Enzymes are proteins that catalyse chemical reactions of other substances without itself being destroyed or altered upon completion of the reactions.

2. → What are enzymes?
→ Structure.
→ Classification.
→ Specificity.
→ Mechanism of action.
→ Ribonuclease and Chymotrypsin.
→ Kinetics.
→ Inhibition.

3. → Factors affecting the rate of enzyme
catalysed reactions.
→ Mechanism of enzymes.
→ Biological and pharmaceutical importance.

4.  Enzymes are proteins that catalyse chemical
reactions of other substances without itself
being destroyed or altered upon completion
of the reactions.
 They increase the reaction rate without
altering the equilibrium.
 They reduce the activation energy.

5.  Enzymes are globular proteins
ranging from 62 – 2500 amino
acid (residues).
 They are long, linear chains of
amino acids that fold to produce
a three dimensional product.
 The region where the substrate
binds and undergoes the reaction
is called the active site of that
enzyme.
 The nature and arrangement of
amino acids in the active site
make it specific for only one type
of substrate.
 The amino acids are linked
together by peptide bonds.

6. Enzymes are classified by reaction types, i.e.
They are named on the basis of the reaction
they catalyse followed by the suffix ‘ase’. For
example:
• Dehydrogenases remove hydrogen atoms.
• Proteases hydrolyse proteins.
• Lipases hydrolyse lipids and fats.

7. Enzymes are grouped into six classes:
1) Oxidoreductases: catalyse oxidations and reductions for
example dehydrogenase.
2) Hydrolases: catalyse the hydrolytic cleavage of C-O, C-N,
C-C etc.
3) Transferases: catalyse transfer of moieties such as a
methyl or glycosyl group from a compound.
4) Lyases: catalyse cleavage of C-C, C-N, C-O, etc. By
atom elimination, leaving double bonds or rings.
5) Isomerases: catalyse geometric or structural changes
within a molecule.
6) Ligases: catalyse the joining together of two molecules
coupled with the hydrolysis of a diphosphate bond in ATP
or a similar triphosphate.

8.  Factors that are
responsible for the
specificity of enzymes are:
 Complementary shape
 Hydrophilic/Hydrophobic
characteristics
 Charge
Enzymes only form the
‘Enzyme – Substrate
complex’ with substrates
that are complementary to
it’s active site.

9. Enzymes act in the following ways:
1) Lower the activation energy.
2) Providing an alternative pathway.
3) Reducing the reaction entropy change by
bringing substrates together in the correct
orientation to react.
4) Increases in temperatures speeds up
reactions.
5) Lowering the energy of the transition state
without distorting the substrate.

10. Lock and key theory: according to
Fildes:
 only a specific substrate can
combine with the active site of a
particular enzyme as a specific
key fits to open a specific lock.
 The enzyme molecule possesses
an active site to fit correctly with
the substrate forming an
‘ENZYME – SUBSTRATE
COMPLEX’.
 When the reaction reaches
completion, the ES complex
breaks into products and
enzymes.
 The enzymes remain intact
throughout the reaction.

11. Induced fit theory: according to
Koshland:
 When a suitable substrate
approaches the active site of an
enzyme, the substrate inducts
some conformational changes in
the enzyme.
 After the suitable ES complex
has formed, the substrate
molecule is held by hydrogen
bonds.
 A strain (nucleophilic attack) hits
the charged catalytic groups of
the active site.
 This strain weakens the bond
which is ultimately broken and
the products are formed.

12.  Ribonuclease is a type
of nuclease that
catalyses the
degradation of RNA
into smaller
components.
 It catalyses the
phosphodiester bond
in RNA chains.
 Chymotrypsin is a
digestive enzyme that
can perform
proteolysis.
 It cleaves the peptide
amide bonds where
the carboxyl side of the
amide bond is tryosine,
tryptophan or
phenylalanine.

13.  Kinetics is the study of
the rates (velocities) of
chemical reactions.
 The rates of biological
reactions are greatly
increased by enzyme
catalysts.
When enzyme
concentration
becomes limiting:
• Enzyme is working at
maximal rate.
• Enzyme becomes
saturated.
• Velocity of reaction at
this substrate
concentration is Vmax.

14.  Michaelis – menton
equation:
 This equation can be
used to demonstrate that
at the substrate
concentration that
produces exactly half of
the minimum reaction
rate i.e. ½ Vmax, the
substrate concentration
is numerically equal to
Km

15.  Competitive inhibition:
 A competitive inhibitor is a substance that
competes with the substrate for the same
active site.
 Hence they cannot bind at the same time.

16.  Uncompetitive inhibition:
 the inhibitor cannot bind to the free enzyme,
instead it binds to the ES complex.
 The EIS complex formed is thus
enzymatically inactive.

17.  Non-competitive inhibition:
 The inhibitor binds to the enzyme at the
same time as the substrate but not at the
active site.
 Both the EI and EIS complexes are
enzymatically inactive.

18.  Temperature:
 Enzymes catalyse reactions by randomly
colliding with substrate molecules.
 Increasing temperature increases the rate
of reaction, forming more product.
 Increasing the temperature also increases
the vibrational energy, which puts strain on
the bonds in the enzyme molecule.
 As the temperature increases, the weak h-
bonds and ionic bonds break.
 As a result, the active site ‘s shape
changes.
 This means that it is no longer
complementary to the shape of the
substrate.
 The enzyme becomes denatured and the
rate of reaction decreases considerably.
 The temperature at which the maximum
rate of reaction occurs is called the
‘optimum temperature’.

19.  pH:
 H+ and OH+ ions are charged
and therefore interfere with
hydrogen and ionic bonds that
hold together an enzyme.
 This interference causes a
change in shape of the enzyme’s
active site.
 As a result the active site is no
longer complementary to the
substrate due hence the rate of
reaction decreases.
 Different enzymes have pH
values at which they work best,
i.e. They work most efficiently.
This pH is known as its ‘optimum
pH’.

20.  Substrate concentration:
 Increasing the substrate
concentration increases the rate
of reaction.
 This is due to more substrate
molecules colliding with enzyme
molecules, hence more product
will be formed.
 However, after a certain
concentration, the increase will
have no affect on the rate of
reaction since substrate
concentration will no longer be
the limiting factor.
 The enzymes will effectively
become saturated and will be
working at their maximum
possible rate.

21.  Enzyme concentration:
 Increasing enzyme
concentration will increase
the rate of reaction, as
more enzymes will be
colliding with substrate
molecules.
 This will also only have an
effect up to a certain
concentration, where the
enzyme concentration is no
longer the limiting factor.

22.  Enzymes play an important
role in the ripening of fruits.
 They help in the coagulation
of blood.
 Enzymes help in digestion of
food, converting it into simple,
soluble and diffusible form.
 Transport of oxygen and
carbon dioxide is rapidly
carried out by RBCs due the
presence of enzymes –
carbonic anhydrase.
 Fermentation of yeast for
example, is a process which
takes place with the help of
enzymes.

23.  Enzymes are used in industry for
maltose degradation and
fermentation.
 Since enzymes are catalyst, they
are the most sensitive target for
drug development.
 Used in pharmaceutical products
such as in the treatment of
enzymatic deficiency caused by
various genetic disorders.
 Used in the production of
synthetic and semi – synthetic
hormones and steroids.
 Help in extraction of medicinally
important compounds such as
heparin.

24. Let’s see how attentive you were during this
presentation. Just a few questions will tell us all:
 What do you mean by limiting factor?
 Which 6 groups are enzymes classified into?
 Define non-competitive inhibition.
Bonus questions:
 What do we mean by allosteric enzymes?
 What is the difference between coenzymes and
cofactors?