Transcript Enzymes

Chapter
Outline
Chapter 30
Enzymes
Enzymes
accelerate
chemical
reactions as the
engine
accelerates this
drag race.
Introduction to General, Organic, and Biochemistry, 10e
John Wiley & Sons, Inc
Morris Hein, Scott Pattison, and Susan Arena
Chapter
Outline
Course 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
Chapter 30 Summary
2
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
Molecular Accelerators
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.
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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).
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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 -
COO -
O
H
C
OH
H
C
O
P
H
C
H
H
C
H
O
O
O
P
O-
O
O
-
O3-phosphoglycerate
H
2-phosphoglycerate
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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 -
COO -
O
H
C
OH
H
C
O
P
H
C
H
H
C
H
O
O
O
P
O-
O
O
-
O3-phosphoglycerate
H
2-phosphoglycerate
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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.
C
Reaction Time (hr)
0.0
1.0
2.0
3.0
4.0
D
D Concentration (M)
0.0
0.5
1.5
4.5
6.0
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Chapter
Outline
Your Turn!
Reaction Time (hr)
0.0
1.0
2.0
3.0
4.0
D Concentration (M)
0.0
0.5
1.5
4.5
6.0
Total change in concentration of D (0.0 M to 6.0 M)
Rate of appearance of D =
=
Total change in time (0.0 hr to 4.0 hr)
6.0 M
4.0 hr
=
1.5 M of D
hr
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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.
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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.
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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 enzymecatalyzed reaction as a function of substrate
concentration is shown here.
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 minutes
=
171
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
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more nutritious.
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
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). 47
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 enzymecatalyzed rate of reaction. Even slight changes in the
pH can have profound effects on enzyme catalysis.
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Chapter
Outline
Temperature and pH Effects on
Enzyme Catalysis
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.
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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
52
Chapter
Outline
Your Turn!
False
Enzymes generally act optimally within a
small temperature range. So, if you
plot the reaction rate of an enzymecatalyzed 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 enzymecatalyzed 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
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increase in activity, enzyme activation.
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
E1
B
E2
C
E3
D
57
Chapter
Outline
Your Turn!
This is an example of feedback inhibition because the
product inhibits the overall reaction.
A
E1
B
E2
C
E3
D
58
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.
59
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.
60
Chapter
Outline
Chapter 30 Summary
• Michaelis–Menten plots show the rates of enzymecatalyzed 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.
61
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.
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