Download Page 20-1 CHAPTER 20: Enzymes 20.2

20.1 GENERAL CHARACTERISTICS
CHAPTER 20: Enzymes
Globular
Globular proteins
proteins that
that act
act as
as
catalysts
catalysts in
in biochemical
biochemical processes
processes
• Describe the general characteristics and
functions of enzymes
H OH
• Identify the general function of cofactors
HC
• Illustrate the mechanism of enzyme function
−OOC
• Identify factors for the regulation of
enzyme activity, including zymogen
precursors and allosteric regulation
C
CITRATE
NO
REACTION
CH2COO−
COO−
aconitase
SUBSTRATE
CATALYST
CATALYST
Increases
Increases reaction
reaction rate
rate
without
without itself
itself undergoing
undergoing
any
net
change
any net change
• Compare competitive and noncompetitive
inhibition
HC
CH2COO−
C
−OOC
COO−
cis-ACONITATE
PRODUCT
ENZMATIC PROCESSES
UNCATALYZED
• EFFICACY
Reaction rates greatly (×109-1020)
increased
Ea
∆E
Ea
CATALYZED
• SPECIFICITY
Rates increased for one specific reaction
or reaction class
REACTANTS
• REGULATION
Reaction rates can be controlled
PRODUCTS
Extent of Reaction
20.2 CLASSIFICATION
• TRANSFERASES
Transfer of a functional group between molecules
Identifying
Identifying suffix
suffix -ase
-ase
COO−
• OXIDOREDUCTASES
Oxidation and reduction
COO−
CH3 C
H
OH
CH NH3+
CH2
COO−
COO−−
−2H
CH3 C
COO−
+
C O
CH2
CH2
COO−
O
COO−
C O
CH2
COO−
COO−
+
CH NH3+
CH2
CH2
COO−
Page 20-1
N+(CH3)3
N+(CH3)3
• HYDROLASES
Hydrolysis or
esterification
CH2
CH2
H2O
CH2
• ISOMERASES
Isomerization
CH2
O
OH
CH3C O
P
O
O
COO−
C
COO−
CH
COO−
P
COO−
O
OH CH2OH
~H
OH
OH
HO
OH
O
+
O C
H CH2 C
CH2
CH2
H C
O
• LIGASES (SYNTHETASES)
Joining or condensation of 2 molecules
COO−
H2O
OH
HO
O−
O−
• LYASES
Addition to a double bond, or elimination
to make a double bond
CH2
=
CH3C OH
CH2O P
CH2O P
S CoA
COO−
COO−
COO−
HO C
HO CH
O
CH2 C
S CoA
CH2
COO−
COO−
20.3 ENZYME STRUCTURE:
20.4 MECHANISM of ENZYME
ACTION
COFACTORS AND COENZYMES
• PROTEIN
Globular polypeptide of 100-200 residues
called an APOENZYME if it also contains
• MODE OF ACTION
E
• COFACTOR(S)
– METAL IONS (Mg2+, Fe2+, Zn2+, Mn2+,
Ni2+, Se0…) and/or
– COENZYMES (organic molecules ←
water-soluble vitamins = B vitamins, etc)
enzyme
+
S
substrate
ES
COMPLEX
E
enzyme
+
P
product(s)
Formed
Formed at
at active
active site
site of
of enzyme
enzyme
APOENZYME
APOENZYME ++ COFACTOR(S)
COFACTOR(S) == HOLOENZYME
HOLOENZYME
CH2 O P
• Active site is a small pocket or cleft in
enzyme surface
ADP
ATP
CH2 O P
CHOH
CHOH
C
C
O
O P
O
O−
• Substrate is held in active site by noncovalent interactions with amino acid side
chains
ACTIVE SITE
• Active site shape is complementary to
substrate shape
• Active site shape determines specificity
ACTIVE
ACTIVE SITE
SITE
Specific
Specific location
location on
on enzyme
enzyme molecule
molecule
where
where the
the chemical
chemical reaction
reaction is
is catalyzed
catalyzed
PHOSPHOGLYCERATE
KINASE
Page 20-2
ENZYME
APOENZYME
COFACTOR
(COENZYME)
ENZYME-SUBSTRATE
COMPLEX
HOLOENZYME
PRODUCTS
ENZYME-SUBSTRATE
COMPLEX
ENZYME-SUBSTRATE
COMPLEX
ENZYME
Effect of Substrate Stereoisomerism:
Lactate Dehydrogenase
CH3
−OOC
−OOC
H
OH
H
CH3
H
OH
• INDUCED-FIT
Reaction of closely-related substrates
(functional group, bond type, stereochemistry) are catalyzed
CH3
ACTIVE SITE
H
OOC
−
OH
• LOCK-AND-KEY
Reaction of only one substrate is catalyzed:
absolute specificity
Complex formed
D-LACTATE
−OOC
Further
Reaction
CH3
L-LACTATE
SPECIFICITY OF ENZYME-SUBSTRATE
COMPLEX
OH
Mismatch — no complex
salt bridge
Some drugs work better
than the natural substance:
Morphine
H-bonds
hydrophobic
interactions
Dopamine
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20.6 FACTORS AFFECTING
ACTIVITY
20.5 ENZYME ACTIVITY
The ACTIVITY of an enzyme, its ability to increase the
rate of a reaction, is reflected in its TURNOVER
NUMBER (mol substrate consumed/min):
ENZYME
Carbonic
anhydrase
• ENZYME CONCENTRATION
• SUBSTRATE CONCENTRATION
TURNOVER Nº
REACTION
36,000,000
CO2 + H20 ↔ H2CO3
• TEMPERATURE
60,000
Pyruvate ↔ Lactate
• pH
900
Growth of DNA
chains
Lactate
dehydrogenase
DNA
polymerase
ENZYME CONCENTRATION
Where [S] >> [E],
• Increased [E] leads to increased [ES]
Initial
Velocity
• Reaction rate increases
[E]
SUBSTRATE CONCENTRATION
Vmax
• Rate initially responsive to [S]
• Rate eventually reaches a maximum (Vmax)
Initial
Velocity
• Vmax reached when the active site of every
enzyme molecule is occupied by a substrate
molecule
[S]
Page 20-4
pH
• Many structural features pH dependent (ex:
COO−, NH3+)
Rate
• Large deviations from pH optimum
irreversibly change 3-D structure
• Enzymes most efficient at pH of normal
environment (ex: cellular cytoplasm pH ~7)
5
7
9
pH
TEMPERATURE
• Enzymes most efficient at temperature of
normal environment (~37°C)
• Below optimum temperature, insufficient
energy for normal reaction rate
• Above optimum temperature, irreversible
changes in 3-D structure (3° structure
destroyed)
Rate
0
20
40
60
T, °C
?!@x!
20.7 ENZYME INHIBITION
• IRREVERSIBLE
– Tight, often covalent bonding at active site
– Not enzyme specific
EX:
Hg2+,
CN−,
INHIBITOR
nerve gases
• REVERSIBLE
– Non-covalent bonding
– Competitve: to enzyme active site
ACTIVE
EX: sulfa drugs
– Non-competitve: to cofactors or on enzyme
away from active site
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INACTIVE
• POSITIVE ALLOSTERISM
20.8 REGULATION OF
ENZYME ACTIVITY
ALLOSTERIC ENZYME
ACTIVATOR
(MODULATOR)
Control of enzymatic processes necessary:
• Conserves a cell’s chemical resources/
energy
• Stops production of excess or unwanted
reaction products
ACTIVATED
ENZYME
REGULATORY
SITE
YES!
• NEGATIVE ALLOSTERISM
• END PRODUCT INHIBITION
Accumulation
Accumulation of
of product
product
promotes
promotes conversion
conversion
back
back to
to substrate
substrate
ACTIVATOR
(MODULATOR)
REGULATORY
SITE
DEACTIVATED
ENZYME
LeChatelier’s
Principle:
Chap 8
WHA?
Most
Most enzymatic
enzymatic reactions
reactions can
can proceed
proceed
in
in both
both the
the forward
forward and
and reverse
reverse directions
directions
• FEEDBACK INHIBITION
A
E1
B
E2
• ZYMOGEN (PROENZYME) CONVERSION
C
E3
D
PROTEOLYTIC
ENZYME
“Downstream”
“Downstream” product
product (D)
(D) inhibits
inhibits own
own
synthesis
synthesis by
by acting
acting as
as negative
negative allosteric
allosteric
inhibitor
inhibitor of
of “upstream”
“upstream” enzyme
enzyme (E
(E11))
ZYMOGEN
INACTIVE
ENZYME
ACTIVE
• Allows transport of enzymes in inactive form
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20.9 ENZYME APPLICATIONS
• FOOD INDUSTRY:
Fermentation, polysaccharide hydrolysis (“lite beer”, Beano™),
meat tenderizers
• MEDICINE:
Post-surgery replacement of
enzymes, protein breakdown as surgical
procedure, clinical diagnosis of pathology
• ANALYSIS: bioprocess monitoring, assaying
serum contents, assaying pathogen or
allergen proteins
Page 20-7