Download Enzyme

ENZYMES
Without Enzymes Reactions are
extremely slow
• The conversion of bread (starch) to
glucose takes about 1000 years if it sits in
water
• If you heat up the mix, it takes about 1
hour.
• But, eat some bread and our body
converts it to glucose in minutes with the
help of Enzymes
Enhancement of Reaction
Rate by Enzymes
Enzyme
Function
Uncatalyse Enzymatic Rate
d Reaction Reaction Enhanceme
-1
Rate (s -1) Rate (s ) nt
Carbonic
Anhydrase
CO2-hydratisation
1.3 x 10 -1
1 x 106
7.7 x 10 6
7,700,000
Carboxypeptidase A
Peptide hydrolysis
3 x 10 -9
578
1.9 x 10 11
190,000,000,000
Staphylococcal
nuclease
Digestion of
nucleic acids in
Staphylococcus
aureus
1.7 x 10 -13
95
5.6 x 10 14
560,000,000,000,
000
Enzymes
 Enzymes are molecules that act as catalysts to
speed up biological reactions.
 Most enzymes are proteins.
 They work by lowering the activation energy
required to start the reaction
Enzymes
 The compound on which an enzyme acts is
called its substrate.
A normal reaction of enzyme and substrate to form product
+
enzyme
substrate Enzyme substrate
complex
+
products
enzyme
 The substance formed are called products
 For an enzyme to speed up a reaction a substrate
must bind to a particular part of the enzyme
called the active site.
 When the substrate binds to its enzyme an
enzyme-substrate complex is formed
 New product is formed
 Product(s) are then released from the enzyme
 Enzymes are unchanged by the reactions they
catalyse ; thus they are REUSABLE.
Anabolic Reaction
 Enzymes can join two or more substrate
molecules together.(synthesis/ anabolism)
e.g Formation of the disaccharide maltose
Catabolic Reaction
• Enzymes can break molecules into smaller
components (decomposition/cataboslism);
e.g digestion of sucrose into the monomers
glucose and fructose.
Enzyme Active Sites
Substrate molecule:
Substrate molecules are the
chemicals that an enzyme
acts on. They are drawn into
the cleft of the enzyme.
Active site:
At the active site, the substrate/s is
drawn to the enzyme’s surface and
is/are are positioned in a way to
promote a reaction: either joining two
molecules together or splitting up a
larger one.
Enzyme molecule:
The complexity of the
active site is what makes
each enzyme so specific
(i.e. precise in terms of the
substrate it acts on).
This model (above) is an enzyme called
Ribonuclease S, that breaks up RNA
molecules.
How do enzymes work?
 For an enzyme to speed up a reaction a
substrate must bind to a particular part of the
enzyme called the active site
 Binding of enzyme and substrate is dependent
on enzyme shape and shape of the active site
substrate
Active site
Enzyme
Lock and key model
substrate
Enzyme
Enzyme (lock)
1
2
3
4
5
Substrate (key)
Lock and Key Model
 The lock and key model of enzyme action
Substrate
Products
Symbolic representation of the lock and key model of enzyme action.
Enzyme
1.A substrate is drawn into the active sites of the enzyme.
2. The substrate shape must be compatible with the enzymes active site
(precisely complementary) in order to fit and enable a reaction to proceed.
3. The enzyme modifies the substrate. In this instance the substrate is
broken down, releasing two products.
Induced fit Model
Active site is NOT perfectly
complementary to the substrate
Active site changes conformation to
match the substrate
Active site reverts back to original
shape
Effective collision
• In order for the substrate to be drawn to the
active site of the enzyme, an effective collision
must occur between enzyme and substrate
• The more collisions that occur between the
enzyme and its substrate the more chance of
an effective collision occurring
What give the enzyme its shape
• How it is folded into its 3-dimensional shape
or (tertiary structure) is vital
• If this structure is changed or altered then the
enzyme is said to have been DENATURED
• Denaturation of the enzyme will change the
shape of the enzymes active site
• This will mean the enzyme will no longer be
able to bind to its substrate
• Denaturation is permanent.
Effect of Temperature
 Enzymes often
Rate of reaction
have a narrow
range of
conditions under
which they
operate properly.
 As temperature
increases
reaction rate
increases up to a
point
 At this point the
enzyme is
denatured and it
can no
longer function.
Optimum temperature
for the enzyme
Rapid
denaturation
at high
temperatures
Too cold for the
enzyme to
operate
Temperature (°C)
At low temperatures is the enzyme
denatured?
• No! Low temperature has no affect on enzyme
structure. You can freeze enzymes and then
defrost them and they will again be functional
• The reaction rate slows with a decrease in
temperature due to a decrease in the kinetic
energy of both enzyme and substrate molecules
• As they are moving more slowly then will
experience fewer effective collisions which results
in a slower rate of catalysis.
Why does reaction rate increase as
temperature increases?
• As temperature increases the kinetic energy of
molecules increase and the enzyme and substrate
molecules both move faster
• Thus more collisions and more effective collisions
and a faster rate of catalysis
• This will continue UNTIL the enzyme’s 3
dimensional shape changes and the active site
becomes deformed
• Then the substrate can NO LONGER bind with the
enzyme
• The enzyme can NO LONGER catalyse the reaction
Protein Denaturation
Effect of pH
Trypsin
Urease
Pepsin
Enzyme activity
Enzymes can be affected
by pH.
Each enzyme will have an
optimal pH as which they
function best
A shift in pH in either
direction from the
optimum will result in
the enzyme being
denatured
Once denatured the
enzyme can no longer
function
Denaturation of the
enzyme is permanent
1
2
3
4
5
Acid
6
7
8
9
10
Alkaline
pH
Enzymes often work over a range of pH
values, but all enzymes have an optimum
pH where their activity rate is fastest.
Substrate Concentration
• Reaction rate increase
as substrate
concentration increases
up to a point
(saturation)
(enzyme concentration
constant)
• Maximum activity
occurs when the
enzyme is saturated
Enzyme Concentration
• Reaction rate
increases as
enzyme
concentration
increase
• (at constant and
very high substrate
concentration)
• At higher enzyme
concentration
more substrate will
bind with enzyme
What determines the final about of
PRODUCT produced in an enzyme driven
reaction?
• The final amount of product is dependent on
the initial amount to substrate
• The amount of time taken to produce the
maximum amount of product can be affected
by factors such as temperature, pH, and
enzyme concentration.
Co Enzymes
• Sometimes help is needed to make the active
site of an enzyme a perfect complementary fit
for its substrate
• Co enzymes are molecules that fulfil this role
• Vitamins are an example
INHIBITION: COMPETITIVE
- AN INHIBITOR BINDS TO THE ACTIVE SITE PREVENTING THE
ENZYME FROM BINDING TO THE SUBSTRATE
HIV protease complexed with the
protease inhibitor ritonavir
INHIBITION: NON-COMPETITIVE
-AN INHIBITOR BINDS TO A SITE OTHER THAN THE ACTIVE
SITE WHICH CAUSES A CHANGE IN THE ACTIVE SITE
examples
• Heavy metals such as lead, mercury, copper or
silver are poisonous
• This is because ions of these metals are noncompetitive inhibitors for several enzymes.
Drugs and desirable Inhibition
• While some enzyme inhibitors are poisonous, others
are beneficial.
• Penicillin acts as an inhibitor for transpeptidase, an
enzyme that bacteria need to build their cell walls.
If the cell wall is lacking, osmotic pressure causes
the bacterial cell to burst and the die.
• However, new strains of bacteria have developed an
enzyme, penicillinase, that inactivates penicillin.
• To destroy these new strains, synthetically modified
penicillins have been prepared so that this antibiotic
remains effective.