Download Chapter 30 HEIN

Enzymes
Chapter 30
Hein * Best * Pattison * Arena
Version 1.0
Colleen Kelley
Chemistry Department
1
Pima Community College
© John Wiley and Sons, Inc.
Chapter 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
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Molecular Accelerators
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•
•
•
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Enzymes are the catalysts of biochemical
reactions.
Enzymes catalyze nearly all the myriad
reactions that occur in living cells.
Uncatalyzed reactions that require hours of
boiling in the presence of a strong acid or
strong base can occur in a fraction of a
second in the presence of the proper enzyme.
The catalytic functions of enzymes are
directly dependent on their threedimensional structures.
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Figure 30.1 A typical reaction-energy profile: The lower activation
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energy in the cell is due to the catalytic effect of enzymes.
•Each organism contains thousands
of enzymes:
1.Some are simple proteins
consisting of only amino acid units.
2.Others are conjugated and consist
of a protein part, or apoenzyme,
and a nonprotein part, or
coenzyme.
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•A functioning enzyme that consists
of both the protein and nonprotein
parts is called a holoenzyme.
•Apoenzyme + Coenzyme =
Holoenzyme
•Often the coenzyme is derived from
a vitamin, and one coenzyme may be
associated with different enzymes.
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•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.
•The activator is analogous to a
coenzyme.
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•Another remarkable property of
enzymes is their specificity of
reaction – that is, a certain enzyme
catalyzes the reaction of a specific
type of substance.
• e.g. lactase
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•The substance acted on by an
enzyme is called the substrate.
•e.g. Sucrose is the substrate of the
enzyme sucrase.
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Classes of Enzymes
1. Oxidoreductases: Enzymes that catalyze
the oxidation-reduction 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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Classes of Enzymes
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 ATP.
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Rates of
Chemical Reactions
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Figure 30.2 The change in product concentration [B] as a
function of time. The reaction rate is determined by
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measuring the slope of this line.
Figure 30.3 An energy profile for the reaction between water
and carbon dioxide.
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•
There are three common ways to
increase a reaction rate:
1. Increasing the reactant concentration
2. Increasing the reaction temperature
3. Adding a catalyst
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Enzyme Kinetics
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Figure 30.4 A
Michaelis-Menten
plot showing the rate
of enzyme-catalyzed
reaction as a function
of substrate
concentration. The
lower left portion of
the graph marks the
approximate area
where an enzyme
responds best to
concentration
changes.
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Figure 30.5 Michaelis-Menten plots for two glucose
metabolic enzymes.
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Turnover Number
• An enzyme’s catalytic speed is also
matched to an organism’s metabolic
needs.
• This catalytic speed is commonly
referred to as turnover number – the
number of molecules an enzyme can
react or “turn-over” in a given time
span.
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Industrial Strength
Enzymes
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•
Enzymes offer two major advantages
to manufacturing processes and in
commercial products:
1. Enzymes cause very large increases
in reaction rates even at room
temperature.
2. Enzymes are relatively specific and
can be used to target selected
reactants.
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•
•
•
Proteases (proteolytic enzymes)
break down proteins.
Lipases digest lipids.
Cellulases, amylases, lactases, and
pectinases break down
carbohydrates, cellulose, amylose,
lactose, and pectin, respectively.
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Enzyme Active Site
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• Catalysis takes place on a small portion
of the enzyme structure called the
enzyme active site.
• Often this is a crevice or pocket on the
enzyme that represents only 1-5% of
the total surface area.
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Figure 30.6 A spacefilling model of the enzyme
hexokinase (a) before and (b) after it binds to the substrate
D-glucose. Note the two protein domains for this enzyme,
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which are colored differently.
Figure 30.7 Enzyme-substrate interaction illustrating both the
lock-and-key hypothesis and the induced-fit model. The correct
substrate (orange square-blue circle) fits the active site (lock-andkey 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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Figure 30.8 Strain
Hypothesis: The
substrate is being
forced toward the
product shape by
enzyme binding.
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Temperature and
pH Effects on
Enzyme Catalysis
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•
•
•
Essentially, any change that affects
protein structure also affects an enzyme’s
catalytic function.
If an enzyme is denatured, its activity will
be lost.
Thus, strong acids and bases, organic
solvents, mechanical action, and high
temperature are examples of treatments
that decrease an enzyme-catalyzed rate of
reaction.
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Figure 30.9 A plot of the enzymecatalyzed rate as a function of pH.
Figure 30.10 A plot of the
temperature dependence of an
enzyme-catalyzed reaction31
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