Enzymes and
How They Work
Enzymes
• Enzymes are proteins.
• They are Biological catalysts (speed up
the rate of reactions in living things
without themselves being changed).
• Without enzymes metabolic reactions
would take months to occur.
• Since they are not changed the enzymes
can be used over and over again.
Enzymes
• Enzymes speed up reactions by lowering
the amount of energy needed to get the
reaction to occur; the Activation
energy barrier.
Enzymes
• Enzymes are specific – each enzyme will
only catalyse one reaction.
• Enzymes are thought to act by a lock
and key method or by an induced fit
method.
• The shape of the enzyme brings the 2
reacting chemicals together at the
enzymes active site so that they react
together quickly.
Lock and Key Hypothesis
• The specific action of an enzyme with a
single substrate can be explained using
a Lock and Key analogy first postulated
in 1894 by Emil Fischer.
• In this analogy, the lock is the enzyme
and the key is the substrate.
• Only the correctly sized key
(substrate) fits into the key hole
(active site) of the lock (enzyme).
Lock and Key Hypothesis
• Smaller keys, larger keys, or incorrectly
positioned teeth on keys (incorrectly
shaped or sized substrate molecules) do
not fit into the lock (enzyme).
• Only the correctly shaped key opens a
particular lock. This is illustrated below.
The Induced Fit Model
• Not all experimental evidence can be
adequately explained by using the socalled rigid enzyme model assumed by
the lock and key theory. For this reason,
a modification called the induced-fit
theory has been proposed.
The Induced Fit Model
• The induced-fit theory assumes that
the substrate plays a role in
determining the final shape of the
enzyme and that the enzyme is partially
flexible.
• This explains why certain compounds can
bind to the enzyme but do not react
because the enzyme has been distorted
too much.
The Induced Fit Model
• Other molecules may be too small to
induce the proper alignment and
therefore cannot react.
• Only the proper substrate is capable of
inducing the proper alignment of the
active site.
Induced Fit Model
Types of Enzymes
• Enzymes which function within the body
cells are called Intracellular Enzymes.
• Enzymes which function mainly outside
the cells e.g. the digestive enzymes are
known as Extracellular enzymes.
• Some enzymes can become dangerous if
they start working at the wrong time or
in the wrong place, so the body has
organised various methods of
containment.
Types of Enzymes
• Secreting the Enzyme in a harmless
form
– E.g. Pepsin is secreted as pepsinogen. It
does not become active pepsin until it is
acted on by stomach acid.
• Lysosomes
– These are small membrane-bound
structures which hold some strong enzymes
inside, they are not released until they are
needed.
Types of Enzymes
• Arranged on a Membrane
– Some enzymes work in an orderly
arrangement (as if moving on a conveyor
belt).
– E.g. the enzymes involved in photosynthesis
and respiration.
Factors Affecting Enzymes
• pH
– Enzymes have a specific ph at which they
work best. This is called the optimum pH.
– E.g. the enzymes in the mouth work at
neutral, those in the stomach work at pH 1
or 2 and those in the intestine work in
alkaline conditions.
Effect of pH
Factors Affecting Enzymes
• Temperature
– At low temperatures enzyme action is slow.
– There is a steady increase in activity as
temperature rises, until an optimum
temperature is reached.
– Above the optimum temperature, the rate
of action slows down.
– At about 45ºC to 50ºC, the protein
becomes denatured (loses its shape).
Effect of Temperature
Factors Affecting Enzymes
• Enzyme Concentration
– In general an increase in the rate of
enzyme concentration will increase the rate
of enzyme action.
Factors Affecting Enzymes
• Substrate Concentration
– In general an increase in substrate
concentration will increase the rate of
enzyme action.
Factors Affecting Enzyme
• Enzymes can be Poisoned
– E.g. by heavy metal ions such as mercury
and lead.
– Cytochrome oxidase, an enzyme in the
electron transport system of respiration, is
poisoned by cyanide.
– Cyanide kills because an affected body
cannot respire.
Factors Affecting Enzyme
• Enzymes can act as Poisons
– If they get into the wrong part of the
body.
– Several types of snake, bee, and scorpion
venom are harmful – they destroy blood
cells or other tissue of other animals.
Factors Affecting Enzyme
• Co-factors and Co-enzymes
– Some enzymes consist solely of protein,
others have 2 parts, a protein and another
chemical component called a co-factor.
– For some enzymes this co-factor is a metal.
– The organic co-factors are called coenzymes, and most are vitamins.