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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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