2. LEARNING GOALS
Describe the role of enzymes and why they are
essential to living organisms
List the factors that may affect the rate of an
enzyme catalysed reaction
Describe the importance of the structure of an
enzyme to its functioning
3. 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.
5. ENZYME EXAMPLES
Enzyme Role
Stomach enzyme used to
break protein down into
Pepsin
peptides. Works at very acidic
pH (1.5).
Digestive enzymes which act
Proteases on proteins in the digestive
system
A family of enzymes which
Amylases assist in the breakdown of
carbohydrates
A family of enzymes which
3D molecular structures for the Lipases
enzymes pepsin (top) and breakdown lipids
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. Finish
ENZYMES
Without enzyme: The activation
energy required is high.
Direction of reaction
Reactant With enzyme: The activation
energy required is lower.
High energy
Product
Low energy
Start
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- Space filling model of the yeast
enzyme hexokinase. Its active
substrate complex is formed. site lies in the groove (arrowed)
10. molecules. It has three active sites (arrowed).
Ribonuclease S, that breaks up RNA
This model (above) is an enzyme called
Enzyme molecule:
The complexity of the
active site is what makes
each enzyme so specific
(i.e. precise in terms of the
Active site: substrate it acts on).
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 the cleft of the enzyme.
together or splitting up a larger one. acts on. They are drawn into
chemicals that an enzyme
Substrate molecules are the
Substrate molecule:
ENZYME ACTIVE SITES
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.
Products
Substrate
Symbolic representation of the lock and key model of enzyme action.
1. A substrate is drawn into the active sites of the enzyme.
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 Two substrate
molecules are
revealed that the process is drawn into the
much more likely to involve cleft of the
enzyme.
an induced fit.
The enzyme
The enzyme or the reactants changes shape,
(substrate) change their shape forcing the substrate
slightly. molecules to
combine.
The reactants become bound to
enzymes by weak chemical
bonds.
This binding can weaken bonds The resulting end
within the reactants product is released
by the enzyme
themselves, allowing the
which returns to its
reaction to proceed more normal shape, ready
readily. 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
Optimum denaturation of enzymes
Temperature also increases at higher
for enzyme Too hot for temperatures.
Enzyme to
work
High temperatures break
the disulphide bonds
holding the tertiary
Too cold structure of the enzyme
for Enzyme together thus changing the
to work shape of the enzyme.
This destroys the active
sites & therefore makes
the enzyme non –
functional.
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.
