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Chapter 20 Enzymes and Vitamins
20.1
Enzymes
• Describe how enzymes function as catalysts; name and classify them.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
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Enzymes are Biological Catalysts
Enzymes are proteins
that
 catalyze nearly all the
chemical reactions
taking place in the cells
of the body
 increase the rate of
reaction by lowering
the energy of activation
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Names of Enzymes
The name of an enzyme
 usually ends in ase
 identifies the reacting substance; for example,
sucrase catalyzes the reaction of sucrose
 describes the function of the enzyme; for example,
oxidases catalyze oxidation
 can be a common name, particularly for the digestive
enzymes (such as pepsin and trypsin)
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Classification of Enzymes
Enzymes are classified by the types of reaction they
catalyze.
Class
Type of Reactions catalyzed
Oxidoreductases
Oxidation–reduction
Transferases
Transfer groups of atoms
Hydrolases
Hydrolysis
Lyases
Add atoms/remove atoms to or
from a double bond
Isomerases
Rearrange atoms
Ligases
Use ATP to combine small molecules
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Oxidoreductases and Transferases
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Hydrolases and Lyases
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Isomerases and Ligases
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Learning Check
Match the type of reaction with an enzyme
1) aminase
2) dehydrogenase
3) isomerase
4) synthetase
A.
B.
C.
D.
converts a cis-fatty acid to a trans-fatty acid
removes 2H atoms to form a double bond
combines two molecules to make a new compound
adds NH3
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Solution
Match the type of reaction with an enzyme
1) aminase
2) dehydrogenase
3) isomerase
4) synthetase
3
2
4
1
A.
B.
C.
D.
converts a cis-fatty acid to a trans-fatty acid
removes 2H atoms to form a double bond
combines two molecules to make a new compound
adds NH3
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9
Chapter 20 Enzymes and Vitamins
20.2
Enzyme Action
• Describe the role of an enzyme in an enzyme–catalyzed
reaction.General, Organic, and Biological Chemistry
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Active Site
How enzyme works p 706
The active site
 is a region within an
enzyme that fits the
shape of the reacting
molecule, called a
substrate
 contains amino acid R
groups that bind the
substrate
 releases products when
the reaction is complete
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Enzyme-Catalyzed Reaction
 The proper fit of a substrate (S) in an active site on
an enzyme (E) forms an enzyme–substrate (ES)
complex.
E+S
ES
 Within the ES complex, the reaction occurs to
convert substrate to product (P).
ES
E+P
 The products, which are no longer attracted to the
active site, are released.
 Overall, substrate is converted to product.
E+S
ES
E+P
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Enzyme-Catalyzed Reaction
(continued)
In an enzyme-catalyzed
reaction,
 a substrate attaches to
the active site
 an enzyme–substrate
(ES) complex forms
 reaction occurs and
products are released
 an enzyme (E) is used
over and over
E+S
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ES
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E+ P
13
Enzyme Specificity
Enzymes may recognize and catalyze
 a single substrate
 a group of similar substrates
 a particular type of bond
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Lock-and-Key Model
In the lock-and-key model of enzyme action,
 the active site has a rigid shape
 only substrates with the matching shape can fit
 the substrate is the key that fits that lock
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Induced-Fit Model
In the induced-fit model of enzyme action,
 enzyme structure is flexible, not rigid
 shapes of enzyme and substrate adjust for best fit at
the active site to improve catalysis of reaction
 substrate specificity increases
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Example of an Enzyme-Catalyzed
Reaction
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Learning Check
A. The active site is
1) the entire enzyme
2) a section of the enzyme
3) the substrate
B. In the induced-fit model, the shape of the enzyme
when substrate binds
1) stays the same
2) adapts to the shape of the substrate
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Solution
A. The active site is
2) a section of the enzyme
B. In the induced-fit model, the shape of the enzyme
when substrate binds
2) adapts to the shape of the substrate
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H.N. P 709
Isoenzymes as Diagnostic Tools
Isoenzymes
 catalyze the same reaction in different tissues in
the body
 such as lactate dehydrogenase (LDH), which
converts lactate to pyruvate, consist of five
isoenzymes
 can be used to identify the organ or tissue involved
in damage or disease
 such as LDH have one form more prevalent in
heart muscle and another form in skeletal muscle
and liver tissue
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Isoenzymes (continued)
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Diagnostic Enzymes
Diagnostic enzymes
 determine the
amount of damage in
tissues
 that are elevated
may indicate damage
or disease in a
particular organ
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Diagnostic Enzymes (continued)
Levels of the enzyme
creatine kinase (CK),
lactic dehydrogenase
(LDH), and aspartate
transaminase (AST)
 are elevated following
a heart attack
 are used to determine
the severity of the
attack
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Chapter 20 Enzymes and Vitamins
20.3
Factors Affecting Enzyme Activity
• Describe the effect of temperature, pH, concentration of enzyme, and
concentration of substrate on enzyme activity.
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Temperature and Enzyme Action
Enzymes
 are most active at an
optimum temperature
(usually 37 °C in
humans)
 show little activity at
low temperatures
 lose activity at high
temperatures as
denaturation occurs
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pH and Enzyme Action
Enzymes
 are most active at
optimum pH
 contain R groups of
amino acids with
proper charges at
optimum pH
 lose activity in low or
high pH as tertiary
structure is disrupted
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Optimum pH Values
Enzymes in
 the body have an optimum pH of about 7.4
 certain organs have enzymes that operate at lower
and higher optimum pH values
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Enzyme Concentration
An increase in enzyme
concentration
 increases the rate of
reaction (at constant
substrate
concentration)
 binds more substrate
with enzyme
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Substrate Concentration
An increase in substrate
concentration
 increases the rate of
reaction (at constant
enzyme concentration)
 eventually saturates an
enzyme with substrate
to give maximum
activity
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Learning Check
Sucrase has an optimum temperature of 37 °C and an
optimum pH of 6.2. Determine the effect of the following
on its rate of reaction.
1) no change
2) increase
3) decrease
A. increasing the concentration of sucrase
B. changing the pH to 4
C. running the reaction at 70 °C
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Solution
Sucrase has an optimum temperature of 37 °C and an
optimum pH of 6.2. Determine the effect of the following
on its rate of reaction
1) no change
2) increase
3) decrease
2 A. increasing the concentration of sucrase
3 B. changing the pH to 4
3 C. running the reaction at 70 °C
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31
Chapter 20 Enzymes and Vitamins
20.4
Enzyme Inhibition
• Describe competitive and noncompetitive inhibition, and reversible and
irreversible inhibition.
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Enzyme Inhibitors
Inhibitors
 are molecules that cause a loss of catalytic activity
 prevent substrates from fitting into the active sites
E+S
ES
E+P
E+I
EI
no P
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Competitive Inhibitor
A competitive inhibitor
 has a structure that is
similar to that of the
substrate
 competes with the
substrate for the active
site
 has its effect reversed by
increasing substrate
concentration
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Noncompetitive Inhibitor
A noncompetitive inhibitor
 has a structure that is
much different than the
substrate
 distorts the shape of the
enzyme, which alters the
shape of the active site
 prevents the binding of the
substrate
 cannot have its effect
reversed by adding more
substrate
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Malonate and Succinate
Dehydrogenase
Malonate
 is a competitive
inhibitor of succinate
dehydrogenase
 has a structure that is
similar to succinate
 inhibition is reversed
by adding succinate
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Irreversible Inhibition
In irreversible inhibition, an inhibitor
 bonds with R groups at the active site
 destroys enzyme activity
Enzyme Inhibition p 714
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Irreversible Enzyme Inhibitors
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Learning Check
Identify each description as an inhibitor that is
1) competitive or 2) noncompetitive.
A.
B.
C.
D.
increasing substrate reverses inhibition
binds to enzyme surface but not to the active site
structure is similar to substrate
inhibition is not reversed by adding more substrate
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Solution
Identify each description as an inhibitor that is
1) competitive or 2) noncompetitive.
1
2
1
2
A. increasing substrate reverses inhibition
B. binds to enzyme surface but not to the active site
C. structure is similar to substrate
D. inhibition is not reversed by adding more
substrate
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Chapter 20 Enzymes and Vitamins
20.5
Regulation of Enzyme Activity
Describe the role of zymogens, feedback control, and allosteric enzymes in
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regulatingGeneral,
enzyme
activity.
Zymogens
Zymogens (proenzymes)
 are inactive forms of enzymes
 are activated when one or more peptides are removed
Example: The zymogen proinsulin is converted to its
active form, insulin, by removing a small peptide chain.
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Digestive Enzymes
Digestive enzymes are
 produced as zymogens in one organ and transported
to another, such as the pancreas, when needed
 activated by removing small peptide sections
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Digestive Enzymes (continued)
Zymogen
(from pancreas)
Active Enzyme
(in small intestine)
Enteropeptidase
Trypsinogen
trypsin + peptide
Trypsin
Chymotrypsinogen
chymotrypsin + 2 dipeptides
Trypsin
Procarboxypeptidase
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carboxypeptidase + peptide
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Summary of Digestive Enzymes
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Allosteric Enzymes
An allosteric enzyme is
 an enzyme in a reaction sequence that binds a
regulator substance
 a positive regulator when it enhances the binding
of substrate and accelerates the rate of reaction
 a negative regulator when it prevents the binding
of the substrate to the active site and slows down
the rate of reaction
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Feedback Control
In feedback control,
 a product acts as a regulator
 an end product binds with the first enzyme (E1) in
a sequence when sufficient product is present so
shuts down the reaction
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Feedback Control by Threonine
Deaminase
When isoleucine accumulates in a cell,
 it binds to threonine deaminase, the first enzyme in
the pathway from threonine to isoleucine
 a change in the shape of threonine deaminase blocks
the binding of threonine
 feedback control decreases the synthesis of
isoleucine
Enzyme Inhibition p 714
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Learning Check
Identify each statement as:
zymogen (Z)
allosteric enzyme (A)
positive regulator (PR)
feedback control (FC)
A. an enzyme in a pathway that controls the rate of the
reaction
B. speeds up a reaction by combining with an
enzyme in the pathway
C. removal of a peptide activates the enzyme
D. some product binds to the first enzyme to limit the
synthesis of product
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Solution
Identify each statement as:
zymogen (Z)
allosteric enzyme (A)
positive regulator (PR)
feedback control (FC)
A
PR
Z
FC
A. an enzyme in a pathway that controls the rate
of the reaction
B. speeds up a reaction by combining with an
enzyme in the pathway
C. removal of a peptide activates the enzyme
D. some product binds to the first enzyme to limit
the synthesis of product
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50
Chapter 20 Enzymes and Vitamins
20.6
Enzyme Cofactors and Vitamins
• Describe the types of cofactors found in enzymes.
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Enzyme Cofactors
 A simple enzyme is an active enzyme that
consists only of protein.
 Many enzymes are active only when they combine
with cofactors such as metal ions or small
molecules.
 A coenzyme is a cofactor that is a small organic
molecule such as a vitamin.
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Enzyme Cofactors (continued)
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Function of Coenzymes
 A coenzyme prepares the active site for catalytic
activity.
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Metal Ions as Cofactors
Many active enzymes
require a metal ion.
For example, Zn2+, a
cofactor for
carboxypeptidase,
stabilizes the carbonyl
oxygen during the
hydrolysis of a peptide
bond.
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Some Enzymes and Their Cofactors
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Learning Check
Identify each enzyme as
1) a simple enzyme
2) an enzyme that required a cofactor
A. requires Mg2+ for hydrolysis of phosphate esters
B. requires vitamin B3 to transfer an acetyl group
C. is active with four polypeptide subunits
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Solution
Identify each enzyme as
1) a simple enzyme
2) an enzyme that required a cofactor
2 A. requires Mg2+ for hydrolysis of phosphate esters
2 B. requires vitamin B3 to transfer an acetyl group
1 C. is active with four polypeptide subunits
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Water-Soluble Vitamins
Water-soluble vitamins are
 soluble in aqueous solutions
 cofactors for many enzymes
 not stored in the body
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Descriptions of Water-Soluble
Enzymes
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Fat-Soluble Vitamins
Fat-soluble vitamins are
 vitamins A, D, E, and K
 soluble in lipids but not in aqueous solutions
 stored in the body
 important in vision, bone formation, antioxidants, and
blood clotting
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Learning Check
Identify each of the following as a water-soluble vitamin
(WS) or fat-soluble vitamin (FS).
A. folic acid
B. retinol (vitamin A)
C. vitamin C
D. vitamin E
E. niacin
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Solution
Identify each of the following as a water-soluble vitamin
(WS) or fat-soluble vitamin (FS)
WS A. folic acid
FS
B. retinol (vitamin A)
WS C. vitamin C
FS
D. vitamin E
WS E. niacin
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Learning Check
Identify the vitamin associated with each
1) riboflavin (B2)
2) vitamin A
3) vitamin K
4) vitamin D
5) ascorbic acid
A.
B.
C.
D.
E.
collagen formation
part of the coenzymes FAD and FMN
absorption of phosphorus and calcium in bone
vision
blood clotting
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Solution
Identify the vitamin associated with each
1) riboflavin (B2)
2) vitamin A
3) vitamin K
4) vitamin D
5) ascorbic acid
5
1
4
2
3
A.
B.
C.
D.
E.
collagen formation
part of the coenzymes FAD and FMN
absorption of phosphorus and calcium in bone
vision
blood clotting
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65