1. Enzymes are biological catalysts that speed up reactions by lowering activation energy. 2. Enzymes have specific active sites that substrates bind to, and can break down or join substrates. 3. Enzymes are proteins but some are RNA molecules called ribozymes, and their activity is affected by temperature, pH, substrate concentration and other factors.

1. Enzymes
An Introductory Presentation on the Rudiments of Biological
Catalysts

2. Enzymes
Enzymes are molecules that act as catalysts
to speed up biological reactions.
The compound on which an enzyme acts is the
substrate.
Enzymes can break a single structure into
smaller components or join two or more
substrate molecules together.
Most enzymes are proteins.
Many fruits contain enzymes that are used in commercial
processes. Pineapple (Ananas comosus, right) contains the
enzyme papain which is used in meat tenderization processes and
also medically as an anti-inflammatory agent.

3. Enzymes
Ribozymes (ribonucleic acid enzymes) are RNA
molecules that are capable of catalyzing specific
biochemical reactions, similar to the action of protein
enzymes.
The 1982 discovery of ribozymes demonstrated that RNA
can be both genetic material (like DNA) and a biological
catalyst (like protein enzymes), and contributed to the
RNA world hypothesis, which suggests that RNA may
have been important in the evolution of prebiotic self-
replicating systems.
Schematic showing ribozyme cleavage of RNA.

4. Enzymes

5. Enzyme Examples
Enzyme Role
Pepsin
Stomach enzyme used to
break protein down into
peptides. Works at very acidic
pH (1.5).
Proteases
Digestive enzymes which act
on proteins in the digestive
system
Amylases
A family of enzymes which
assist in the breakdown of
carbohydrates
Lipases
A family of enzymes which
breakdown lipids
3D molecular structures for the
enzymes pepsin (top) and
hyaluronidase (bottom).

6. Enzyme Examples
 One of the fastest enzymes in the body is catalase.
Catalase breaks down hydrogen peroxide, a waste
product of cell metabolism, into water and oxygen.
Accumulation of hydrogen peroxide is toxic so this
enzyme performs an important job in the body.

7. Enzyme Power!
 All reactants need to have a certain energy
before they will react. This is like an energy
barrier that it has to overcome before a reaction
will occur. It is called the activation energy.
 Enzymes are organic catalysts.
 All catalysts lower the energy barrier, allowing
the reactants (substrates) to react faster forming
the products.
 Enzymes do not participate in the reaction.

8. Reactant
Product
Without enzyme: The activation
energy required is high.
With enzyme: The activation
energy required is lower.
Enzymes
High
Low
Start Finish
Direction of reaction
Amountofenergystoredinthe
chemicals
Low energy
High energy

9. Enzymes
Enzymes have a specific region
where the substrate binds and
where catalysis occurs. This is
called the active site.
Enzymes are substrate-specific,
although specificity varies from
enzyme to enzyme.
When a substrate binds to an
enzyme’s active site, an enzyme-
substrate complex is formed.
Space filling model of the yeast
enzyme hexokinase. Its active
site lies in the groove (arrowed)

10. Enzyme Active Sites
This model (above) is an enzyme called
Ribonuclease S, that breaks up RNA
molecules. It has three active sites (arrowed).
Active site:
The active site contains both binding
and catalytic regions. The substrate
is drawn to the enzyme’s surface and
the substrate molecule(s) 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).
Substrate molecule:
Substrate molecules are the
chemicals that an enzyme
acts on. They are drawn into
the cleft of the enzyme.

11. Lock and Key Model
The lock and key model of enzyme action, proposed earlier this
century, proposed that the substrate was simply drawn into a closely
matching cleft on the enzyme molecule.
Substrate
Enzyme
Products
Symbolic representation of the lock and key model of enzyme action.
1. A substrate is drawn into the active sites of the enzyme.
2. The substrate shape must be compatible with the enzymes active site in
order to fit and be reacted upon.
3. The enzyme modifies the substrate. In this instance the substrate is
broken down, releasing two products.

12. Induced Fit Model
More recent studies have
revealed that the process
is much more likely to
involve an induced fit.
The enzyme or the reactants
(substrate) change their shape
slightly.
The reactants become bound to
enzymes by weak chemical
bonds.
This binding can weaken bonds
within the reactants themselves,
allowing the reaction to proceed
more readily.
The enzyme
changes shape,
forcing the substrate
molecules to
combine.
Two substrate
molecules are
drawn into the cleft
of the enzyme.
The resulting end
product is released
by the enzyme
which returns to its
normal shape, ready
to undergo more
reactions.

13. Changing the Active Site
 Changes to the shape of the active site will result in a
loss of function. Enzymes are sensitive to various
factors such as temperature & pH.
 When an enzyme has lost its characteristic 3D shape,
it is said to be denatured. Some enzymes can regain
their shape while in others, the changes are
irreversible.

14. The Effect of Temperature on
Enzyme Action  Speeds up all reactions, but
the rate of denaturation of
enzymes also increases at
higher temperatures.
 High temperatures break the
disulphide bonds holding the
tertiary structure of the
enzyme together thus
changing the shape of the
enzyme.
 This destroys the active
sites & therefore makes the
enzyme non – functional.
Too cold for
Enzyme to
work
Too hot for
Enzyme to
work
Optimum
Temperature
for enzyme

15. The Effect of Temperature on
Enzyme Action
 The curve in the blue represents an enzyme isolated from an organism
living in the artic. These cold dwelling organisms are called psychrophiles.
 The curve in red represents an enzyme isolated from the digestive tract of
humans.
 The curve in green represents an enzyme isolated from a thermophile
bacteria found growing in geothermal sea beds.

16. The Effect of pH on Enzyme Action
 Like all proteins, enzymes are
denatured by extremes of pH
(acidity/alkalinity).
 The green curve is for pepsin
that digests proteins in the
stomach.
 The red curve represents the
activity of arginase that
breaks down arginine to
ornithine & urea in the liver.

17. The Effect of Enzyme
Concentration on Enzyme Action
 Assuming that the
amount of substrate is
not limiting, an
increase in enzyme
concentration causes
an increase in the
reaction rate.

18. The Effect of Substrate
Concentration on Enzyme Action
 Assuming that the amount of
enzyme is constant, an increase in
substrate concentration causes a
diminishing increase in the reaction
rate.
 A maximum rate is obtained at a
certain concentration of substrate
when all enzymes are occupied
substrate (the rate cannot increase
any further).

19. The Effect of Cofactors on Enzyme
Action
 Cofactors are substances
that are essential to the
catalytic activity of some
enzymes.
 Cofactors may alter the
shape of enzymes slightly to
make the active sites
functional or to complete the
reactive site.
 Enzyme cofactors include
coenzymes (organic
molecules) or activating ions
(eg. Na+, K+..)
 Vitamins are often
coenzymes (eg. Vit B1, Vit
B6…)

20. The Nature of Enzyme Inhibitors
Enzyme inhibitors may or may not act reversibly:
Reversible: the inhibitor is temporarily bound to the
enzyme, thereby preventing its function (used as a
mechanism to control enzyme activity).
Irreversible: the inhibitor may bind permanently to
the enzyme causing it to be permanently
deactivated.

21. The Nature of Enzyme Inhibitors
Reversible Enzymes work in one of two ways:
Competitive inhibitors: the inhibitor competes with
the substrate for the active site, thereby blocking it
and preventing attachment of the substrate.
Non-competitive: the inhibitor binds to the enzyme
(but not at the active site) and alters its shape. It
markedly slows down the reaction rate by making
the enzyme less able to perform its function
(allosteric inhibition).

22. Summary: Enzymes
1. Enzymes work very rapidly and help to speed up
biological reactions.
2. Enzymes can be used multiple times (however they
do degrade eventually).
3. Enzymes can work in both directions of a chemical
reaction.
4. Enzymes have optimal temperatures and pH that
they will operate. Beyond these optimum ranges
they will either not work or become denatured
(unfolded/breakdown).
5. Enzymes are usually specific to one particular
substrate.