Download Amino Acids, Proteins, and Enzymes

Chapter 20 Enzymes and Vitamins
20.1
Enzymes
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Publishing as Benjamin Cummings
1
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.
2
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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.
3
Classification of Enzymes
Enzymes are classified by the reaction they catalyze.
Class
Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases
Type of Reactions Catalyzed
Oxidation–reduction
Transfer groups of atoms
Hydrolysis
Add atoms/remove atoms to or
from a double bond
Rearrange atoms
Use ATP to combine small molecules
4
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. Converts a cis-fatty acid to a trans-fatty acid.
B. Removes 2 H atoms to form a double bond.
C. Combines two molecules to make a new compound.
D. Adds NH3.
8
Solution
Match the type of reaction with an enzyme.
1) aminase
2) dehydrogenase
3) isomerase
4) synthetase
A. 3
B. 2
C. 4
D. 1
Converts a cis-fatty acid to a trans-fatty acid.
Removes 2 H atoms to form a double bond.
Combines two molecules to make a new compound.
Adds NH3.
9
Chapter 20 Enzymes and Vitamins
20.2
Enzyme Action
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Active Site
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
In an enzyme-catalyzed
reaction
 A substrate attaches to
the active site.
 An enzyme-substrate
(ES) complex forms.
 Reaction occurs and
products are released.
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 An enzyme is used
over and over.
E+S
ES
E+ P
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Enzyme Specificity
Enzymes may recognize and catalyze
 A single substrate.
 A group of similar substrates.
 A particular type of bond.
TABLE 20.2
13
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Lock and Key Model
In the lock-and-key model of enzyme action,
 The active site has a rigid shape.
 An enzyme only binds substrates that exactly fit
the active site.
 Only substrates with the matching shape can fit.
 The substrate is the key that fits that lock.
14
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Induced-fit Model
In the induced-fit model of enzyme action,
 Enzyme structure is flexible, not rigid.
 Enzyme and substrate adjust the shape of the active
site to bind substrate.
 The range of substrate specificity increases.
 Shape changes improve catalysis during reaction.
15
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Enzyme Catalyzed Reaction
 The proper fit of a substrate (S) in an active site
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
16
Example of An Enzyme Catalyzed
Reaction
17
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Learning Check
A. The active site is
(1) the 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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Isoenzymes
Isoenzymes
 Catalyze the same reaction in different tissues in
the body.
 Such as lactate dehydrogenase (LDH), which
converts lactate to pyruvate, consists 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.
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Isoenzymes
21
Diagnostic Enzymes
Diagnostic enzymes
 Determine the
amount of
damage in
tissues.
 That are elevated
may indicate
damage or
disease in a
particular organ.
TABLE 20.4
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Diagnostic Enzymes
Levels of enzymes
CK, LDH, and 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
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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, enzymes operate at lower and higher
optimum pH values.
TABLE 20.5
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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 sucrose
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
A. 2
B. 3
C. 3
Increasing the concentration of sucrase
Changing the pH to 4
Running the reaction at 70°C
31
Chapter 20 Enzymes and Vitamins
20.4
Enzyme 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.
34
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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, a substance
 Bonds with R groups at the active site.
 Destroys enzyme activity.
TABLE 20.6
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Learning Check
Identify each description as an inhibitor that is:
1) Competitive
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.
38
Solution
Identify each description as an inhibitor that is:
1) Competitive
2) Noncompetitive
A. 1 Increasing substrate reverses inhibition.
B. 2 Binds to enzyme surface, but not to the
active site.
C. 1 Structure is similar to substrate.
D. 2 Inhibition is not reversed by adding more
substrate.
39
Chapter 20 Enzymes and Vitamins
20.5
Control of Enzyme Activity
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Zymogens
Zymogens (proenzymes)
 Are inactive forms of
enzymes.
 Are activated when
one or more peptides
are removed.
 Such as proinsulin is
converted to insulin by
removing a small
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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.
TABLE 20.7
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42
Digestive Enzymes
Zymogen
(from pancreas)
Active Enzyme
(in small intestine)
enteropeptidase
trypsinogen
trypsin + peptide
trypsin
chymotrypsinogen
chymotrypsin + 2 dipeptides
trypsin
procarboxypeptidase
carboxypeptidase + peptide
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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.
44
Feedback Control
In feedback control
 A product acts as a negative regulator.
 An end product binds with the first enzyme (E1) in
a sequence when sufficient product is present.
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Learning Check
Identify each statement as:
zymogen (Z)
allosteric enzyme (A)
positive regulator (PR)
feedback control (FC)
1. An enzyme in a pathway that controls the rate of the
reaction.
2. Speeds up a reaction by combining with an
enzyme in the pathway.
3. Removal of a peptide activates the enzyme.
4. Some product binds to the first enzyme to limit the
synthesis of product.
46
Solution
Identify each statement as:
zymogen (Z)
allosteric enzyme (A)
positive regulator (PR)
feedback control (FC)
1. A
An enzyme in a pathway that controls the rate of
the reaction.
2. PR Speeds up a reaction by combining with an
enzyme in the pathway.
3. Z Removal of a peptide activates the enzyme.
4. FC Some product binds to the first enzyme to limit the
synthesis of product.
47
Chapter 20 Enzymes and Vitamins
20.6
Enzyme Cofactors and Vitamins
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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.
49
Enzyme Cofactors
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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.
Zn2+, a cofactor for
carboxypeptidase,
stabilizes the carbonyl
oxygen during the
hydrolysis of a peptide
bond.
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52
Some Enzymes and Their Cofactors
TABLE 20.8
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53
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.
54
Solution
Identify each enzyme as
1) A simple enzyme
2) An enzyme that required a cofactor
A. 2 Requires Mg2+ for hydrolysis of phosphate esters.
B. 2 Requires vitamin B3 to transfer an acetyl group.
C. 1 Is active with four polypeptide subunits.
55
Water-Soluble Vitamins
Water-soluble vitamins are
 Soluble in aqueous solutions.
 Cofactors for many enzymes.
 Not stored in the body.
TABLE 20.9
56
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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.
 Important in vision, bone formation, antioxidants, and
blood clotting.
 Stored in the body.
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Learning Check
Identify each compound as a
water-soluble vitamin (W) fat-soluble vitamin (F)
1. Folic acid
2. Retinol (Vitamin A)
3. Vitamin C
4. Vitamin E
5. Niacin
58
Solution
Identify each compound as a:
water-soluble vitamin (W) fat-soluble vitamin (F)
1. W Folic acid
2. F Retinol (Vitamin A)
3. W Vitamin C
4. F Vitamin E
5. W Niacin
59
Thiamin (Vitamin B1)
Thiamin
 Was the first B vitamin identified.
 Is part of the coenzyme thiamin pyrophosphate (TPP).
 Is used to decarboxylate -keto carboxylic acids.
 RDA is 2 mg; deficiencies include fatigue, poor appetite,
weight loss, nerve degeneration, heart failure
 Sources are liver, yeast, whole grains, cereals, and milk.
60
Riboflavin (Vitamin B2)
Riboflavin
 Is found in the coenzymes flavin adenine dinucleotide
(FAD) and flavin mononucleotide (FMN).
 Is needed for good vision and healthy skin.
 RDA is 1.7 mg, deficiencies include dermatitis, dry
skin, tongue inflammation, cataracts
 Sources are liver, chicken, eggs, green leafy
vegetables, dairy foods, peanuts, and whole grains.
O
H3C
N
H3C
N
N
flavin
N
H
O
CH2 CH CH CH CH2 OH
OH OH OH
ribitol
61
Niacin (Vitamin B3)
Niacin
 Is part of the coenzyme
nicotinamide adenine
dinucleotide (NAD+) involved in
oxidation-reduction reactions.
 RDA is 13-18 mg.
 Deficiency can result in
dermatitis, muscle fatigue, and
loss of appetite.
 Sources are brewer’s yeast,
chicken, beef, fish, liver, brown
rice, and whole grains.
O
C
OH
N
62
Pantothenic Acid (Vitamin B5)
Pantothenic acid
 Is part of coenzyme A needed for energy
production as well as glucose and cholesterol
synthesis.
 RDA is 10 mg; deficiency can result in fatigue,
retarded growth, cramps, and anemia.
 Is found in salmon, meat, eggs, whole grains, and
vegetables.
CH3 OH O
HO CH2 C
CH3
O
CH C N CH2
CH2 C OH
H
63
Pyridoxine (Vitamin B6)
Pyridoxine and pyridoxal are
 Two forms of vitamin B6, which are converted to the
coenzyme pyridoxal phosphate (PLP).
 PLP is required in the transamination of amino acids and
decarboxylation of carboxylic acids.
 RDA is 1 mg; deficiency may lead to dermatitis, fatigue,
and anemia.
 Sources are meat, liver, fish, nuts, whole grains, spinach.
64
Cobalamin (Vitamin B12)
Cobalamin
 Consists of four pyrrole rings
with a Co2+.
 Is a coenzyme for enzymes
that transfer methyl groups
and produce red blood cells.
 RDA is3 μg; deficiencies are
pernicious anemia, nerve
damage, and malformed red
blood cells.
 Sources are liver, beef,
kidney, chicken, fish, milk
products.
65
Ascorbic Acid (Vitamin C)
Vitamin C
 Is required in collagen
synthesis, healing of wounds.
 RDA is 60 mg; deficiencies
are scurvy, weakened
connective tissue, slowhealing wounds, and anemia.
 Sources include: blueberries,
citrus fruits, tomatoes,
peppers, broccoli, red and
green vegetables.
O
HO
O
CH2OH
CHOH
OH
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Folic Acid (Folate)
Folic acid (folate)
 Consists of pyrimidine, p-aminobenzoic acid, and
glutamate.
 Forms the coenzyme THF used in the transfer of
methyl groups and the synthesis of nucleic acids.
 RDA is 0.4 mg; deficiencies are abnormal red blood
cells, anemia, and poor growth.
 Sources are green leafy vegetables, beans, meat,
seafood, yeast, asparagus, and whole grains enriched
with folic acid.
67
Folic Acid (Folate)
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68
Vitamin A
Vitamin A
 Is needed for retinol (vision); synthesis of RNA.
 RDA is 3 mg; deficiencies include night blindness,
immune system repression, and slowed growth.
 Sources are meats and beta-carotenes in plants,
yellow and green fruits and vegetables.
H3C
CH3
CH3
H3C
CH3
CH3
CH3
H3C
CH3
CH3
CH3
CH3
H3C
CH3
beta-carotene
CH3
CH2OH
retinol
69
Vitamin D
Vitamin D (D3)
 Is synthesized in skin
exposed to sunlight.
 Regulates the absorption
of phosphorus and calcium
during bone growth.
 RDA is 10 μg; deficiencies
are weakened bones.
 Sources are sunlight, cod
liver oil, eggs, and
enriched milk.
70
Vitamin E
Vitamin E
 Is an antioxidant in cells.
 May prevent the oxidation of unsaturated fatty acids.
 Is found in whole grains, and vegetables.
 RDA is 10 mg; deficiencies are hemolysis and anemia.
 Sources are meats, whole grains, vegetables, and
vegetable oils.
CH3
HO
CH3
H3C
O
CH3
CH3
CH3
CH3
CH3
71
Vitamin K
 Vitamin K1 in plants has a saturated side chain.
 Vitamin K2 in animals has a long unsaturated side
chain.
 Vitamin K2 is needed for the synthesis of zymogens
for blood clotting.
 RDA is 80 μg; deficiencies are prolonged bleeding
time, and bruising.
 Sources are liver, spinach, and cauliflower.
O
O
CH3
CH3
CH3
O
CH3
Vitamin K1 (phylloquinone)
3
CH3
CH3
O
CH3
Vitamin K2 (menaquinone)
n
CH372
Learning Check
Identify the vitamin associated with each
1) Thiamin (B1)
2) Vitamin A
3) Vitamin K
4) Vitamin D
5) Ascorbic Acid
A.
B.
C.
D.
E.
Collagen formation
Beriberi
Absorption of phosphorus and calcium in bone
Vision
Blood clotting
73
Solution
Identify the vitamin associated with each
1) Thiamin (B1)
2) Vitamin A
3) Vitamin K
4) Vitamin D
5) Ascorbic Acid
A.
B.
C.
D.
E.
5
1
4
2
3
Collagen formation
Beriberi
Absorption of phosphorus and calcium in bone
Vision
Blood clotting
74