Enzymes are protein catalysts that greatly increase the rate of biochemical reactions. They work by lowering the activation energy of reactions. The active site of an enzyme complements the substrate and catalyzes its conversion to products. Factors like temperature, pH, substrate and enzyme concentration can affect the rate of enzyme-catalyzed reactions. Inhibitors bind to enzymes and decrease reaction rates, with competitive inhibitors binding the active site and non-competitive inhibitors binding elsewhere.

1. Enzymes

2. 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.

3. Characteristics of Enzymes
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
Substrate
molecule in
the ACTIVE
SITE
Enzyme molecule

4. The Active Site
Substrate
molecules
(complementary
shape to active
site)
Enzyme
molecule
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
Enzyme remains
unchanged
Active
site
Reaction
occurs

5. The Lock And Key Model
In an enzyme - catalysed reaction, the enzyme binds
to the substrate to form a complex
An enzyme - substrate complex
forms
A reaction
occurs
forming an
enzyme - product
complex
Products diffuse
away from the
active site
Enzyme
molecule
The lock & key model
proposes that the substrate
binds to the active site
which it fits exactly, like a
key in a lock
S

6. The Induced Fit Model
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

7. Energy level
of substrate
Energy level
of the products
Energy barrier
without enzyme
Energy barrier
with enzyme
Lower activation
energy
Enzymes are catalysts
because they lower the
ACTIVATION
ENERGY
needed to drive a
reaction
Substrates need to overcome an energy
barrier before they will convert to
products

8. Factors Affecting The Rate Of
Enzyme Catalysed Reactions
Temperature
pH
Substrate Concentration
Enzyme Concentration
Inhibitors
Activators

9. Molecules are
constantly in motion
and colliding with
one another.
The speed of motion
and number of
collisions is affected
by the temperature
AT LOW
TEMPERATURES
AT HIGHER
TEMPERATURES
More enzyme-substrate
complexes and hence
more product
molecules are formed
at the higher
temperature

10. Temperature and Enzyme Activity
Rate of
Reaction
Temperature (oC)
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 around 37 to 40oC.
This is the optimum temperature.
The reaction
rate doubles for
every 10oC rise
in 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

11. In the temperature range
4oC to 40oC, the
rate of reaction doubles
for every 10oC rise in
temperature
The temperature coefficient
(Q10) is the effect of a 10oC
rise in temperature on the
rate of a chemical reaction
When the rate of reaction
doubles for every 10oC rise
in temperature then
the Q10 = 2
For an enzyme controlled
reaction, in the temperature
range 4oC to 40oC, an increase
of 10oC doubles the rate of
reaction. Therefore the
Q10 = 2
Temperature and Enzyme Activity

12. Enzymes and pH
INCREASING ACIDITY INCREASING ALKALINITY
1 3
0 2 4 5 6 7 8 9 10 11 12 13 14
The acidity of a solution is measured by the concentration of
hydrogen ions (H+) and is expressed in terms of pH
The pH scale ranges from 0 to 14
Pure water has a pH of 7.0, which is the pH
of a neutral solution with equal numbers of H+ and OH- ions
If an acid is added
to pure water, the
hydrogen
ion concentration
increases, causing
the solution
to become acidic,
which is measured
as a lower pH
If a base is added to
pure water, the hydrogen
ion concentration
decreases and the
hydroxyl ion (OH-)
concentration increases.
The solution becomes
more basic (alkaline) and
is measured as a higher pH
NEUTRAL

13. Each specific enzyme can only work
over a particular range of pH
Each enzyme has its own optimum pH
where the rate of reaction is maximum
The effects of pH on the rate of enzyme controlled reactions display
characteristically bell shaped curves
A
B 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
Enzymes and pH

15. Naming Enzymes
Enzymes are classified according to the type
of chemical reaction that they catalyse
HYDROLASES are enzymes that catalyse hydrolysis reactions
maltose is a disaccharide
consisting of two alpha
glucose molecules joined
by a glycosidic bond
maltase is a hydrolase
enzyme that catalyses
the hydrolysis of maltose
into two
glucose molecules
H2O MALTASE

16. TRANSFERASES are enzymes that catalyse reactions
involving the transfer of atoms or groups of atoms from
one molecule to another
During cellular respiration, a phosphate group
is transferred from a molecule of ATP to a glucose molecule
This process activates the glucose
P
P
A
P
three phosphate groups
ATP
ADENOSINE
TRIPHOSPHATE
The type of enzyme that
catalyses this reaction
is a TRANSFERASE
Glucose Phosphate ADP
ADENOSINE
DIPHOSPHATE
+
Glucose
P
Naming Enzymes

17. OXIDOREDUCTASES are enzymes that catalyse
reactions involving oxidation and reduction
What is Oxidation?
Oxidation reactions can occur in three main ways:
•addition of oxygen
•removal of hydrogen atoms
•removal of electrons
What is Reduction?
Oxidation reactions can occur in three main ways:
•removal of oxygen
•addition of hydrogen atoms
•addition of electrons
Naming Enzymes

18. EXAMPLES OF OXIDATION AND REDUCTION
C + O2 CO2 Oxidation
Fe3+ + e- Fe2+
Reduction
NAD + 2H NADH2 Reduction
The enzymes that catalyse the above reactions are classed
as OXIDOREDUCTASES
OXIDOREDUCTASES PLAY AN IMPORTANT ROLE
IN THE BIOCHEMISTRY OF RESPIRATION
Naming Enzymes

19. OXIDOREDUCTASES AND RESPIRATION
During the process of respiration, a cycle of reactions, called
The Krebs Cycle takes place
The molecules shown
in the cycle are organic
acids
The cycle involves
the stepwise oxidation
of a 6 carbon acid
in to a 4C acid
Oxidation in the cycle
involves the removal of
pairs of hydrogen atoms
from the acids
NADH2
2H
The class of oxidoreductase
that catalyses such a reaction
is a DEHYDROGENASE
The hydrogen atoms are
then accepted by the
hydrogen carrier NAD
This example illustrates
the point that when
one substance is
oxidised another
is reduced
The organic acid is
oxidised and NAD
is reduced
Such reactions are called REDOX REACTIONS
Naming Enzymes

20. Metabolic Pathways and
Feedback Inhibition
Metabolic pathways are sequences of chemical reactions
each controlled by a specific enzyme
A D
B C E
enzyme
1
enzyme
2
enzyme
3
enzyme
4
The initial
substrate
into the
final product
is converted by a series of
intermediate compounds
It is wasteful for a sequence of chemical reactions to
continue if the end product is being produced at a rate surplus
to requirements
When the end product of the pathway begins to accumulate,
it may act as an inhibitor of the first enzyme in the pathway
Further production of the end product is prevented in a process called
FEEDBACK INHIBITION
INHIBITS

22. The Effect Of Substrate Concentration
On The Rate Of Enzyme - Catalysed Reactions
Low Substrate
Concentration
Low product
concentration per
unit time
Increased Substrate
Concentration
More product
formation;
increased reaction rate

23. Further increase
in substrate
concentration
Excess substrate
concentration
Maximum product
formation; maximum
rate of reaction
No further increase
in product formation;
maximum reaction rate
maintained
Enzyme
concentration
is the LIMITING
FACTOR
The Effect Of Substrate Concentration
On The Rate Of Enzyme - Catalysed Reactions

24. The Effect Of Substrate Concentration
On The Rate Of Enzyme - Catalysed Reactions
Increasing concentration of substrate
Rate of
reaction
A
Rate of reaction
increases as the
substrate
concentration
increases
Rate of reaction reaches
a maximum at substrate
concentration A
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

25. The Effect Of Enzyme Concentration
On The Rate Of Enzyme - Catalysed Reactions
Rate of
reaction
Increasing concentration of enzyme
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

26. The Effect Of Reversible Inhibitors
On Enzyme Activity
The presence of inhibitor molecules decreases the rate
of enzyme reactions by reversible combination with the enzyme
Normal substrate
Molecule similar
in shape to the
normal substrate
This molecule
competes
with the normal
substrate
for the active site
This molecule
is an example of
a COMPETITIVE
INHIBITOR

27. The Effect Of Reversible Inhibitors
On Enzyme Activity
Normal
substrate
converted
into products
This inhibitor molecule attaches
to the enzyme at a position
away from the active site
The substrate
molecule can still
bind to the active
site
Substrate cannot
be converted into
product. The
inhibitor molecule
changes the shape of
the active site
preventing induced
fit
This inhibitor is
a NON-COMPETITIVE
INHIBITOR
This inhibitor
is not competing
for the active
site

28. The Effect Of Competitive Inhibitors
On Enzyme Activity
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

29. The Effect Of Competitive Inhibitors
On Enzyme Activity
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

30. 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
At low substrate
concentrations, the
rate of reaction is reduced
in the presence of the inhibitor
The Effect Of Competitive Inhibitors
On Enzyme Activity

31. substrate binds to the enzyme
when a non-competitive inhibitor
is present but cannot be converted
to product - The rate of reaction is reduced
The Effect Of Non-Competitive Inhibitors
On Enzyme Activity
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

32. Inhibitor molecule
substrate molecules converted
into product when no inhibitor is
attached to the enzyme
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
High substrate concentration
X
X
X X
X X
The Effect Of Non-Competitive Inhibitors
On Enzyme Activity

33. 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
The Effect Of Non-Competitive Inhibitors
On Enzyme Activity
Non-competitive inhibitors act by
preventing bound substrate being
converted into product