1. © SSER Ltd.

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. Substrate molecule in the active site Enzyme molecule
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

4. Substrate molecules bind with enzyme molecules at the active
(complementary
shape to active site)
Enzyme
molecule
Active
site
Reaction
occurs
Product
molecules
diffuse away
from the
active site
Enzyme
remains
unchanged
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

5. In an enzyme-catalysed reaction, the enzyme
binds to the substrate to form a complex
Enzyme
molecule
An enzyme-substrate complex
forms S
A reaction
occurs
forming an
enzyme-product
complex
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

6. This model takes into account the fact that proteins (enzymes)
have some three-dimensional flexibility
Substrate binds to the enzyme
at the active site
SUBSTRATE
Binding of the substrate
induces the enzyme to change shape and form an exact fit
Enzyme Molecule
According to this model, reactions can only take place after induced fit has occurred

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

9. Substrate Concentration
Temperature pH
Substrate Concentration
Enzyme Concentration
Inhibitors
Activators

10. At low temperatures At higher Temperatures
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

11. 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 and secondary structure
are broken; the enzyme loses
its shapes and the active site is
altered; substrate can no longer
bind to the enzyme and the
enzyme has been denatured

12. The reaction rate doubles for every 10°C rise in temperature
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, molecular motion and thus molecular collisions increase
More product molecules are formed in a given time and hence the reaction rate increases
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

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

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
NEUTRAL
INCREASING ACIDITY
INCREASING ALKALINITY
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

15. A B C Enzyme A = amylase optimum pH = 7.2 Enzyme B = pepsin
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

16. increased reaction rate
Low product
concentration per
unit time
Low Substrate
Concentration
Increased Substrate
Concentration
More product
formation;
increased reaction rate

17. Further increase in substrate concentration Excess substrate
Maximum product
formation; maximum
rate of reaction
Further increase
in substrate
concentration
No further increase
in product formation;
maximum reaction rate
maintained
Excess substrate
concentration
Enzyme
concentration
is the limiting factor

18. A Increasing concentration of substrate
Rate of
reaction
Rate of reaction reaches
a maximum at substrate
concentration A
No further increase in
the reaction rate despite the increasing substrate concentration
Rate of reaction
increases as the
substrate
concentration
increases
All the active sites of the enzymes are occupied -enzyme concentration
is the limiting factor A
Increasing concentration of substrate

19. Substrate concentration is rarely a limiting factor
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

20. The presence of inhibitor molecules decreases the rate
of enzyme reactions by reversible combination with the enzyme
Molecule similar
in shape to the
normal substrate
Normal substrate
This molecule competes
with the normal substrate
for the active site
This molecule
is an example of
a competitive
inhibitor

21. converted into products
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 not competing
for the active site
This inhibitor is a non-competitive inhibitor

22. 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

23. 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

24. At low substrate concentrations, the rate of reaction is reduced
in the presence of the inhibitor
maximum rate
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
without
inhibitor
inhibitor
present

25. 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

26. X High substrate concentration Inhibitor molecule
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 X
Substrate molecules converted
into product when no inhibitor is
attached to the enzyme
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

27. 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

28. A B C D E Metabolic pathways are sequences of chemical reactions
each controlled by a specific enzyme
INHIBITS
enzyme 1 2 3 4 A B C D E
The initial
substrate
is converted by a series of
intermediate compounds
into the
final product
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

29. 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

30. + ATP ADP 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
Glucose P
three phosphate groups
ATP
ADENOSINE
TRIPHOSPHATE A P P P
Glucose Phosphate
ADP
ADENOSINE
DIPHOSPHATE +
The type of enzyme that
catalyses this reaction
is a transferase

31. What is Oxidation? What is Reduction?
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

32. 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

33. 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 hydrogen atoms are
then accepted by the
hydrogen carrier NAD
The cycle involves
the stepwise oxidation
of a 6-carbon acid
in to a 4C acid
This example
illustrates the point
that when one
substance is oxidised
another is reduced
Oxidation in the cycle
involves the removal of
pairs of hydrogen atoms
from the acids
The organic acid is
oxidised and NAD
is reduced 2H
The class of oxidoreductase
that catalyses such a reaction
is a dehydrogenase
NADH2
Such reactions are called redox reactions

34. Copyright © 2008 SSER Ltd. and its licensors. All rights reserved
Copyright © 2008 SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.