© SSER Ltd.
Of all the functions of proteins, one of the most
important is that of catalysis
In the absence of catalysis, most reactions in biological systems
would take place far too slowly to provide products at an
adequate pace for metabolising organisms
The catalysts that serve this function in living
organisms are called ENZYMES
All enzymes are globular proteins and are the most
efficient catalysts known
Enzymes are able to increase the rate of reaction by a factor
of up to 1020 over uncatalysed reactions
Substrate
molecule in
the ACTIVE
SITE
• They are proteins of high
molecular weight
• They are biological catalysts
• They are sensitive to
temperature changes,
being denatured at
high temperatures
• They are sensitive to pH
• They are generally specific in
the reactions they catalyse
• Enzymes possess an active
site within which chemical
reactions take place
Enzyme molecule
Active
site
Enzyme
molecule
Reaction
occurs
Enzyme
remains
unchanged
Substrate
molecules
(complementary
shape to active site)
Product
molecules
diffuse away
from the
active site
Substrate molecules bind with enzyme molecules at the active
site as a consequence of their complementary shapes. This is the
basis of the LOCK AND KEY MODEL of enzyme activity
In an enzyme-catalysed reaction, the enzyme
binds to the substrate to form a complex
Enzyme
molecule
A reaction
occurs
forming an
enzyme-product
complex
S
An enzyme-substrate complex
forms
Products diffuse
away from the
active site
The lock & key model proposes that the substrate binds to
the active site which it fits exactly, like a key in a lock
This model takes into account the fact that proteins (enzymes)
have some three-dimensional flexibility
SUBSTRATE
Substrate binds to the enzyme
at the active site
Binding of the substrate
induces the enzyme to change
shape such that there is an
exact fit once the substrate
has bound
Enzyme Molecule
According to this model, reactions can only
take place AFTER induced fit has occurred
Energy barrier
without enzyme
Energy barrier
with enzyme
Energy level
of substrate
Lower activation
energy
Energy level
of the products
Substrates need to overcome an energy
barrier before they will convert to products
Enzymes are catalysts
because they lower the
ACTIVATION
ENERGY
needed to drive a
reaction
Temperature
pH
Substrate Concentration
Enzyme Concentration
Inhibitors
Activators
The reaction
rate doubles for
every 10°C rise
in temperature
As the temperature
increases, molecular
motion and thus
molecular collisions
increase
More product
molecules are
formed in a given
time and hence
the reaction
rate increases
For many enzymes, the maximum
rate of reaction is reached at a
temperature between 37°C to 40°C
This is the optimum temperature
As the temperature increases
beyond the optimum, bonds
that stabilise the enzyme’s
tertiary structure are broken
The enzyme loses its shapes
and the active site is altered
Substrate can no longer bind to the enzyme
The enzyme has been DENATURED
Each specific enzyme can only work Each enzyme has its own optimum pH
over a particular range of pH
where the rate of reaction is maximum
The effects of pH on the rate of enzyme controlled reactions display
characteristically bell shaped curves
B
A
C
Enzyme A = amylase
optimum pH = 7.2
Enzyme B = pepsin
optimum pH = 2.0
Enzyme C = lipase
optimum pH = 9.0
Changes in pH can affect the ionic and hydrogen
bonds responsible for the specific tertiary shape of enzymes
Extremes of pH break these bonds and denature the enzyme
Low Substrate
Concentration
Low product
concentration per
unit time
Increased Substrate
Concentration
More product
formation;
increased reaction rate
Further increase
in substrate
concentration
Maximum product
formation; maximum
rate of reaction
Excess substrate
concentration
Enzyme
concentration
is the LIMITING
FACTOR
No further increase
in product formation;
maximum reaction rate
maintained
Rate of Rate of reaction reaches
reaction a maximum at substrate
concentration A
Rate of reaction
increases as the
substrate
concentration
increases
No further increase in
the reaction rate despite
the increasing substrate
concentration
All the active sites of the
enzymes are occupied enzyme concentration
is the limiting factor
A
Increasing concentration of substrate
Rate of
reaction
The rate of reaction
is directly proportional to the
enzyme concentration
As enzyme concentration
increases, the rate of
reaction increases
In living cells, enzyme
concentrations are usually
much lower than substrate
concentrations
Substrate concentration is
rarely a limiting factor
Increasing concentration of enzyme
Low substrate concentration
Inhibitor molecule
When the substrate concentration is low, the
inhibitor competes successfully for the active
site; fewer substrate molecules are converted
into product and the rate of reaction is reduced
High substrate concentration
Inhibitor molecule
The effect of the competitive inhibitor is overcome
when the high concentration of substrate molecules compete
successfully for the active sites of the enzymes; at high substrate
concentration, maximum reaction rate is achieved
At low substrate concentrations,
the rate of reaction is reduced
in the presence of the inhibitor
maximum rate
without
inhibitor
inhibitor
present
The effect of the inhibitor
is overcome by very high
substrate concentrations
At high substrate concentrations,
the inhibitor is out-competed by the
substrate and the maximum rate
of reaction is achieved
Low substrate concentration
Inhibitor molecule
Substrate
molecules not
converted to
product when
inhibitor
molecules are
bound to the
enzyme
Substrate molecules converted
into product when no inhibitor is
attached to the enzyme
Substrate binds to the enzyme when a
non-competitive inhibitor is present but
cannot be converted to product; the rate
of reaction is reduced
High substrate concentration
Inhibitor molecule
X
X
Substrate molecules converted
into product when no inhibitor is
attached to the enzyme
X X
X X
At high substrate
concentration
all enzyme active
sites are occupied
Substrate molecules
bound to enzymes
with attached
inhibitor are NOT
converted into
product - maximum
reaction rates are
never achieved
The effect of the inhibitor is not overcome
by increasing the substrate concentration.
All the enzyme molecules with bound
non-competitive inhibitor do NOT
convert substrate to product; the effect
is equivalent to lowering
enzyme concentration
no inhibitor; maximum
reaction rate achieved
at high substrate
concentration
with inhibitor; maximum
reaction rate never achieved the effect of the inhibitor cannot
be overcome by increasing the
substrate concentration
Non-competitive inhibitors act by preventing bound
substrate being converted into product