enzymes. enzymes speed up metabolic reactions by lowering energy barriers a catalyst is a chemical...
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ENZYMES
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Enzymes speed up metabolic reactions by lowering energy barriers
• A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
• An enzyme is a catalytic protein– Hydrolysis of sucrose by the enzyme sucrase is an example
of an enzyme-catalyzed reaction
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Sucrase
Sucrose(C12H22O11)
Fructose(C6H12O6)
Glucose(C6H12O6)
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The Activation Energy Barrier
• Every chemical reaction between molecules involves bond breaking and bond forming
• The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
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Figure 8.13
Course ofreactionwithoutenzyme
EA
withoutenzyme EA with
enzymeis lower
Course ofreactionwith enzyme
Reactants
Products
G is unaffectedby enzyme
Progress of the reaction
Fre
e en
erg
y
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Substrate Specificity of Enzymes• The reactant that an enzyme acts on is called the
enzyme’s substrate • The enzyme binds to its substrate, forming an
enzyme-substrate complex• The active site is the region on the enzyme
where the substrate binds
• Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction
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Figure 8.14
Substrate
Active site
Enzyme Enzyme-substratecomplex
(a) (b)
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The Enzyme’s Active Site• In an enzymatic reaction, the substrate binds to the active site of the
enzyme via weak interactions, such as hydrogen bonds and ionic bonds.
• The active site can lower an EA barrier by– Orienting substrates correctly– Straining substrate bonds– Providing a favorable microenvironment– Covalently bonding to the substrate A singles enzyme can
catalyze thousands of reactions in a second.• Enzymes are unaffected by the reaction and are reusable.• Most metabolic enzymes can catalyze a reaction in both the forward
and reverse direction.– The actual direction depends on the relative concentrations of
products and reactants.– Enzymes catalyze reactions in the direction of equilibrium.
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Figure 8.15-1
Substrates
Substrates enter active site.
Enzyme-substratecomplex
Substrates are heldin active site by weakinteractions.
12
Enzyme
Activesite
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Figure 8.15-2
Substrates
Substrates enter active site.
Enzyme-substratecomplex
Substrates are heldin active site by weakinteractions.
Active site canlower EA and speedup a reaction.
12
3
Substrates areconverted toproducts.
4
Enzyme
Activesite
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Figure 8.15-3
Substrates
Substrates enter active site.
Enzyme-substratecomplex
Enzyme
Products
Substrates are heldin active site by weakinteractions.
Active site canlower EA and speedup a reaction.
Activesite is
availablefor two new
substratemolecules.
Products arereleased.
Substrates areconverted toproducts.
12
3
45
6
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Effects of Local Conditions on Enzyme Activity
• The three-dimensional structure of enzymes are influenced by environmental conditions.
• An enzyme’s activity can be affected by– General environmental factors, such as temperature
and pH– Chemicals that specifically influence the enzyme
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Effects of Temperature and pH• Each enzyme has an optimal temperature in which it can function
– As temperature increases, collisions between substrates and active sites occur more frequently as molecules move faster.
– TOO much of an increase in temperature will disrupt the weak bonds that stabilize the protein’s active conformation and the protein will denature.
• Each enzyme has an optimal pH in which it can function– Most enzymes have an optimal pH between 6-8.– Some digestive enzymes in the stomach work best at lower pH
values (pH=2), while those in the intestine work best at a pH of 8.
– The working environments influence the optimal pH of these enzymes.
• Optimal conditions favor the most active shape for the enzyme molecule
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Figure 8.16Optimal temperature fortypical human enzyme (37°C)
Optimal temperature forenzyme of thermophilic
(heat-tolerant)bacteria (77°C)
Temperature (°C)(a) Optimal temperature for two enzymes
Ra
te o
f re
ac
tio
nR
ate
of
rea
cti
on
120100806040200
0 1 2 3 4 5 6 7 8 9 10pH
(b) Optimal pH for two enzymes
Optimal pH for pepsin(stomachenzyme)
Optimal pH for trypsin(intestinal
enzyme)
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Figure 8.16a
Optimal temperature fortypical human enzyme (37°C)
Optimal temperature forenzyme of thermophilic
(heat-tolerant)bacteria (77°C)
Temperature (°C)(a) Optimal temperature for two enzymes
Rat
e o
f re
acti
on
120100806040200
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Figure 8.16b
Rat
e o
f re
acti
on
0 1 2 3 4 5 6 7 8 9 10pH
(b) Optimal pH for two enzymes
Optimal pH for pepsin(stomachenzyme)
Optimal pH for trypsin(intestinal
enzyme)