Enzyme Activity
The properties of enzymes related to their
tertiary structure.The effects of change in
temperature,pH,substrate concentration,and
competitive and non-competitive inhibition on
the rate of enzyme action
HOW ENZYMES WORK
• Enzymes are ORGANIC
CATALYSTS. A CATALYST is
anything that speeds up a
chemical reaction that is
occurring slowly.
How does a catalyst work?
• The explanation of what
happens lies in the fact that
most chemical reactions that
RELEASE ENERGY
(exothermic reactions) require
an INPUT of some energy to
get them going. The initial
input of energy is called the
ACTIVATION ENERGY
Enzymes
•
•
•
•
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An enzyme is a biological
catalyst
The pockets formed by tertiary
and quaternary structure can
hold specific substances
(SUBSTRATES).
These pockets are called
ACTIVE SITES.
When all the proper substrates
are nestled in a particular
enzyme's active sites, the
enzyme can cause them to
react quickly
Once the reaction is complete,
the enzyme releases the
finished products and goes back
to work on more substrate.
Properties of Enzymes relating to
their tertiary structure.
• The activity of enzymes is strongly
affected by changes in pH and
temperature. Each enzyme works best
at a certain pH and temperature,its
activity decreasing at values above
and below that point. This is because
of the importance of tertiary
structure (i.e. shape) in enzyme
function and forces, e.g., ionic
interactions and hydrogen bonds, in
determining that shape.
The effects of change in
temperature.
Temperature: enzymes work best at an optimum
temperature.
Below this, an increase in temperature provides more
kinetic energy to the molecules involved. The numbers of
collisions between enzyme and substrate will increase so
the rate will too.
Above the optimum temperature, and the enzymes are
denatured. Bonds holding the structure together will be
broken and the active site loses its shape and will no
longer work
The effect of change in pH.
• pH: as with temperature, enzymes have an
optimum pH. If the pH changes much from the
optimum, the chemical nature of the amino acids
can change.
This may result in a change in the bonds and so the
tertiary structure may break down. The active site
will be disrupted and the enzyme will be
denatured.
The effect of change in
concentration
•
Enzyme concentration: at low enzyme concentration there is
great competition for the active sites and the rate of reaction
is low. As the enzyme concentration increases, there are more
active sites and the reaction can proceed at a faster rate.
Eventually, increasing the enzyme concentration beyond a
certain point has no effect because the substrate concentration
becomes the limiting factor.
•
Substrate concentration: at a low substrate concentration there
are many active sites that are not occupied. This means that
the reaction rate is low.
When more substrate molecules are added, more enzymesubstrate complexes can be formed. As there are more active
sites, and the rate of reaction increases.
Eventually, increasing the substrate concentration yet further
will have no effect. The active sites will be saturated so no
more enzyme-substrate complexes can be formed.
Competitive and non-competitive inhibition
•
Inhibitors slow down the rate of a reaction. Sometimes this is a
necessary way of making sure that the reaction does not
proceed too fast, at other times, it is undesirable
• Reversible inhibitors:
• Inhibitors
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Competitive reversible inhibitors: these molecules have a similar
structure to the actual substrate and so will bind temporarily
with the active site. The rate of reaction will be closer to the
maximum when there is more ‘real’ substrate, (e.g. arabinose
competes with glucose for the active sites on glucose oxidase
enzyme).
Non-competitive reversible inhibitors: these molecules are not
necessarily anything like the substrate in shape. They bind with
the enzyme, but not at the active site. This binding does
change the shape of the enzyme though, so the reaction rate
decreases.
Irreversible inhibitors:
These molecules bind permanently with
the enzyme molecule and so effectively
reduce the enzyme concentration, thus
limiting the rate of reaction, for
example, cyanide irreversibly inhibits
the enzyme cytochrome oxidase found
in the electron transport chain used in
respiration. If this cannot be used,
death will occur
Enzymes are the tools of life.