Chapter Outline Chapter 30 Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Enzymes Enzymes accelerate chemical reactions as the engine accelerates this drag race.
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Chapter Outline
Chapter 30
Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc
Morris Hein, Scott Pattison, and Susan Arena
Enzymes Enzymes accelerate chemical reactions as the engine accelerates this drag race.
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Chapter Outline
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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
Chapter 30 Summary
Course Outline
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Chapter Outline
Molecular Accelerators
Enzymes are molecules that catalyze biochemical reactions and a large majority of these catalysts are proteins. Enzymes catalyze nearly all the myriad reactions that occur in living cells.
Enzymes are essential to life. In the absence of enzymes
biochemical reactions occur too slowly to maintain life. The typical biochemical reaction occurs more than a million times faster when catalyzed by an enzyme.
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Chapter Outline
Molecular Accelerators
In biological cells enzymes lower the activation energy and enable reactions to occur rapidly. Here is the reaction-energy profile of sucrose with oxygen that is catalyzed (in a cell) and uncatalyzed (in a sugar bowl).
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Chapter Outline
Molecular Accelerators
Each organism contains thousands of enzymes. Some are simple proteins consisting only of amino acid units. Others are conjugated and consist of a protein part, or apoenzyme, and a nonprotein part, or coenzyme.
Both parts are essential. A functioning enzyme that
consists of both the protein and nonprotein parts is called a holoenzyme.
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Chapter Outline
Molecular Accelerators
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.
From the standpoint of function, an activator is analogous
to a coenzyme, but inorganic components are not called coenzymes.
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Chapter Outline
Another remarkable property of enzymes is their specificity. That means that a certain enzyme catalyzes the reaction of a specific type of substance. For example each of these similar reactions requires a specific enzyme.
Molecular Accelerators
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Chapter Outline
Molecular Accelerators
The substance acted on by an enzyme is called the substrate. Enzymes are named by adding the suffix -ase to the root of the substrate name. Note the derivations of maltase, sucrase, and lactase from maltose, sucrose, and lactose.
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Chapter Outline
Your Turn!
An enzyme that converts the amino acid serine to phosphoserine would be called which of the following.
• Serinase • Serine decarboxylase • Serine dehydrogenase • Serine kinase • Serine phosphatase
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Chapter Outline
Your Turn!
Serine phosphatase The names of enzymes generally refer to the types of
reactions they catalyze and the substrates they act on. So, of the names listed, your best guess would be that the enzyme serine phosphatase catalyzes the reaction that converts serine to phosphoserine.
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Chapter Outline
Your Turn!
What can be deduced about the reaction catalyzed by the enzyme lysine decarboxylase?
• The enzyme adds a CO2 group to lysine • The enzyme removes a CO2 group from lysine • The enzyme oxidizes lysine • The enzyme reduces lysine • The enzyme hydrolyzes lysine
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Chapter Outline
Your Turn!
The enzyme removes a CO2 group from lysine The name of the enzyme is lysine decarboxylate. The
“lysine” part of the name indicates that the enzyme acts on lysine. The prefix “de-“ in the name of an enzyme generally means that something is being removed during the chemical reaction. The “carbox” refers to carbon dioxide (CO2). So, a decarboxylase is an enzyme that removes carbon dioxide.
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Chapter Outline
Molecular Accelerators
In the International Union of Biochemistry (IUB) System, enzymes are assigned to one of six classes based on the reactions they catalyze.
1. Oxidoreductases: Enzymes that catalyze the oxidation–
reduction reactions 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). 13
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Chapter Outline
Molecular Accelerators
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 adenosine triphosphate (ATP).
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Chapter Outline
Your Turn!
What class of enzyme would catalyze the following reaction?
COO-
CH OH
CH H
O
PO O-
O-
COO-
CH O
CH H
O
H
P
O
O-
O-
3-phosphoglycerate 2-phosphoglycerate 15
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Chapter Outline
Your Turn!
The two molecules have the same molecular formula and are structural isomers of each other. The enzyme that would catalyze this reaction is an isomerase.
COO-
CH OH
CH H
O
PO O-
O-
COO-
CH O
CH H
O
H
P
O
O-
O-
3-phosphoglycerate 2-phosphoglycerate 16
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Chapter Outline
Rates of Chemical Reactions
Enzymes catalyze biochemical reactions and thus increase the rates of these chemical reactions.
Every chemical reaction starts with at least one reactant
and finishes with a minimum of one product. As the reaction proceeds, the reactant concentration
decreases and the product concentration increases . . .
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Chapter Outline
Rates of Chemical Reactions
We can plot these changes as a function of time as shown for the hypothetical conversion of reactant A into product B.
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Chapter Outline
Rates of Chemical Reactions
A reaction rate is defined as a change in concentration with time. This is the rate at which the reactants of a chemical reaction disappear and the products form.
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Chapter Outline
Your Turn!
Calculate the reaction rate of the appearance of product D with the following reaction data.
Reaction Time (hr) D Concentration (M) 0.0 0.0 1.0 0.5 2.0 1.5 3.0 4.5 4.0 6.0
C D
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Chapter Outline
Your Turn!
Reaction Time (hr) D Concentration (M) 0.0 0.0 1.0 0.5 2.0 1.5 3.0 4.5 4.0 6.0
6.0 M
4.0 hr=
1.5 M of D
hr
Total change in concentration of D (0.0 M to 6.0 M)
Total change in time (0.0 hr to 4.0 hr)Rate of appearance of D = =
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Chapter Outline
Rates of Chemical Reactions
The reactant must pass through a high-energy transition state to be converted into a product. This transition state is an unstable structure with characteristics of both the reactant and the product.
The energy necessary to move a reactant to the transition
state is called the activation energy. The larger this energy barrier is the slower the reaction rate will be.
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Chapter Outline
Rates of Chemical Reactions
This is an energy profile for the reaction between water and carbon dioxide showing the transition state.
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Chapter Outline
Rates of Chemical Reactions
There are three common ways to increase a reaction rate. • Increasing the reactant concentration
• Increasing the reaction temperature
• Adding a catalyst
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Chapter Outline
Your Turn!
For the reaction of carbon dioxide with water predict which set of conditions 1) or 2) will yield a faster reaction.
1) CO2 pressure = 100 torr, T = 37°C, Activation energy =
31 kcal/mol.
versus 2) CO2 pressure = 150 torr, T = 37°C, Activation energy =
31 kcal/mol.
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Chapter Outline
Your Turn!
1) CO2 pressure = 100 torr, T = 37°C, Activation energy = 31 kcal/mol.
versus
2) CO2 pressure = 150 torr, T = 37°C, Activation energy =
31 kcal/mol.
Condition 2) will yield a faster reaction because the CO2 pressure (concentration) is greater while the other conditions remain constant. 26
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Chapter Outline
Your Turn!
For the reaction of carbon dioxide with water predict which set of conditions 1) or 2) will yield a faster reaction.
1) CO2 pressure = 100 torr, T = 37°C, Activation energy =
31 kcal/mol.
versus 2) CO2 pressure = 100 torr, T = 37°C, Activation energy =
28 kcal/mol.
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Chapter Outline
Your Turn!
1) CO2 pressure = 100 torr, T = 37°C, Activation energy = 31 kcal/mol.
versus
2) CO2 pressure = 100 torr, T = 37°C, Activation energy =
28 kcal/mol.
Condition 2) will yield a faster reaction because the activation energy is lower while other conditions remain constant. 28
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Chapter Outline
Enzyme Kinetics
Two German researchers, Leonor Michaelis and Maud Menten measured enzyme-catalyzed reaction rates as a function of substrate (reactant) concentration.
They observed that most enzyme-catalyzed reactions
show an increasing rate with increasing substrate concentration, but only to a specific maximum velocity, Vmax.
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Chapter Outline
Enzyme Kinetics
A Michaelis–Menten plot showing the rate of an enzyme-catalyzed reaction as a function of substrate concentration is shown here.
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Notice that the enzyme has limited enzyme capacity. The rate of catalysis doesn’t continue to increase with substrate concentration but reaches a maximum (Vmax).
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Chapter Outline
Enzyme Kinetics
Michaelis–Menten plots for two glucose metabolic enzymes. Kexokinase has a stronger attraction for glucose than glucokinase.
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Chapter Outline
Enzyme Kinetics
An enzyme’s catalytic speed is also matched to an organism’s metabolic needs.
This catalytic speed is commonly measured as a turnover
number which is the number of molecules an enzyme can convert, or “turn over,” in a given time span.
Turnover number is a convenient way to compare enzymes
to each other or to the effect of reaction conditions.
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Chapter Outline
Your Turn!
The enzyme lactase can break down 1197 molecules of lactose in 7 minutes. Calculate the turnover number per minute.
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Chapter Outline
Your Turn!
The enzyme lactase can break down 1197 molecules of lactose in 7 minutes. Calculate the turnover number per minute.
1197 lactose molecules
7 minutes171
min=
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Chapter Outline
Industrial-Strength Enzymes
Not only are enzymes important in biology, they are increasingly important in industry. About 75% of industrial enzymes have a digestive or “breakdown” function.
They are hydrolases but are known by common names . . .
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Chapter Outline
Industrial-Strength Enzymes
• Proteases (proteolytic enzymes) break down proteins and comprise about 40% of all industrial enzymes.
• Lipases digest lipids. • Cellulases, amylases, xylanases, lactases, and
pectinases break down cellulose, amylose, xylans, lactose, and pectin, respectively.
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Chapter Outline
Industrial-Strength Enzymes
Enzymes have long been used in food processing. For example.
• Amylases and other carbohydrate hydrolases act to soften
the dough.
• About 25% of all industrial enzymes are used to convert cornstarch into syrups.
• The plant products used in animal feed are also commonly treated with industrial enzymes to make them more nutritious. 37
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Chapter Outline
Industrial-Strength Enzymes
Industrial enzymes offer solutions to environmental pollution problems for some industries.
• A number of industries use cellulases instead of strong
base to give a soft appearance to denim.
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Chapter Outline
Industrial-Strength Enzymes
Industrial enzymes are used in consumer goods like detergent additives. About 37% of all industrial enzymes are found in laundry detergents.
• Proteases digest clothing stains like grass, blood, and
sweat.
• Lipases hydrolyze the fats.
• Amylases digest starchy residues.
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Chapter Outline
Industrial-Strength Enzymes
Enzymes are used in medicine primarily because of their specificity.
Several proteases are used to dissolve blood clots in
patients with such diseases as lung embolism (clot in the lung), stroke (clot in the brain), and heart attack (clot in the heart).
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Chapter Outline
Your Turn!
Explain why cellulases are not used to soften nylon fabrics.
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Chapter Outline
Your Turn!
Explain why cellulases are not used to soften nylon fabrics.
Cellulases soften fabrics that contain cellulose such as
cotton fabrics. Nylon doesn’t contain cellulose. Nylon is a man-made polyamide.
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Chapter Outline
Enzyme Active Site
Catalysis takes place on a small portion of the enzyme structure called the enzyme active site.
A space-filling model of the enzyme hexokinase is below.
The substrate glucose enters the site and binds. The enzyme changes its shape before the reaction takes place.
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Chapter Outline
Enzyme Active Site
An enzyme must attract and bind the substrate. Once the substrate is bound, a chemical reaction is catalyzed. This two-step process is as follows.
• Enzyme (E) and substrate (S) combine to form an
enzyme–substrate intermediate (E–S).
• The intermediate decomposes to give the product (P) and the enzyme.
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Chapter Outline
Enzyme Active Site
Each different enzyme has its own unique active site whose shape determines which substrates can bind. Enzymes are said to be stereospecific. Each enzyme catalyzes reactions for only a limited number of different reactant structures.
Enzyme-substrate interaction is explained by two
hypotheses. • The lock-and-key hypothesis • The induced-fit hypothesis
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Chapter Outline
Enzyme Active Site
The lock-and-key hypothesis envisions the substrate as a key that fits into the appropriate active site (the lock).
The induced-fit model proposes that the active site adjusts
its structure before the reaction can take place. Both of these hypotheses are demonstrated by the figure on
the next slide . . .
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Chapter Outline
Enzyme Active Site
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The correct substrate 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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Chapter Outline
Enzyme Active Site
Three additional ideas about enzyme-substrate interaction and catalysis are as follows.
• Proximity catalysis refers to an enzyme bringing the
reactants close together.
• The productive binding hypothesis explains how the enzyme works to make sure that the correct bonds are broken and formed during the reaction.
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Chapter Outline
Enzyme Active Site
• The strain hypothesis explains how the enzyme forces the substrate to change shape which allows catalysis to occur as shown in this figure.
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Chapter Outline Temperature and pH Effects on
Enzyme Catalysis Essentially any change that affects protein structure also
affects an enzyme’s catalytic function. Catalytic activity will be lost when an enzyme is denatured.
Strong acids and bases, organic solvents, mechanical
action, and high temperature decrease an enzyme-catalyzed rate of reaction. Even slight changes in the pH can have profound effects on enzyme catalysis.
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Chapter Outline
Enzymes have optimal operating temperatures and pHs. These graphs show how enzyme catalysis is affected by changes in temperature and pH for enzymes that operate most effectively at physiological conditions.
Temperature and pH Effects on Enzyme Catalysis
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Chapter Outline
Your Turn!
If you plot the reaction rate of an enzyme-catalyzed reaction on the vertical axis and the temperature of the reaction on the horizontal axis you will almost always get a straight line.
• True • False
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Chapter Outline
Your Turn!
False Enzymes generally act optimally within a
small temperature range. So, if you plot the reaction rate of an enzyme-catalyzed reaction on the vertical axis and the temperature of the reaction on the horizontal axis you will almost never get a straight line. The type of graph you would get is shown here.
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Chapter Outline
Enzyme Regulation
Enzyme catalysis is carefully controlled in cells. Cells use a variety of mechanisms to change the rates of enzyme-catalyzed reactions to meet metabolic needs.
Sometimes a new group of atoms covalently bond to the
enzyme in a process called covalent modification. In other cases, another molecule is noncovalently bound to the enzyme to affect catalytic activity.
The protein structural change that results can cause a
decrease in enzyme activity, enzyme inhibition, or an increase in activity, enzyme activation. 54
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Chapter Outline
Enzyme Regulation
The product binding to the enzyme and inhibiting catalysis is called product inhibition.
Feedback inhibition and feedforward activation are two
common forms of enzyme control. • Feedback inhibition affects enzymes at the beginning of
the reaction assembly line. In feedback inhibition the final product inhibits the enzyme.
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Chapter Outline
Enzyme Regulation
• Feedforward activation controls enzymes at the end of the molecular assembly line. Here an excess of starting materials will “feedforward” and activate enzymes.
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Chapter Outline
Your Turn!
In the reaction scheme below product D inhibits enzyme E1. What type of enzyme regulation is this an example of?
A B C DE1 E2 E3
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Chapter Outline
Your Turn!
This is an example of feedback inhibition because the product inhibits the overall reaction.
A B C DE1 E2 E3
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Chapter Outline
Chapter 30 Summary
• Enzymes are proteins that catalyze biochemical reactions.
• Some enzymes are conjugated proteins. The protein part is the apoenzyme and the nonprotein part is the coenzyme. The conjugated protein is termed a holoenzyme.
• The substance acted on by an enzyme is called the substrate.
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Chapter Outline
Chapter 30 Summary
• An enzyme is commonly studied by measuring a reaction rate which is the change in concentration of reactants or products with time.
• The transitions state in a reaction is the highest energy point during a reaction.
• Three common ways to increase a reaction rate are to increase the reactant concentration, increase the reaction temperature, or add a catalyst.
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Chapter Outline
Chapter 30 Summary
• Michaelis–Menten plots show the rates of enzyme-catalyzed reactions.
• The turnover number, the number of substrates an enzyme can react with in a given time span, is a measure of an enzyme’s catalytic ability.
• Catalysis takes place on a small portion of the enzyme
surface called the active site.
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Chapter Outline
Chapter 30 Summary
• The lock-and-key hypothesis and the induced-fit hypothesis describe how substrates and enzymes interact.
• Many enzymes use proximity catalysis, productive binding, or strain to affect catalysis.
• Enzyme structure and function are effected by pH and temperature.
• Enzymes are regulated by enzyme inhibition and enzyme activation. 62
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