Download 10.4 Factors That Affect Enzyme Activity, Continued

Chapter 10 Lecture
General, Organic, and Biological
Chemistry: An Integrated Approach
Laura Frost, Todd Deal and Karen Timberlake
by Richard Triplett
Chapter 10
Enzymes—Nature’s Chemists
© 2011 Pearson Education, Inc.
Chapter Outline
10.1 Enzymes and Their Substrates
10.2 Thermodynamics of Chemical Reactions
10.3 Enzymes and Catalysis
10.4 Factors That Affect Enzyme Activity
© 2011 Pearson Education, Inc.
Chapter 10
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Introduction
• Enzymes are biologically active proteins that
accelerate the breakdown of food that is eaten.
• Enzymes are biological catalysts. They
accelerate reactions, but are not consumed or
changed in reactions.
• Discussions on the production or consumption of
energy, specifically heat, during chemical
reactions is called thermodynamics.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates
• Enzymes are large proteins with complex, threedimensional structures.
• Enzymes work in an aqueous environment in
our body so that the protein chain folds such that
the polar amino acids are on the surface.
• Consider hexokinase, an enzyme whose job is
to transfer a phosphate group from the high
energy molecule, adenosine triphosphate,
ATP, to D-glucose.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
• In this equation, the enzyme name is written
above or below the reaction arrow.
• The phosphate group is represented by a P in a
circle.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
The Active Site
The folded structure for hexokinase is shown here.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
• When in its proper three-dimensional shape,
hexokinase has an indentation on one side of
the structure.
• This indentation is known as the active site, and
it is lined with amino acid side chains.
• The active site is the functional part of an
enzyme where catalysis occurs.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
• Glucose, the reactant for hexokinase, fits snugly
in the active site. In an enzyme reaction, the
reactant is called the substrate.
• Enzymes have specific substrates, a property
known as substrate specificity. For example,
the active site of hexokinase reacts with
D-glucose, but will not react with L-glucose.
• Enzymes are specific for one enantiomer of the
substrate.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
Some enzymes, like hexokinase, have nonprotein helpers. Two categories of helpers are
as follows:
1. Cofactors are inorganic substances like
magnesium ions.
2. Coenzymes are small organic molecules derived
from vitamins. Riboflavin found in the coenzyme
flavin adenine dinucleotide (FAD) is a coenzyme.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
Enzyme–Substrate Models
• A substrate is drawn into the active site by
intermolecular attractions like hydrogen bonding.
• Hydrogen bonding orients the substrate properly
within the active site.
• The initial interaction of the enzyme with the
substrate is called the enzyme–substrate
complex (ES). This complex forms prior to
catalysis.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
There are two enzyme–substrate models:
1. In the Lock-and-key model, the active site
is thought to be a rigid, inflexible shape that
is an exact complement to the shape of the
substrate. The substrate fits in the active site
much like a key fits in a lock.
2. In the induced-fit model, the active site is
flexible, has a shape roughly complementary
to the shape of its substrate, and undergoes
a conformational change, adjusting to the
shape of the substrate when the substrate
interacts with the enzyme.
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
© 2011 Pearson Education, Inc.
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10.1 Enzymes and Their Substrates, Continued
A good example of an induced-fit model is when
hexokinase and glucose form an enzyme–
substrate complex as shown.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions
• As chemical reactions occur, some bonds are
formed and some are broken, and in the
process, the amount of energy changes.
• Some reactions release energy as heat
(exothermic reactions), and some absorb
energy as heat (endothermic reactions).
• A collision of reactant molecules must occur for
a chemical reaction to occur.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
• Energy is required to cause reactant molecules
to collide.
• Reactant molecules must be aligned properly in
order for a reaction to occur.
• Activation energy is required to properly align
reactant molecules and to cause them to collide
to produce products.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
If the energy that is available is lower than the
activation energy, the molecules will not collide
with enough force to form products.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
• The activation energy that must be overcome
before products are formed is shown as:
• The heat of reaction is the difference between the
energy of reactants and the energy of products.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
• In an exothermic reaction, the energy of
reactants is higher than the energy of products,
so heat is released.
• In an endothermic reaction, the energy of
products is higher than the energy of reactants,
so heat is absorbed.
• The height of the activation energy peak gives
an indication of how fast the reaction proceeds.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
• Reactions with a low activation energy will
proceed at a faster rate than reactions with a
high activation energy.
• Activation energy can be lowered with a catalyst,
which will cause the reaction to proceed at a
faster rate.
• A catalyst will not affect the energy of products
or reactants.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
An enzyme-catalyzed reaction increases the rate
of a reaction by forming ES before forming a
product.
© 2011 Pearson Education, Inc.
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10.2 Thermodynamics of Chemical Reactions,
Continued
• Consider the addition of hydrogen peroxide to a
cut. The area bubbles considerably because
oxygen is produced by the enzyme catalase
found in blood.
• A high concentration of oxygen is produced at
the wound site that kills germs.
• Without catalase, this reaction occurs very
slowly.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis
• Enzymes lower the activation energy by forming
ES complex.
• ES is formed through the interactions between the
enzyme and substrate. Each interaction releases a
small amount of energy to stabilize the complex.
• These interactions combine to lower the activation
energy of the reaction.
• Some interactions that help lower the activation
energy are discussed in the next several slides.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
Proximity
• The active site of enzymes has a small volume.
• When ES forms, the active site is filled with
substrate.
• The reacting molecules are in close proximity to
each other, and the closer they are the more likely
a reaction will occur.
• Amino acid side chains in the active site are used
to facilitate the reaction.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
Orientation
• In the active site, substrate molecules are held
at the appropriate distance and in correct
alignment to each other for the reaction to occur.
• Amino acid side chains in the active site create
interactions that orient the substrates.
• This lowers the activation energy needed for the
reaction to occur.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
Proper orientation is shown as:
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
Bond Energy
• When an enzyme interacts with substrate to
form ES, the bonds of the substrate molecule
are weakened (strained).
• Strained bonds in the substrate means that the
reaction will proceed more rapidly because the
activation energy is lowered by this effect.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
• Consider the hexokinase reaction that catalyzes
glucose to glucose-6-phosphate.
• Mg2+ (a coenzyme) holds ATP in one area of the
active site and glucose interacts with another
area.
• Amino acid side chains in the active site form
multiple hydrogen bonds with glucose, which
stabilizes ES and lowers the activation energy.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
• A conformational change in the enzyme occurs
when glucose enters the active site.
• ATP is in close proximity to the glucose and is in
proper orientation for the reaction.
• Glucose-6-phosphate is formed along with ADP.
• The enzyme is less attracted to the products, so
the lobes of the enzyme move apart and the
products are released.
© 2011 Pearson Education, Inc.
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10.3 Enzymes and Catalysis, Continued
This figure shows the formation of
glucose-6-phosphate by hexokinase.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity
• If allowed to sit untouched, the flesh of sliced
apples will turn brown by a process known as
oxidation, caused by an enzyme.
• If lemon juice is sprinkled on the sliced apple,
the vitamin C in the lemon juice will inhibit the
formation of this brown color by changing the pH
of the environment of the enzyme.
• Enzyme reactions are affected by reaction
conditions such as substrate concentration, pH,
temperature, and the presence of inhibitors.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
Substrate Concentration
• Recall that the first step in an enzyme-catalyzed
reaction is the formation of ES.
• At a constant concentration of enzyme, an
increase in substrate concentration will cause an
increase in the enzyme activity up to the point
where the enzyme becomes saturated with
substrate.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Increasing substrate concentration will not affect
the rate of the reaction.
• A condition known as steady state is when an
enzyme is operating under maximum activity.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
pH
• When the enzyme environment is changed by
pH, its tertiary structure is disrupted, altering the
active site and causing the enzyme’s activity to
decrease.
• Enzymes are most active at a pH known as their
optimum pH.
• At optimum pH, the enzyme maintains its tertiary
structure and its active site.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Changes in pH will also affect the nature of the
amino acid side chains in the active site.
• The optimum pH for enzymes is based on the
location of the enzymes as shown:
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
Temperature
• Enzymes have an optimum temperature at
which they are most active.
• The optimum temperature for most human
enzymes is normal body temperature, 37 oC.
• Above optimum temperature, enzymes lose
activity due to disruption of intermolecular forces
stabilizing the tertiary structure.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• At high temperatures, enzymes denature, which
modifies the active site.
• At low temperatures, enzyme activity is low due
to a lack of energy for the reaction to occur.
• Food is stored in a refrigerator or freezer to slow
spoilage brought on by enzymes.
• Boiling contaminated water will destroy enzymes
in bacteria that are present in the water.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
Inhibitors
• Inhibitors are types of molecules that will cause
enzymes to lose activity.
• Enzyme inhibitors prevent the active site from
interacting with substrate to form ES.
• Some inhibitors cause temporary loss of activity,
while others cause permanent loss of activity.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Reversible inhibition occurs when the inhibitor
causes a temporary loss of activity. However,
activity is regained if the inhibitor is removed.
• Reversible inhibitors can be competitive or
noncompetitive.
• Competitive inhibitors are molecules that
compete with a substrate for the active site, and
have a structure similar to the substrate.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
As long as an inhibitor remains in the active site,
the enzyme cannot react with the substrate to form
product.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• An example of a medical therapy that involves a
competitive inhibitor involves liver alcohol
dehydrogenase (LAD). This enzyme oxidizes
ethanol, the alcohol found in alcoholic
beverages.
• This enzyme will also react with ethylene glycol
and methanol, which are found in antifreeze,
and will compete with ethanol for the active site.
• If a pet is poisoned by drinking antifreeze, a slow
intravenous infusion of ethanol is administrated.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Administration of ethanol slows the production of
the toxic metabolites of ethylene glycol and
methanol, giving the kidneys time to eliminate
these two substrates.
• Noncompetitive inhibitors do not resemble the
substrate. They do not compete for the
enzyme’s active site.
• Noncompetitive inhibitors bind at a site on the
enzyme that is usually remote to the active site.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• When a noncompetitive inhibitor binds to an
enzyme, it causes a conformational change in
the enzyme. This change in shape causes the
active site to no longer interact with the
substrate.
• As long as this type of inhibitor is bound to the
enzyme, it will no longer function effectively.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
This figure diagrams how a noncompetitive inhibitor
functions.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Inhibitions caused by competitive and
noncompetitive inhibitors can be reversed.
• Inhibition by competitive inhibitors can be
reversed by adding more substrate. The higher
the concentration of substrate, the more likely it
will overcome the competition for the active site.
• Adding more substrate with noncompetitive
inhibitors has no effect on overcoming inhibition.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Reversing a noncompetitive inhibitor requires a
special chemical reagent to remove the inhibitor
and restore catalytic activity.
• An irreversible inhibitor forms a covalent bond
with an amino acid side chain in the enzyme’s
active site.
• Irreversible inhibition causes the substrate to be
excluded from the active site.
• Irreversible inhibition is a permanent inhibition.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
• Irreversible inhibition is demonstrated in this
figure.
• Heavy metals like silver, mercury, and lead are
examples of irreversible inhibitors.
© 2011 Pearson Education, Inc.
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10.4 Factors That Affect Enzyme Activity,
Continued
Antibiotics Inhibit Bacterial Enzymes
• Enzyme inhibitors are used to fight bacterial
infections.
• Penicillin is an example of an irreversible inhibitor.
It binds to the enzyme that bacteria use to
synthesize cell walls, and slows the growth of cell
walls.
• Without a cell wall, bacteria cannot survive and the
infection stops.
© 2011 Pearson Education, Inc.
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Chapter Summary
10.1 Enzymes and Their Substrates
• Enzymes are large, globular proteins that serve
as biological catalysts.
• The functional part of an enzyme is the active
site.
• Substrates are the reactants for an enzyme
reaction, and they bind to the active site to form
ES.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.1 Enzymes and Their Substrates, Continued
• Enzymes are specific for one substrate that will
bind to the active site and react.
• Two theories, lock-and-key and induced-fit,
explain how an enzyme interacts with its
substrate to form ES.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.2 Thermodynamics of Chemical Reactions
• Thermodynamics is a study of the energy changes
that occur during a chemical reaction.
• Activation energy is the energy required to start a
reaction, and plays a role in the rate of reaction.
• The lower the activation energy, the faster the rate
of reaction.
• Heat of reaction is a measure of the production or
consumption of energy in a reaction.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.2 Thermodynamics of Chemical Reactions,
Continued
• An exothermic reaction releases heat to its environment.
• An endothermic reaction absorbs heat from its
environment.
• Catalysts lower the activation energy causing an increase
in the rate of reaction.
• Enzymes form ES before catalysis, which causes a
lowering of the activation energy.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.3 Enzymes and Catalysis
• Formation of the ES complex lowers the
activation energy for a catalyzed reaction.
• In the active site, atoms are brought close
together and aligned with amino acid side
chains.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.3 Enzymes and Catalysis, Continued
• The active site also aligns the reactants with the
optimal orientation for a reaction to occur.
• The interaction of substrate with the active site
weakens bonds between atoms in the substrate
so the reaction can form products easier.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.4 Factors That Affect Enzyme Activity
• Factors such as pH, temperature, substrate
concentration, and the presence of inhibitors can
affect the activity of an enzyme.
• An increase in substrate concentration increases
the rate of an enzyme-catalyzed reaction. When
substrate concentration is increased, the active
site becomes saturated with substrate.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.4 Factors That Affect Enzyme Activity,
Continued
• When the active site is saturated, the enzyme is
operating at steady state.
• Enzymes have an optimum pH and temperature at
which they function best.
• Inhibitors decrease or eliminate an enzyme’s
catalytic abilities.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
10.4 Factors That Affect Enzyme Activity,
Continued
• The effect of inhibitors can be reversible or
irreversible.
• Reversible inhibitors can be competitive inhibitors,
which compete with the substrate for the active site.
• Reversible inhibitors can also be noncompetitive,
which bind to a site on the enzyme other than the
active site, and causes a conformational change in the
enzyme.
© 2011 Pearson Education, Inc.
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