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Structure and Function of an Enzyme

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Structure and Function of an Enzyme
Enzymes are large proteins that speed up chemical reactions. In their
globular structure, one or more polypeptide chains twist and fold,
bringing together a small number of amino acids to form the active
site, or the location on the enzyme where the substrate binds and the
reaction takes place. Enzyme and substrate fail to bind if their
shapes do not match exactly. This ensures that the enzyme does not
participate in the wrong reaction. The enzyme itself is unaffected by
the reaction. When the products have been released, the enzyme is
ready to bind with a new substrate. As a rule, enzymes do not attack
living cells. As soon as a cell dies, however, enzymes that break down
protein rapidly digest it. The resistance of the living cell is due to
the enzyme's inability to pass through the membrane of the cell as
long as the cell lives. When the cell dies, its membrane becomes
permeable, and the enzyme can then enter the cell and destroy the
protein within it. Some cells also contain enzyme inhibitors, known as
anti-enzymes, which prevent the action of an enzyme upon a substrate.

Most enzymes work very rapidly. The turnover numbers of enzymes can be
from 100 to several million per minute. The fastest known enzyme is
catalase which turns over 6 million enzymes per minute. It is found in
tissues where it speeds up the decomposition of hydrogen peroxide into
water and oxygen. Inorganic catalases are not as efficient as organic
ones. A piece of liver in a beaker of hydrogen peroxide is a dramatic
demonstration of an organic enzyme in action. Finely divided iron
fillings also decompose hydrogen peroxide but not as effectively.

Enzymes are not destroyed in the reactions they speed up. Enzymes can
be reused but this does not mean that they can be reused without end
because enzymes are unstable and can be inactivated by heat, acid etc.
inorganic catalysts can be used indefinitely and are completely

[IMAGE]Enzymes can reverse reactions. The reaction will proceed from
left to right until equilibrium between substrate and products is


If a large amount of C is present when the reverse action occurs then
C is split up until again equilibrium is re-established. The enzyme
does not affect the equilibrium point. It just speeds up the reaction
until equilibrium is reached.

Enzymes are inactivated by excess heat. This is called denaturation.
This usually occurs above temperatures of 45 degrees upwards because
only a few cells can withstand temperatures this high.

Enzymes are sensitive to pH. Every enzyme has a temperature at which
it is most efficient at working at. Many intracellular enzymes work
best in the region of neutral. Parts of the digestive system where it
is excessively acidic or alkalinised need an enzyme that can work at
these specific conditions. For example pepsin, a protein-splitting
enzyme, only functions at around pH 2.0 acidity. This is because it is
found in the stomach where it is very acidy. Trypsin on functions an
alkaline medium of about pH 8.5 and is found in the duodenum where
conditions are alkaline.

Enzymes are specific. Generally, a given enzyme will only catalyse one
type of reaction or reaction. Most intracellular enzymes only work on
one particular substrate. Certain digestive enzymes work on a somewhat
wide range of related substrates. So, catalase will only split
hydrogen peroxide and it is ineffective on any other natural
substrate. Pancreatic lipase is less specific and will digest a
variety of different fats.


Kinetic theory states that when a substance is heated, energy is given
to the particles and they speed up. Therefore when heat is applied to
an enzyme and substrate, the particles speed up, increasing the rate
at which they bind with each other. This would suggest that the rate
of reaction should increase as the temperature is increased. This is
not quite true, as there is a limit to the temperature at which an
enzyme can work because excessive heat causes an enzyme to become
denatured and stop working. Also, there is a minimum temperature at
which an enzyme can function. Every chemical reaction requires
activation energy in order to get started. Although enzyme catalysis
greatly reduces this, some energy is still required. Because of this
the reaction is still unable to happen below a given temperature (this
varies depending on the type of enzyme and reaction, as does the
maximum temperature). If warmed to above the activation temperature,
an enzyme will work again as normal. A denatured enzyme, however, is
damaged and will not work again even if cooled below the optimum


· Water baths (4)

· Test tubes (6)

· Test tube rack

· Hydrochloric acid

· Water

· Measuring cylinder (2)

· Bowl

· Potato

· Ruler

· Knife

· Borer

· Cutting tile

· Safety goggles

· Stopper and tube

· Stopwatch timer


1. Fill test tubes with 20 ml of hydrochloric acid and put into test
tube rack.

2. Cut out five pieces of potato at equal lengths of at least 4cm
using cutting tile for safety.

3. Put a test tube in a water bath at the correct temperature, 30, 40,
50 or 60 degrees, and leave for two- three minutes so the hydrochloric
acid can adjust to the temperature of the water.

4. Fill the clean measuring cylinder with water to its brim.

5. Fill the bowl with water.

6. Put the tube end of the tube and stopper into the measuring

7. Tip measuring cylinder upside down into the bowl before all the
water has run out of measuring cylinder.

8. Add the potato to the warmed measuring cylinder and quickly cover
with the stopper.

9. Start the stopwatch and keep track of the time and how much water
is being displaced by oxygen.

10. Record results on a table.

When using apparatus there is always a risk of danger. When using the
knife there should be a lot of concentration and cautiousness. When
cutting with the knife it should be used nowhere near your fingers.
The water bath is dangerous also because it is electricity and water
together. The water bath should be used carefully so as to not spill
the water near the plug. The temperature of the water can be very hot
so it is important hands are away from the water when it is hot. The
hydrochloric acid is corrosive and an irritant and therefore should
not have contact with the skin.


The dependent variable is the volume of Oxygen. The volume of oxygen
produced depends on the temperature of the water, the volume of
hydrochloric acid, the length of the potato, and the duration of the
reaction. The independent variables are the temperature of the water,
the volume of hydrochloric acid, the length of the potato and the
duration of the reaction. These are independent because these have to
be measured and are not what the experiment was conducted to find out
about. The control variables are the factors that could affect the


Table summary

My table of results are based on two of my experiments conducted over
two days. I also have class results then an average of the class
results per four minutes. I then had to have the class per minute
because that is how results are read and examined. I found that the
class results although showing the same results generally, were a lot
lower than mine. So at 40 degrees I got an average of 26 degrees where
the class average was 20.57 degrees. I think this was because I shook
the test tube while in the water bath where as my class may not have.
This could affect the reliability of my results but as they more or
less reflected the same thing as the rest of the class I think they
are reliable.


My prediction and the results are in the region of each other but my
prediction did not fully coincide with the results. In my prediction I
said that enzymes work best at 30- 40 degrees but my results show that
it is in fact between 40-50 and then they eventually begin to denature
at 50- 60 degrees. Enzymes are proteins and therefore they usually
would become denatured at approximately 45 degrees. Some organisms
living in conditions higher than 45 degrees can either regulate their
body temperature or have heat-restricting enzymes.

My graph shows that up to about 40 degrees the rate increases
smoothly, a ten degree rise in temperature being accompanied by an
approximate doubling of the rate of the action. Above this temperature
the rate begins to fall of, and 60 degrees the reaction ceases all
together. This is where it is denatured. Most enzymes in a human body
shut down beyond certain temperatures. This can happen if body
temperature gets too low (hypothermia), or too high (hypothermia).
From my background knowledge it is evident that as temperature
increases, the rate of reaction also increases. However, the stability
of the protein also decreases due to thermal degradation. Holding the
enzyme at a high enough temperature for a long period of time may cook
the enzyme. Reaction rate is the speed at which the reaction proceeds
toward equilibrium. For an enzyme-catalysed reaction, the rate is
usually expressed in the amount of product produced per minute. The
energy barrier between reactions and products governs reaction rate.
In general, energy must be added to the reactants to overcome the
energy barrier. This added energy is termed "activation energy", and
is recovered as the reactants pass over the barrier and descend to the
energy level of the products. Enzymes can accelerate the rate of a
reaction. Catalysts accelerate the rates of reactions by lowering the
activation energy barrier between reactants and products. All chemical
reactions speed up as the temperature is raised. As the temperature
increases, more of the reacting molecules have enough kinetic energy
to undergo the reaction. Since enzymes are catalysts for chemical
reactions, enzyme reactions also tend to go faster with increasing
temperature. However, if the temperature of an enzyme catalysed
reaction is raised still further, an optimum is reached: above this
point the kinetic energy of the enzyme and water molecules is so great
that the structure of the enzyme molecules starts to be disrupted. The
positive effect of speeding up the reaction is now more than offset by
the negative effect of denaturing more and more enzyme molecules. I
did not get an anomalous result but my graph did not reflect how
enzymes work. My graph goes up steadily between 20 and 40 degrees and
gradually descends between 40 and 60 degrees. This is unusual as
enzymes normally gradually increase it's turnover number and denature


Overall my experiment was not a failure as it did show the effect on
temperature on the rate of an enzyme-controlled reaction. I conducted
my experiment as fairly as possible according to the amount of time
and equipment I had. Although I felt the test was as fair as possible
there are few aspects of the test, which I feel, could have been
conducted a lot better with the most recent equipment and a lot more
time. For example, the seconds lost between dropping the potato to the
warmed hydrochloric acid and placing the stopper over it. My results
table shows that an enzymes turn over number is greater at the
beginning of an experiment and the volume of oxygen could have been
affected by the few seconds lost. This could be make some of my
results anomalous because we may have taken longer to cover the test
tube in the water bath at 30 degrees than the test tube in the water
bath at 40 degrees. The water baths used were nTo make my experiment a
little more accurate and reliable I would increase the range at which
I worked at. On this experiment I worked between 20 degrees and 60
degrees where as I could work between 10 and 70 degrees this wider
range would help me see more clearly the collision theory and the
kinetic theory also it would be a lot clearer on a graph to see these
theories at work and also the subject of denaturation. To make it more
accurate I could work at temperatures of fives i.e. 10, 15, 20, 25
etc. the accuracy would prove very good because the temperature rise
in relation to the rate of reaction would be a lot smoother and
clearer on a graph. With more time to conduct the experiment I could
have repeated it at least three times this would give me a very
accurate average result. Although repeating the experiment twice would
help to eliminate anomalous results, repeating the experiment three
times would completely eliminate anomalous results and therefore I
would end up with very precise results.

My results were a little unreliable because they did not correspond to
the majority of my class and they also did not correspond to the
theory of proteins, enzymes in this case, being denatured at 45
degrees upwards.

I also think it would have been better if I had used the same potato
from the whole experiment but was unable to due to the time
restrictions. I had to conduct the experiment over a number of days
and could not therefore use the same potato. This is a source of error
because the concentration of catalase in the potatoes may have been
different which may have produced an inconsistent rate of reaction. To
remove this problem, I could repeat the experiment not only with three
readings at each temperature, but also with three different potatoes,
which would provide an even more accurate reading, as I could
calculate an average. It was essential for the hydrochloric acid to
acclimatise to the temperature the water bath was set at. Although all
the test tubes containing acid spent an amount of time in the water
bath to acclimatise, some were in the baths more than others and
therefore would have a temperature closer to the correct one.

I did not get any anomalous results but my graph seemed to be
different to an enzyme's nature.

How to Cite this Page

MLA Citation:
"Structure and Function of an Enzyme." 19 Apr 2014

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