CH. 6 Factors Affecting
ENZYME Activity
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Catabolic and Anabolic Reactions
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The energy-producing reactions within cells
generally involve the breakdown of complex organic
compounds to simpler compounds. These reactions
release energy and are called catabolic reactions.
Anabolic reactions are those that consume energy
while synthesizing compounds.
ATP produced by catabolic reactions provides the
energy for anabolic reactions. Anabolic and catabolic
reactions are therefore coupled (they work together)
through the use of ATP.
Diagram: next slide
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An anabolic reaction
Energy
Catabolic and Anabolic
Reactions
ATP
ADP + Pi
Energy
A catabolic reaction
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Enzymes lower the amount of
activation energy needed for a reaction.
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Energy Released
Energy Supplied
Enzymes Lower Activation Energy
Activation energy
without enzyme
Activation energy
with enzyme
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Substrate
Enzymes
Enzymes are organic
catalysts.
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Active Site
Cofactor/Coenzyme
Enzyme
Product
Enzyme-Substrate Complex
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Enzyme
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Enzymes
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Catalysts are substances that speed up chemical
reactions. Organic catalysts (contain carbon) are
called enzymes.
Enzymes are specific for one particular reaction or
group of related reactions.
Many reactions cannot occur without the correct
enzyme present.
They are often named by adding “ASE" to the name
of the substrate. Example: Dehydrogenases are
enzymes that remove hydrogen.
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Induced Fit Theory – Most current
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An enzyme-substrate complex forms
when the enzyme’s active site binds with
the substrate like a key fitting a lock.
The substrate molecule does not fit
exactly in the active site. This induces a
change in the enzymes conformation
(shape) to make a closer fit.
After the reaction, the products are
released and the enzyme returns to its
normal shape.
Only a small amount of enzyme is needed
because they can be used repeatedly.
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Rate of Reaction
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Reactions with enzymes are up to 10 billion times
faster than those without enzymes.
Enzymes typically react with between 1 and 10,000
molecules per second. Fast enzymes catalyze up to
500,000 molecules per second.
Substrate concentration, enzyme concentration,
Temperature, and pH affect the rate of enzyme
reactions.
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Substrate Concentration
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Enzyme
Active Site is
Saturated
Rate of Reaction
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At lower concentrations, the active sites on most of the enzyme
molecules are not filled because there is not much substrate. Higher
concentrations cause more collisions between the molecules. With
more molecules and collisions, enzymes are more likely to encounter
molecules of reactant.
The maximum velocity of a reaction is reached when the active sites
are almost continuously filled. Increased substrate concentration after
this point will not increase the rate. Reaction rate therefore increases
as substrate concentration is increased but it levels off.
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Substrate Concentration
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Enzyme Concentration
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Rate of Reaction
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If there is insufficient enzyme present, the reaction will
not proceed as fast as it otherwise would because there is
not enough enzyme for all of the reactant molecules.
As the amount of enzyme is increased, the rate of reaction
increases. If there are more enzyme molecules than are
needed, adding additional enzyme will not increase the
rate. Reaction rate therefore increases as enzyme
concentration increases but then it levels off.
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Enzyme Concentration
Even when adding
more enzymes, there
isn’t any more
available substrate to
create product at a
faster rate
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Rate of Reaction
Effect of Temperature on Enzyme
Activity
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Temperature
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Increasing the temperature causes
more collisions between substrate
and enzyme molecules. The rate of
reaction therefore increases as
temperature increases.
Rate of Reaction
Effect of Temperature on Enzyme
Activity
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40
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Temperature
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Effect of Temperature on Enzyme
Enzymes denature when
Activity
the temperature gets too
Rate of Reaction
high. The rate of reaction
decreases as the enzyme
becomes nonfunctional.
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Temperature
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Temperature
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Rate of Reaction
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Higher temperature causes more collisions between the atoms,
ions, molecules, etc. It therefore increases the rate of a reaction
– “Turnover Rate”. More collisions increase the likelihood that
substrate will collide with the active site of the enzyme.
Above a certain temperature, activity begins to decline because
the enzyme begins to denature (unfold).
The rate of chemical reactions therefore increases with
temperature but then decreases.
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Temperature
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Denaturation
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If the hydrogen bonds within an enzyme are broken, the
enzyme may unfold or take on a different shape. The enzyme
is denatured.
A denatured enzyme will not function properly because the
shape of the active site has changed.
If the denaturation is not severe, the enzyme may regain its
original shape and become functional.
The following will cause denaturation:
– Heat
– Changes in pH
– Heavy-metal ions (lead, arsenic, mercury)
– Alcohol
– UV radiation
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Effect of pH on Enzyme Activity
Rate of Reaction
Each enzyme has its own optimum pH.
Pepsin
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Trypsin
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pH
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pH
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Rate of Reaction
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Each enzyme has an optimal pH. Pepsin, an enzyme found in the
stomach, functions best at a low pH. Trypsin, found in the intestine,
functions best at a neutral pH.
A change in pH can alter the ionization of the R groups of the amino
acids. When the charges on the amino acids change, hydrogen bonding
within the protein molecule change and the molecule changes shape.
The new shape may not be effective.
The diagram shows that pepsin functions best in an acid environment.
This makes sense because pepsin is an enzyme that is normally found in
the stomach where the pH is low due to the presence of hydrochloric
acid. Trypsin is found in the duodenum (small intestine), and therefore,
its optimum pH is in the neutral range to match the pH of the
duodenum.
Pepsin
Trypsin
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3
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5
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pH
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Metabolic Pathways
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Metabolism refers to the chemical reactions that
occur within cells.
Reactions occur in a sequence and a specific enzyme
catalyzes each step.
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Notice that C can produce either D or F.
This substrate has two different enzymes
that work on it.
Metabolic Pathways
A
enzyme 1
B
enzyme 2
C
enzyme 3
enzyme 5
D
enzyme 4
E
F
Enzymes are very specific. In this case
enzyme 1 will catalyze the conversion of
A to B only.
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A Cyclic Metabolic Pathway
In this pathway, substrate “A” enters
the reaction. After several steps,
product “E” is produced.
A
B
F
A+FB
C
BCD
DF+E
E
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D
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Regulation of Enzymes
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The next several slides illustrate how cells
regulate enzymes. For example, it may be
necessary to decrease the activity of certain
enzymes if the cell no longer needs the product
produced by the enzymes.
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Regulation of Enzymes
Cell can turn on
DNA genes to build
more enzymes when
needed
genetic
regulation
competitive
inhibition
regulation of enzymes
already produced
noncompetitive
Inhibition
Cells can use
certain chemicals
to slow down
existing enzymes
(next slide)
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Competitive Inhibition
In competitive inhibition,
a similar-shaped molecule
competes with the
substrate for active sites.
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Competitive Inhibition
Active site is
being occupied
by competitive
inhibitor
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This substrate
cannot get into
active site at
this time
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Noncompetitive Inhibition
Active site
Inhibitor
Altered active site
Enzyme
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Another form of inhibition involves an inhibitor that
binds to an allosteric site of an enzyme. An
allosteric site is a different location than the active
site.
The binding of an inhibitor to the allosteric site alters
the shape of the enzyme, resulting in a distorted
active site that does not function properly.
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Noncompetitive Inhibition
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The binding of an inhibitor to an allosteric
site is usually temporary. Poisons are
inhibitors that bind irreversibly. For example,
penicillin inhibits an enzyme needed by
bacteria to build the cell wall. Bacteria
growing (reproducing) without producing
cell walls eventually rupture.
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Feedback Inhibition
The goal of this hypothetical metabolic
pathway is to produce chemical D from A.
A
enzyme 1
B
enzyme 2
C
enzyme 3
D
B and
are intermediates.
Enzyme regulation
by C
negative
feedback inhibition is
similar to the thermostat example. As an enzyme's product
The next several slides will show how
accumulates, it turns off the enzyme just as heat causes a
feedback inhibition regulates the amount
thermostat to turn off the production of heat.
of D produced.
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Feedback Inhibition
C and D will decrease
because B is needed to
produce C and C is
needed to produce D.
The amount of B in the cell will
decrease if enzyme 1 is inhibited.
A
enzyme 1
X
B
X
enzyme 2
C
X
enzyme 3
D
X
Enzyme 1 is structured in a way that causes it
to interact with D. When the amount of D
increases, the enzyme stops functioning.
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Feedback Inhibition
B, C, and D can now be synthesized.
A
enzyme 1
B
X
enzyme 2
C
X
enzyme 3
D
X
When the amount of D
drops, enzyme 1 will no
longer be inhibited by it.
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Feedback Inhibition
A
enzyme 1
X
B
enzyme 2
C
enzyme 3
D
As D begins to increase, it inhibits enzyme
1 again and the cycle repeats itself.
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The End
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