5. Effect of temperature - What is the optimum temperature
for an enzyme catalysed reaction?
Enzymes perform best in their 'optimum' ambient conditions
investigating the effect of temperature on enzyme activity, three
factors must be kept constant, (i) the pH and the concentrations of both
the (ii) substrate and (iii) enzyme must all be kept constant (see
General description of graphs: For any enzyme,
initially, as you increase the temperature, the rate of reaction
Then as the temperature increases, the reaction rate
reaches a maximum at the optimum temperature and then decreases
with further increase in temperature.
The optimum temperature varies from one enzyme to
another, but for many warm blooded animals like us, it is often close to
37oC - but beware of enzymes in extremophile organisms in hot
springs, volcanic vents and 10 km down at the bottom of an ocean,
Temperature: The structure of the protein enzyme can
depend the temperature. If the enzyme does not have the correct 'lock'
structure, it cannot function efficiently. The shape of the graph is
due to two factors.
A too high a temperature can affect the shape of the
active site or the ability of the substrate molecule to 'dock in' to the
active site - some kind of change is promoted by a high temperature - a
denaturing effect due to interfering with the bonds holding the enzyme
together in its unique 3D shape - the active site is damaged and the
substrate molecule can't fit in.
Explaining the left graph of enzyme reaction
rate versus temperature
(a) The initial rise in rate of reaction is what you
normally expect for any chemical reaction
The increase in temperature
increases the average kinetic energy (KE) of the molecules to increase the chance of
the product forming from the higher KE substrate-enzyme effective 'fruitful' collisions.
words, at a higher temperature, more molecules have sufficient energy to overcome the activation
energy to break bonds and change from reactants to products.
The minimum energy needed for the reaction change is called the
activation energy, and is much lower for the enzyme catalysed
reaction, than the uncatalysed reaction (the green and black
profiles in the diagram above), its the size of the hump compared to
the reactant potential energy baseline that is important).
There is also an increase in the rate of
collision of the substrate and enzyme molecules.
For more details see the GCSE chemistry
changing the temperature of reactants
(b) At higher temperatures the rate goes through a
maximum and then slows down!
However as the
temperature rises further, the increasing thermal vibration of the enzyme
molecule causes its structure to break down (denature) and so the 'lock'
is damaged so the enzyme is less efficient in interacting with the
substrate molecule (see key-lock below). It means the substrate molecule
cannot properly dock into the active site on the enzyme.
be due to the failure of weak intermolecular bonding forces or actual
ionic or covalent bonds being broken, but the 3D molecular structure of the
enzyme is changed so that the substrate molecule cannot 'dock into' the
active site on the enzyme to be
changed into products.
to the enzyme's active site at higher temperatures - the denaturing of the enzyme, is
NOT reversible. The enzyme will not go back to its normal shape even if the
reaction mixture is cooled down.
The above diagram shows the effect of high temperatures on an enzyme
molecule - the crucial and effective 3D shape is destroyed when bonds in
the protein molecule are disrupted.
The optimum temperature for
the fastest rate of reaction is often around 30-40oC (note our
body temperature is about 37oC, no coincidence!). Eventually at high
temperatures the enzyme completely ceases to function.
Note: NOT every enzyme reaction has
an optimum of ~30-40oC!, some organisms exist and survive at
very low temperatures and some at very high temperatures - e.g. extremophile
organisms that live in very hot water near volcanic vents deep in the
ocean on the sea bed.
Adaptations, lots of examples explained including extremophiles
By combining points made in (a)
and (b) we can completely explain the shape of the graph.
The actual graph that you obtain
from experiments is effectively the result
of adding two trends together,
(a) The increase in rate due to increase
in temperature - 'normal chemical behaviour,
and (b), the decrease in rate as denaturing of the enzyme
increases with increase in temperature.
The resulting graph then has two
minimums at the lower and higher temperatures, and one maximum - the hump in the graph is the point of maximum
speed of the reaction, 'highlighting' the optimum most effective
So, the first graph diagram is typical
for temperature controlled enzyme activity.
The 2nd temperature graph shows what happens to the speed of the
enzyme catalysed process of photosynthesis as the temperature is
As the temperature increase the
rate of catalysis increases (normal effect on the speed of reaction as the
average kinetic energy of the molecules increases), but at high temperatures the protein structure
of the enzyme is destroyed, so the active site on the enzyme is damaged.
More extreme condition: Some
enzymes in bacteria found in:
(i) hot springs have an
optimum temperature of over 80oC - at this temperature most our
enzymes would be denatures,
(ii) bacteria living in cold
deep ocean water have optimum temperatures as low as 0oC - at
this temperature, most of our enzymes would only function very
'decay' investigation using milk and lipase gcse
biology revision notes