|
Enzymes
Joanna
George-Johnson, Contributor
Enzymes are one of the most important
topics at the CSEC level. A firm grasp
on the topic will allow you to understand
other areas of biology, as well as
to better handle some of those short
answer and essay questions. The reason
for this is the important function
that enzymes play in all living organisms.
Please pay keen attention and aim
at mastering all the concepts associated
with enzymes.
What
are enzymes?
Picture
a situation where a young lady has
washed her hair at the hairdresser
and now wants it to be dried. Without
a hairdryer, the process of drying
the lady's hair would be extremely
long and possibly lead to getting
a very bad hair-do! In an organism,
reactions taking place need to occur
at an efficient rate in order to sustain
life, therefore, the enzyme acts as
a catalyst to speed up the rate of
the reaction. So just as the hair
dryer acts to speed up the process
of hairdrying, so do enzymes speed
up metabolic reactions in organisms.
By
definition, we can say that an enzyme
is a biological molecule that speeds
up the rate of chemical reactions
in a living organism.
Such
reactions occurring in an organism's
cells are known as metabolism. Generally,
enzymes are found in cells, however,
there are cases in which they are
released outside the cells and their
effects are felt outside. An enzyme
is actually a type of protein and
therefore, has all the properties
of proteins. So, just like proteins,
they are a three-dimensional structure
of amino acids. Enzymes can be described
as globular proteins because their
molecules are usually round in shape.
Many
people have the misconception that
enzymes just take part in the process
of digestion. This is totally wrong!
In fact, enzymes take part in just
about any body process you can imagine.
Some
of these processes include: respiration,
photosynthesis, DNA replication and
many more.
If
you can appreciate that, since enzymes
are important to DNA processes - the
molecule which determines how all
the cells in the body work, enzymes
are very important to the sustenance
of life.
Another
misconception persons have about enzymes
is that they tend to think they participate
in the reaction that drive biological
processes, but they do not! Just like
the lady's hair - it will dry eventually,
however, this would not be as efficient.
So it is with enzymes, they do not
get used up in the reaction, they
just allow it to take place faster!
At the end of the reaction, they are
still available to catalyse more reactions,
just as the hairdryer can be used
to speed up the drying of the hair
of other customers.
The
following table classifies some enzymes
and states their main functions.
|
Type
|
Enzyme
|
Functions
|
| Digestive |
Amylase |
Converts
starch into maltose |
| Pepsin |
Break
bonds between amino acids in proteins,
producing, shot chain polypeptides |
| Trypsin |
Breaks
down proteins into peptides |
| Lipase |
Breaks
down triglycerides into fatty
acids and alcohol |
| Metabolic
enzymes |
Catalase |
Breaks
down Hydrogen Peroxide to liberate
oxygen. |
Can
you name any more enzymes and their
function?
How
do enzymes work?
An
enzyme works on a substance called
a substrate. A substrate is a reacting
molecule that binds to the enzyme.
An enzyme combines with a substrate
forming an enzyme-substrate complex.
In the breakdown of starch in the
cells to maltose, the substrate would
be starch. The enzyme that it binds
to is known as amylase. There is a
particular area on the enzyme that
this substrate binds to, called the
active site. At the active site, the
enzymes bind to the reactants and
in doing so, they increase the amount
of energy needed to start the reaction;
this speeds up the reaction.
It
is important to note that enzymes
do not affect the products formed
during a reaction. Therefore, the
enzymes are unchanged, and only the
substrates are changed during a reaction.
The diagram below shows how an enzyme
controlled reaction proceeds.
More
properties of enzymes
1)
Enzymes are specific: This property
is essential in understanding the
work of enzymes. Enzymes are usually
responsible for catalysing one type
of a reaction. This means that the
enzyme maltase, which breaks down
maltose into glucose, cannot break
down sucrose. This can be explained
in terms of a 'lock and key' structure.
Have you ever tried to open a lock
with the wrong key? It doesn't open!
The lock would behave like an enzyme
which the key (the substrate) is trying
to open. If the substrate (key) doesn't
'fit', the lock won't open at all!
Therefore, the active site of the
enzyme must fit with the substrate
binding to it in order for the enzyme
to work.
2)
Enzymes work best at a particular
pH. The pH the enzyme works in is
particularly important - pH refers
to how acidic or basic a solution
is. The enzyme reaction will not take
place efficiently if the pH is greater
or lesser than the pH required for
the enzyme reaction to occur. Different
enzymes work best at different pH.
For example, amylase works at a pH
of eight and pepsin works at a pH
of about two. The pH at which the
enzyme takes place most efficiently
is known as the optimum pH. The graph
below shows how pH affects enzyme
actions.
3)
Enzymes work at a particular temperature.
Enzymes work best at an optimum temperature.
Body temperature is the optimum temperature
at which most enzymes work.
An
increase in the temperature increases
the kinetic energy associated with
the movement of the enzymes and substrates.
Such an increase would increase the
chances of the enzymes colliding,
thus increasing the chance of reactions
to occur. Note that an increase in
temperature of about 10 degrees Celsius
doubles the rate of the reaction.
There comes a point, however, where
enzyme activity reduces because the
temperature is too high. The enzymes
are denatured at high temperatures.
Denatured means that their active
sites have been destroyed, thus, the
enzymes become inactive because the
substrate can no longer fit into the
active site of the enzyme. In low
temperatures, enzyme-controlled reactions
occur very slowly. This is due to
the low probability of collisions
occurring between substrate and enzymes.
Can you think of why this may be so?
Denaturation of enzymes is irreversible,
however, this is not the case for
enzymes working at low temperatures.
The graph below demonstrates how temperate
influences the rate of a particular
enzyme-controlled reaction.
 |
Questions:
1.
Explain why there was very little
reaction occurring after a student
placed a small piece of fat into an
aqueous solution of lipase for 24
hours in a beaker that was left in
the refrigerator.
2.
What is the main function of enzymes?
3.
State three reasons enzymes are important.
4.
With the aid of diagrams, describe
the 'lock and key' hypothesis.
5.
Why is an enzyme described as being
globular?
6.
explain the following terms:
a)
Active Site
b)
Activation energy
c)
Catalyst
d)
Denaturation
e)
Optimum temperature
f)
Optimum pH
Joanna
George-Johnson teaches at Ardenne
High School.
|
