enzymes. enzymes speed up metabolic reactions by lowering energy barriers a catalyst is a chemical...
TRANSCRIPT
ENZYMES
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)
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
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)
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
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
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
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)
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
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)