Download Lecture Notes Ch21

Chapter
Twenty One
Enzymes
and Vitamins
Enzymes
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_____________ for biological reactions
Proteins
Lower the __________ _________
___________ the rate of reaction
Have specific shapes that ______
the ___________ of reactants
• Activity lost if _____________
• May be simple __________ or complex
• May contain _________________ such as metal ions or
__________ _____________
Ch 21 | # 2 of 47
Names of Enzymes
• End in –ase
• Identifies a reacting substance
sucrase – reacts sucrose
lipase - reacts lipid
• Describes function of enzyme
oxidase – catalyzes oxidation
hydrolase – catalyzes hydrolysis
• Common names of digestion enzymes still end –in
pepsin, trypsin
Ch 21 | # 3 of 47
Classification of Enzymes
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Class
Oxidoreductoases
Transferases
Hydrolases
Lyases
• Isomerases
• Ligases
Reactions catalyzed
oxidation-reduction
transfer group of atoms
hydrolysis
add/remove atoms
to/from a double bond
rearrange atoms
combine molecules
using ATP
Ch 21 | # 4 of 47
Examples of Enzyme Classification
• Oxidoreductoases
oxidases - oxidize
reductases – reduce
• Transferases
transaminases – transfer amino groups
kinases – transfer phosphate groups
• Hydrolases
proteases - hydrolyze peptide bonds
lipases – hydrolyze lipid ester bonds
• Lyases
carboxylases – add CO2
hydrolases – add H2O
Ch 21 | # 5 of 47
Enzyme Structure
• The shape/structure of enzymes is related to their
functions
– Simple enzymes
• Enzymes that consist only of protein
– Conjugated enzymes
• Enzymes that consist of protein and a nonprotein part
• Apoenzyme + cofactor = holoenzyme
• Coenzyme: small organic molecule that serves as a cofactor in
a conjugated enzyme (needed to prepare the active site for
catalytic activity)
Ch 21 | # 6 of 47
Vitamins
• __________ compounds that are essential for the
proper functioning of the human body
– Many function as cofactors
• ___________ be synthesized by the human body
• Must be obtained from dietary sources
• Two main ___________:
– Water-soluble
– Fat soluble
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Water-Soluble Vitamins
• Soluble in aqueous solutions
• Used as cofactors by many enzymes
• Not store in the body
• Vitamin C
• Vitamin B
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Fat-Soluble Vitamins
• 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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Enzyme Action
• An enzyme binds a substrate in a region called the
_____________ _________
– Only certain substrates can fit the _________ ____________
– Amino acid R groups in the active site help substrate bind and
align correctly
• Enzyme-substrate ____________ forms
• Substrate reacts to form product
• Product is released
Ch 21 | # 14 of 47
The active site
of an enzyme
can be a
crevice-like
region formed as
a result of the
protein’s
secondary and
tertiary structural
characteristics.
Ch 21 | # 15 of 47
Enzyme Specificity
• Enzymes may recognize and catalyze:
– 1. A single substrate: ___________ Specificity
• Example: Urease (only catalyzes the hydrolysis of
urea)
– 2. A single stereoisomer: Stereochemical
Specificity
• Example: L-amino-acid oxidase (catalyzes the
oxidation of L-amino acids, but not D-amino acids)
Ch 21 | # 16 of 47
Enzyme Action: 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 a key that fits the lock of the active site
Ch 21 | # 17 of 47
Enzyme Action: Lock and Key Model
Ch 21 | # 18 of 47
Lock and Key Model
P
+
S
+
S
P
E
+
S
ES complex
E
+
P
Ch 21 | # 19 of 47
Enzyme Specificity
• Enzymes may recognize and catalyze:
– 3. A group of similar substrates: Group
Specificity
• Example: Hexokinase (adds a phosphate to
hexoses)
– 4. A particular type of bond: Linkage Specificity
• Example: Chymotrypsin (catalyzes the hydrolysis of
peptide bonds)
Ch 21 | # 20 of 47
Enzyme Action: Induced Fit Model
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Enzyme structure flexible, not rigid
Enzyme and active site adjust shape to bind substrate
Increases range of substrate specificity
Shape changes also improve catalysis during reaction
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Enzyme Action
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Enzyme Action: Induced Fit Model
P
S
S
P
E
+
S
ES complex
E
+
P
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Enzyme Activity
Ch 21 | # 25 of 47
Factors Affecting Enzyme Action:
Temperature
• Little activity at low
temperature
• Rate ____________ with
temperature
• Most active at optimum
temperatures (usually 37°C
in humans)
• Activity lost with
______________ at ______
temperatures
Ch 21 | # 26 of 47
Factors Affecting Enzyme Activity: pH
• Maximum activity at optimum
pH
• Narrow range of activity
• Most lose activity in low or
high pH
• Why is one pH better than
another?
– R groups of amino acids have
proper charge at certain pH
values
– Tertiary structure of enzyme is
correct
Ch 21 | # 27 of 47
Factors Affecting Enzyme Activity:
Substrate Concentration
• Increasing substrate
concentration increases
the rate of reaction
(enzyme concentration is
constant)
• Maximum activity
reached when all of
enzyme combines with
substrate
Ch 21 | # 28 of 47
Factors Affecting Enzyme Activity:
Enzyme Concentration
• The rate of reaction
increases as enzyme
concentration increases
(at constant substrate
concentration)
• At higher enzyme
concentrations, more
substrate binds with
enzyme
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Ch 21 | # 30 of 47
Enzyme Inhibition
Inhibitors
• Molecules that cause a ________ of catalytic activity
• Change the protein structure of an enzyme to prevent
substrates from fitting into the active sites
• May be “competitive” or “noncompetitive”
• Some effects are irreversible
Ch 21 | # 31 of 47
Competitive Inhibition
A competitive inhibitor
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Has a structure similar to substrate
Occupies active site
Competes with substrate for active site
Has effect reversed by increasing substrate concentration
http://www.eccentrix.com/members/chempics/Slike/Enzyme/2Competitive_inhibition.jpg
Ch 21 | # 32 of 47
Noncompetitive Inhibition
A noncompetitive inhibitor
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Does not have a structure like substrate
Binds to the enzyme but not active site
Changes the shape of enzyme and active site
Substrate cannot fit altered active site
No reaction occurs
Effect is not reversed by adding substrate
Substrate activity is restored when inhibitor is no
longer bonded to the enzyme
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Heavy metal poisoning is an example of
noncompetitive inhibition of an enzyme.
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Irreversible Inhibition
• In irreversible inhibition, a substance destroys enzyme activity by
bonding with R groups at the active site
• Inhibitor permanently blocks substrate binding
– One of the ways antibiotics kill bacteria
Ch 21 | # 37 of 47
Sulfa Drugs
Structures of selected
sulfa drugs in use
today as antibiotics.
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Regulating Enzyme Activity
• Four main mechanisms to regulate enzyme
activities
– Genetic Control
– Feedback Control
– Zymogen Activation
– Allosteric Regulation
• Activation – Positive Regulation
• Deactivation – Negative Regulation
Ch 21 | # 40 of 47
Feedback Control
• In feedback control:
– A product of a reaction acts as a negative regulator
– An end product binds with the first enzyme in a
sequence when sufficient product is present
A
E1

B
E2

C
E3
 D
Inhibition of enzyme
1 by product D
Ch 21 | # 41 of 47
Zymogens
– Inactive forms of enzymes
– Activated when one or more peptides are removed
– Example: Proinsulin is converted to insulin by removing a small
peptide chain
– Digestive enzymes are produced in one organ as zymogens, but
not activated until they are needed; Ex. trypsinogen / trypsin
Ch 21 | # 42 of 47
Allosteric Enzymes
• An enzyme with two or more protein chains (quaternary
structure) and two kinds of binding sites (substrate and
regulator)
– Activity is influenced by “regulators” (found or produced in cells
normally)
• Positive regulator
– Enhances the binding of substrate and accelerates the rate of
reaction
• Negative regulator
– Prevents the binding of the substrate to the active site and slows
down the rate of reaction
Ch 21 | # 43 of 47
Enzymes and Vitamins
• Levels of
enzymes in
blood used
as a
diagnostic
tool.
Ch 21 | # 44 of 47
Diagnostic Enzymes
• The levels of diagnostic enzymes determine the
amount of damage in tissues
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Enzymes and Vitamins
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