Enzymes
Pathway organisers
The ushers of chemical reactions
What is an enzyme?
 A biological catalyst, speeding up the
rate of a reaction.
 Catalysts are chemical substances
that help to speed up a reaction
without being used up. (They are not
reactants.)
 They are only needed in small
amounts and are re-useable
Enzymes - Biological
Catalysts
 Enzymes are all proteins
 They allow biochemical reactions to
happen in conditions that the body
can tolerate
 Some biological reactions would
never happen without enzymes, they
would require very high temperatures
that living things could not survive in
Enzymes - Proteins
 To understand how enzymes work, you
will need to know about protein
structure.
 Proteins are very long polymers that
become folded up to form 3D shapes
 This folding occurs in a series of steps:
 Primary
 Secondary
 Tertiary
Primary
structure refers
to the order of
amino acids
that make up
the chain
Tertiary
structure
refers to how
the helices and
pleats interact
to make the
protein fold in
to a 3D
globular
shape. This is
due to many
different types
of bonding
Secondary
structure refers
the folding of
the structure
into coils
(helix) or
pleated sheets.
This is caused
by hydrogen
bonds between
the amino
acids
Quaternary
structure refers
to the
interaction of
one protein
with another
Enzymes – Biological
Catalysts
 Enzymes work by bringing molecules
together or positioning them in a way
that makes the reaction happen more
quickly.
 The molecules that enzymes act on
are called substrates
 Enzymes are very specific
 The part of the enzyme that comes
into contact with the substrate is
called the active site
For
enzymes,
tertiary is
the most
important
aspect of the
protein
structure. It
creates
grooves in
specific
shapes.
Such
grooves
then form
the active
site which
fits the
substrate(s)
perfectly
If anything
disrupts the
tertiary
structure of
the protein,
the active
site may be
changed. If
the active
site is
changed, the
enzyme will
no longer be
functional
Enzyme + Substrate
Enzymes for building
Enzymes for breaking
Jobs that enzymes perform
 Enzymes work on a variety of
reactions; both catabolic and anabolic:
 The breakdown of glucose
 The production of ATP (cellular
respiration)
 The destruction (lysis) of worn out cells
 Breakdown of toxic substances
 Cell division
 Digesting food that is eaten
How do enzymes speed up
reactions?
 Enzymes are
able to lower
the ‘activation
energy’
required to get
a reaction
started
Reaction Rate
Enzymes – ‘ase’
 The suffix ‘–ase’ is often used in enzyme
names.






Amylase
Maltase
Lactase
Catalase
Lipase
Protease
 Exceptions: pepsin
Factors affecting enzyme activity
 Enzyme concentration – more enzyme = faster
reaction
 Substrate concentration – more substrate = faster
reaction
 pH (acid or base) – depends on enzyme, mostly pH
7
 Temperature – depends on enzyme, usually best at
25-40˚C
 Denaturation – irreversible destruction of active
site due to exposure to extreme conditions
 Inhibition – presence of other molecules which
slow down or prevent enzyme activity by blocking
active site
Substrate & Enzyme Conc.
Temperature & pH:
Optimal Conditions
 Each enzyme will work best at a
specific temperature and specific pH
 This is because pH and temperature
can greatly affect the tertiary structure
of the enzyme and change the active
site
Examples of Optimum pH
Denaturation
 All proteins (including enzymes) can
have their tertiary structure disrupted
permanently. This is called
denaturation
 This can happen if the enzyme is:
 Overheated (beyond ideal temperature)
 Exposed to extreme changes in pH
 Exposed to a toxin which irreversibly
binds to the active site
Denaturation – the irreversible disruption of an enzyme’s tertiary
structure, causing it to become totally inactive
Inhibition
 Enzymes can be affected by the
presence of other molecules
 Molecules which decrease enzyme
activity are called inhibitors
 There are 3 main types of inhibitors:
 Competitive
 Non-competitive
 Toxins/poisons which bind irreversibly to
the active site
Competitive Inhibitors
 Competitive inhibitors bind to the active
site of the enzyme, preventing the
substrate from binding
 This is reversible
 Competitive inhibitors usually have a
similar structure to the substrate for
that enzyme and  fit into the active site
easily
Non-competitive inhibitors
 Non-competitive inhibitors also bind to
the enzyme, but not the active site
 When the inhibitor binds to the
enzyme, it alters the tertiary structure
slightly, causing a change in the shape
of the active site
 The change in the active site prevents
the substrate from binding to it
 This is reversible
Toxins
 Many toxins can bind irreversibly to
enzymes, changing the active site
permanently
 Some examples are:
 Snake venom
 Nerve gases
 Heavy metals
Cofactors and coenzymes
 Some enzymes only function when another
chemical component is able to attach to them
 There are 2 types of these chemical
components:
 Cofactors (inorganic molecules)
 Coenzymes (organic molecules)
 These cofactors and coenzymes are usually
carrier molecules and transfer electrons or
ions from one molecule to another (eg. NADH,
FADH2 and NADPH)