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ENZYMES
• A catalyst
– Is a chemical agent that speeds up a reaction
without being consumed by the reaction
– An enzyme is an organic catalyst
• Enzymes are proteins
1. An Enzyme
Active Site
Substrate
(Reactants)
Enzyme
Inhibitor
Allosteric Site
2. Enzyme Specificity
• Enzymes can only
work with certain
substrates
• The shape of the
enzyme must
match the shape
of its substrate
- The root of the enzyme’s name typically
indicates the substrate which it acts upon
e.g. ATPase, Amylase, Sucrase
3. Induced Fit Model
Substate
Active site
Enzyme- substrate
complex
Enzyme
Figure 8.16
(a)
Figure 8.16
(b)
• When the substrate binds to the active site, the enzyme
changes conformation (shape) to make a better fit.
• Interactions between chemical groups on substrate and those
of the amino acids as well as the shape of the active site
cause the induced fit
Sucrose
Sucrase
Glucose + Fructose
Sucrose + Sucrase  Glucose + Fructose + Sucrase
Activation Energy
• The initial amount of energy needed to start a
chemical reaction (i.e. break the bonds)
A
B
C
D
Free energy
Transition state
A
B
C
D
EA
activation energy  EA
Reactants
A
B
C
D
Products
Progress of the reaction
Figure 8.14
8.15
What do enzymes do?
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
Products
Progress of the reaction
• Enzymes lower the
activation energy
• How?
– Orienting substrates
correctly
– Putting stress on
substrate bonds
– Providing a favorable
environment
• This increases the
rate of the reaction.
5. Increasing the Rate of
Reaction
(1)Increase the number of substrate
molecules in solution (increase conc’n)
(2)Increase the number of enzymes in
solution (increase conc’n)
(3)Increase the temperature of the solution
(up to a certain point)
6. Saturation
• A reaction is said to be “saturated” when
100% of enzymes have their active sites
filled with substrate.
• Vmax is maximum
velocity (speed) of rxn
Question 7.
• If there are left-over
reactants (substrates),
then you could add more
enzymes.
• If there are no more leftover reactants
(substrates), then adding
more enzymes will not
increase the rate.
8. Stopping a Reaction
• Change the pH so it
is above or below its
optimal value.
• This changes the
enzyme’s
conformation
(shape) making it
lose it dysfunctional
• e.g. add H2SO4(aq) –
Sulfuric Acid
9. Effect of Increasing
Temperature
• Above a certain
temperature,
enzymes’ activity
starts to decline
because the
enzyme begins
to denature
(unravel)
Metabolic Pathways
• Reactions occur in a
sequence and
specific enzymes
catalyze each step
10. Cyclic Metabolic Pathways
Z
U
Y
V
X
W
(a) Z + Y  U
UVW
WX+Y
(b) Initial Reactant: Z
End product: X
(desired product)
11. Competitive vs. Non-competitive Inhibition
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
• If the inhibitor
attaches by weak
bonds, it is usually
reversible.
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
Figure 8.19
(b) Competitive inhibition
Figure 8.19
(c) Noncompetitive inhibition
• If the inhibitor attaches by covalent bonds,
it is usually irreversible (usually (c) is)
0
12. Allosteric Regulation:
Activation and Inhibition
Allosteric enyzme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Allosteric activater
stabilizes active from
Activator
Active form
Stabilized active form
Oscillation
Allosteric activater
stabilizes active form
NonInactive form Inhibitor
functional
active
site
Stabilized inactive
form
– When one
activator or
inhibitor bind to
an allosteric site,
and have an
effect on all four
subunits of an
enzyme
– Bonds are noncovalent, so they
are reversible
(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors
dissociate when at low concentrations. The enzyme can then oscillate again.
Feedback Inhibition
• The end product
of a metabolic
pathway shuts
down the pathway
• Prevents the cell
from wasting
chemical
resources
Active site
available
Initial substrate
(threonine)
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off
Enzyme 2
Intermediate B
Enzyme 3
Intermediate C
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
Figure 8.21
End product
(isoleucine)