Chapter 5

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Chapter 5
Lecture Outline
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5-1
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How Cells Use Enzymes
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5-2
Organisms need energy and nutrients to
grow and reproduce.
Organisms mobilize nutrients for energy
through biochemical reactions.
Catalysts are chemicals that speed up the
rate of biochemical reactions.
Enzymes are catalytic proteins.
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How Enzymes Speed Chemical
Reactions
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Enzymes lower the activation energy of
biochemical reactions.
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Enzymes have a specific shape that fits with
the substrate shape.
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5-3
The reactants in an enzyme-catalyzed reaction
are called substrates.
When the substrate and enzyme interact, an
enzyme-substrate complex is formed.
This destabilizes the bonds in the substrate,
speeding up the reaction.
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Lowering Activation Energy
5-4
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How Substrates Bind to Enzymes
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Enzymes only catalyze one or a few reactions.
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The enzyme has a binding site for the substrate.
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Called the active site
Induced fit
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5-5
They are specific because they have a particular shape that only
fits particular substrates.
When the substrate binds to the active site, the enzyme changes
shape to fit it perfectly.
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How Enzymes are Usually
Named
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Since each enzyme catalyzes a specific reaction
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The first part of an enzyme’s name
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Examples:
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Indicates the type of reaction it will catalyze
All enzyme names end in the suffix
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Is the name of the substrate
The second part of an enzyme’s name
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Each has a unique name
DNA polymerase
Glycogen synthetase
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Why Enzymes Need Vitamins
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Some enzymes need special molecules to help them
function correctly.
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Cofactors can be inorganic ions, such as zinc or iron.
Some cofactors are organic molecules.
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5-7
Called coenzymes
Vitamins are the precursors for many coenzymes.
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Called cofactors
Vitamin B2 is made into FAD.
Niacin is made into NAD.
Vitamins must be acquired from the diet, since cells
cannot make them.
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The Role of Coenzymes
5-8
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How the Environment Affects
Enzymes
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The rate at which an enzyme can bind to a
substrate is called the turnover number.
The turnover number of an enzyme is
maximized under the ideal conditions for that
enzyme.
Each enzyme has ideal conditions that
include:
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5-9
Temperature
pH
Substrate concentration
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Temperature
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Temperature has two effects on enzymes.
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Changes the rate of molecular motion
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Increasing temperature increases molecular motion.
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Optimum temperature-the temperature at which the enzyme
has the highest rate of catalysis.
Decreasing temperature decreases molecular movement.
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Decreases the rate of catalysis
Causes changes in the shape of an enzyme
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5-10
Increases the rate of catalysis
Temperature changes above optimum will denature the
enzyme.
This changes its shape, and it can no longer bind substrate
and catalyze the reaction.
This is why a high fever is potentially dangerous.
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The Effect of Temperature
on Turnover Number
5-11
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pH
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In the three-dimensional shape of an enzyme
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In a basic environment
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The basic side chains could accept protons.
Both of these events will change the shape of the
enzyme
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The acidic side chains could donate protons.
In an acidic environment
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Some amino acid side chains are exposed to the
environment.
Making it less able to bind substrate, thus less able to
catalyze the reaction
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The Effect of pH on the Turnover
Number
5-13
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Enzyme-Substrate Concentration
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The rate of catalysis increases as the amount
of
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However, once all of the enzymes are
occupied, the rate of catalysis will not
increase
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5-14
Enzyme increases
Substrate increases
Even if more substrate is added
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
Once all enzymes (blue) are occupied,
adding more substrate (green) will not
increase reaction
E
E
E
5-15
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How the Cell Controls Enzymes
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A cell must be able to control when and how often its
biochemical reactions take place.
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Coordination ensures that reactions happen in the
correct order.
Regulation ensures that reactions happen at the
correct rate and controls the amount of product that
is made.
Cells coordinate and regulate the activity of their
enzymes by:
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5-16
It does this by controlling enzymes.
Enzymatic competition for substrate
Gene regulation
Enzyme inhibition
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Enzymatic Competition for Substrate
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Enzymatic competition
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Occurs when more than
one enzyme interacts
with the same substrate
Each enzyme converts
the substrate to a
different product.
The enzyme that “wins”
is the one that is the
most abundant at the
time.
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Gene Regulation
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Enzymes are proteins.
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Protein production is controlled by genes.
Certain chemicals in the cell turn particular
enzyme-producing genes on or off
depending on the situation.
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Called gene-regulator proteins
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Those that decrease the amount of an enzyme made
are called gene-repressor proteins.
Those that increase the amount of an enzyme made are
called gene-activator proteins.
Example: Malate synthetase
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Enzyme Inhibition
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Inhibitors are molecules
that attach to enzymes
and make them unable
to bind to substrate.
Many drugs, pesticides
and herbicides target
enzymes.
Types of inhibition
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5-19
Competitive inhibition
Negative-feedback
inhibition
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Competitive Inhibition
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Competitive
inhibitors closely
resemble the
substrate.
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Example:
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Therefore, they bind
to the active site of
the enzyme.
They block the
substrate from
binding.
Anti-herpes drugs
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Enzymes and Inhibition- Animation
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5-21
http://www.youtube.com/watch?v=PILzvT3sp
CQ&feature=related
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Negative-Feedback Inhibition
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5-22
Occurs within enzyme-catalyzed reactions
that occur in a sequence
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Negative-Feedback Inhibition
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As the end-product of the sequence accumulates,
– Those molecules feedback and bind to an
enzyme early in the sequence.
– They inhibit that enzyme, and stop the sequence.
– This decreases the amount of end-product made.
This functions to keep levels of the end-product
within a certain range.
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Feedback Inhibition Animation
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5-24
http://www.youtube.com/watch?v=rHDp4wJ1
U0w&feature=related
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Cells Use Enzymes to Process
Energy and Matter
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Organisms obtain
energy through
enzyme-catalyzed
biochemical reactions.
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5-25
These reactions break
chemical bonds,
releasing their internal
potential energy.
Example: burning wood
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Biochemical Pathways
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A series of enzyme-catalyzed reactions
Also called metabolic pathways
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Catabolism-the breakdown of compounds
Anabolism-the synthesis of new, larger compounds
Examples: photosynthesis, respiration, protein synthesis, etc.
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Generating Energy in a Useful
Form: ATP
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ATP
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Is the molecule that
organisms use to fuel
anabolic reactions
Is an adenosine + three
phosphates
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The bonds between the
phosphates contain a lot of
potential energy.
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Breaking those bonds
releases a lot of energy.
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Called high energy
phosphate bonds
ATP - 1 phosphate = ADP
ADP – 1 phosphate = AMP
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ATP: The Power Supply for Cells
5-28
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Electron Transport
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Electrons in the outer energy level of atoms can be
lost to the other atoms
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If they receive energy and become excited
Special molecules can receive the excited electrons
and harness that energy.
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They are called electron carriers.
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The transfer reactions are called oxidation-reduction
reactions.
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NAD, FAD
Molecules losing electrons become oxidized. (OIL)
Molecules receiving electrons become reduced.
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Proton Pumps
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The energy released by the transfer of electrons can
be used to pump protons.
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This is accomplished by proton pumps.
This concentrates protons in a small space.
The “pressure” created by this concentration gradient drive
the diffusion of the protons.
The protons diffuse through a special protein called
ATP synthase.
The ATP synthase uses the energy released from
the diffusion of the protons to make ATP.
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Electron Transport and Proton
Gradient
5-31
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