2. Enzymes
o Enzymes are specific proteins which act as biological catalysts that
speed up the rate of chemical reactions without undergoing any
change themselves
o A catalyst is a substance which speeds up the rate of reactions
without undergoing any chemical changes itself.
o The molecule on which an enzyme acts is called a substrate. The
substrate binds with the active site of the enzyme.
o Active site is the part of the enzyme to which the substrate
molecule attaches.
o The substance formed as a result of enzyme action is the product.
3. Examples of some enzyme catalyzed reactions
• Some of the reactions result in the synthesis of complex substances
from simpler once. Eg- amino acids are joined together to form
proteins.
• Some complex substances may be broken down to simple
substances for example oxidation of glucose molecule to release
energy, carbon dioxide and water.
• Some chemical reaction in cells produce poisonous hydrogen
peroxide. The cells produce enzyme catalase which catalyses the
breakdown of hydrogen peroxide to water and oxygen.
4. Characteristics/Properties of enzymes
• All enzymes are proteins.
• Enzymes are sensitive to pH. Enzymes work at its best at the
optimum pH.
• Enzymes are sensitive to temperature. To every 10o rise in
temperature the enzyme activity doubles. As the temperature
increases, the kinetic energy of enzyme and substrate molecules
also increases so that more effective collisions can be made. At very
high temperature the enzymes are denatured and they lose their
shape and active site.
5. Characteristics/Properties of enzymes
• Enzymes are specific in their action. Since each enzyme has
particular shape in its active site, it can act only on substrates
that have a complementary shape.
• Enzymes do not undergo any permanent change in the reaction
they catalyze. So they can be used over and over again. A small
amount of enzymes are needed to convert a large amount of
substrate into products.
6. The Lock and Key hypothesis of enzyme action
• Each enzyme is a protein molecule with a specific shape.
• The section of the enzyme amylase (‘lock’) where its substrate starch
molecule (‘key’) fits exactly is the active site.
• When the complementary substrate binds with the enzyme, it acts
on the substrate forming the product maltose.
• The product molecule(s) then drifts away from the enzyme, leaving
the active site free to operate again
• This ensures that the enzymes are used again and again without
being altered or used up in the reaction. So a small amount (minute
quantity) of enzymes can produce a rapid change in the rate of
chemical reactions.
7. The current Lock and Key hypothesis
Currently, biologist think that when a substrate molecule fits into an
enzyme molecule, the enzyme molecule alters its shape slightly so
that it fits more tightly around the substrate molecule. This
facilitates chemical reaction.
9. Effect of Temperature on enzyme activity
• In general, enzyme activity is speed up by an increase in temperature.
• A rise in 10o C doubles the speed of the reaction until the temperature
reaches the optimum.
• The temperature at which enzymes work best is the optimum temperature.
• For each enzyme there is an optimum temperature at which it acts fastest.
• Most enzymes work best between 30o C and 37o C.
• For most of the enzymes found in a mammal’s body (eg: human) the optimum
temperature is at around 37o C.
• Human body temperature is 37o C.
• Collisions between all molecules increase as temperature increases due to the
increase in kinetic energy.
10. Effect of Temperature on enzyme activity
• As the temperature rises above the optimum,
the rate of activity begins to slow down &
falls to zero (enzyme activity is stopped) as
enzymes become progressively denatured at
temperatures above optimum.
• At very low temperatures, approximately
below 5oC, enzymes are inactivated & enzyme
activity ceases.
• When enzymes are denatured, they cannot
be reversed back but inactivated enzymes
become activated when exposed to suitable
temperatures. Most of the enzymes
denatured above 500C
11. Effect of pH on enzyme activity
• Enzymes are influenced by the acidity and alkalinity (pH) of the medium in
which they function.
• The pH at which the enzyme work best is its optimum pH
• If the pH level falls on either side of the optimum, the rate of enzyme activity
gradually decreases.
• Pepsin which digests protein in the stomach works most efficiently in an acidic
medium, approximately pH 2.
• For most enzymes found in a mammals body, the optimum pH is 7 or slightly
above (alkaline)
• Changes in pH also alter an enzyme’s shape and slow down its activity, but this
can usually be reversed if the optimum pH is restored.
13. Effects of substrate and enzyme concentration on
reaction rate
• Enzyme concentration: Increasing enzyme concentration will speed up the
reaction, as long as there is substrate available to bind to. Once all of the
substrate is bound, the reaction will no longer speed up, since there will be
nothing for additional enzymes to bind to.
• Substrate concentration: Increasing substrate concentration also increases
the rate of reaction to a certain point. Once all of the enzymes have bound,
any substrate increase will have no effect on the rate of reaction, as the
available enzymes will be saturated and working at their maximum rate.
14. Effect of Temperature on the Enzyme Amylase
• Amylase digests starch into maltose
• Starch can be tested for easily using iodine solution
• Starch solution is heated to a set temperature
• Iodine is added to wells of a spotting tile
• Amylase is added to the starch solution and mixed well
• Every minute, droplets of solution are added to a new well of iodine solution
• This is continued until the iodine stops turning blue black (this means there
is no more starch left in the solution as the amylase has broken it all down)
• Time taken for the reaction to be completed is recorded
• Experiment is repeated at different temperatures
• The quicker the reaction is completed, the faster the enzyme is working
16. Effect of pH on the Enzyme Amylase
• Place single drops of iodine solution in rows on the tile
• Label a test tube with the pH to be tested
• Use the syringe to place 2cm3 of amylase in the test tube
• Add 1cm3 of buffer solution to the test tube using a syringe
• Use another test tube to add 2cm3 of starch solution to the amylase and buffer
solution, start the stopwatch whilst mixing using a pipette
• After 10 seconds, use a pipette to place one drop of mixture on the first drop of
iodine, which should turn blue black
• Wait another 10 seconds and place another drop of mixture on the second drop of
iodine
• Repeat every 10 seconds until iodine solution remains orange brown
• Repeat experiment at different pH values – the less time the iodine solution takes to
remain orange brown, the quicker all the starch has been digested and so the better
the enzyme works at that pH
