Figure 30.2 The change in product concentration [B] as a function of time. The reaction rate is determined by measuring the slope of this line.
15Figure 30.3 An energy profile for the reaction between water and carbon dioxide.
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• There are three common ways to increase a reaction rate:
1. Increasing the reactant concentration
2. Increasing the reaction temperature
3. Adding a catalyst
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Enzyme KineticsEnzyme Kinetics
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Figure 30.4 A Michaelis-Menten plot showing the rate of enzyme-catalyzed reaction as a function of substrate concentration. The lower left portion of the graph marks the approximate area where an enzyme responds best to concentration changes.
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Figure 30.5 Michaelis-Menten plots for two glucose metabolic enzymes.
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Turnover Number
• An enzyme’s catalytic speed is also matched to an organism’s metabolic needs.
• This catalytic speed is commonly referred to as turnover number – the number of molecules an enzyme can react or “turn-over” in a given time span.
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Industrial Strength Industrial Strength EnzymesEnzymes
Industrial Strength Industrial Strength EnzymesEnzymes
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• Enzymes offer two major advantages to manufacturing processes and in commercial products:
1. Enzymes cause very large increases in reaction rates even at room temperature.
2. Enzymes are relatively specific and can be used to target selected reactants.
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• Proteases (proteolytic enzymes) break down proteins.
• Lipases digest lipids.• Cellulases, amylases, lactases, and
pectinases break down carbohydrates, cellulose, amylose, lactose, and pectin, respectively.
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Enzyme Active SiteEnzyme Active Site
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• Catalysis takes place on a small portion of the enzyme structure called the enzyme active site.
• Often this is a crevice or pocket on the enzyme that represents only 1-5% of the total surface area.
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Figure 30.6 A spacefilling model of the enzyme hexokinase (a) before and (b) after it binds to the substrate D-glucose. Note the two protein domains for this enzyme, which are colored differently.
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Figure 30.7 Enzyme-substrate interaction illustrating both the lock-and-key hypothesis and the induced-fit model. The correct substrate (orange square-blue circle) fits the active site (lock-and-key hypothesis). This substrate also causes an enzyme conformation change that positions a catalytic group (*) to cleave the appropriate bond (induced-fit model).
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Figure 30.8 Strain Hypothesis: The substrate is being forced toward the product shape by enzyme binding.
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Temperature and Temperature and pH Effects on pH Effects on
Enzyme CatalysisEnzyme Catalysis
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• Essentially, any change that affects protein structure also affects an enzyme’s catalytic function.
• If an enzyme is denatured, its activity will be lost.
• Thus, strong acids and bases, organic solvents, mechanical action, and high temperature are examples of treatments that decrease an enzyme-catalyzed rate of reaction.
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Figure 30.9 A plot of the enzyme-catalyzed rate as a function of pH.
Figure 30.10 A plot of the temperature dependence of an enzyme-catalyzed reaction
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