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The Effect of pH on Enzyme Activity

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Length: 602 words (1.7 double-spaced pages)
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The Effect of pH on Enzyme Activity

pH is a measure of the concentration of hydrogen ions in a solution.
The higher the hydrogen ion concentration, the lower the pH. Most
enzymes function efficiently over a narrow pH range. A change in pH
above or below this range reduces the rate of enzyme reaction
considerably.

Changes in pH lead to the breaking of the ionic bonds that hold the
tertiary structure of the enzyme in place. The enzyme begins to lose
its functional shape, particularly the shape of the active site, such
that the substrate will no longer fit into it, the enzyme is said to
be denatured. Also changes in pH affect the charges on the amino acids
within the active site such that the enzyme will not be able to form
an enzyme-substrate complex.

The pH at which an enzyme catalyses a reaction at the maximum rate is
called the optimum pH. This can vary considerably from pH 2 for pepsin
to pH 9 for pancreatic lipase.



Results
=======

pH

1

2

3

Average

Rate of Reaction (cm3/s)

3

0

0

0

0

0.000

5

0

0

0

0

0.000

7

63

106

55

74.7

1.245

9

70

135

90

98.3

1.638

11

85

135

70

96.8

1.613

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Conclusion
==========

From looking at the results I can conclude that when the pH was 3 and
5 no oxygen was produced, therefore no reactions were taking place.
This was because the pH had a high hydrogen ion content, which caused
the breaking of the ionic bonds that hold the tertiary structure of
the enzyme in place. The enzyme lost its functional shape,
particularly the shape of the active site, such that the substrate no
longer fit into it, the enzyme is denatured. The ions also effected
the charges on the amino acids within the active site such that the
enzyme was unable to form an enzyme-substrate complex. Between 5 and 7
the amount of oxygen produced increased and therefore the amount of
reactions taking place was increasing as the pH rose. This was because
the amount of hydrogen ions caused greater amounts of denatured
enzymes and greater losses of enzymes being able to form an
enzyme-substrate complex. At pH 9 the optimum pH was reached as the
enzyme was catalysing the reaction at the maximum rate and greatest
amount of oxygen was produced. This was because the amount of hydrogen
ions did not affect the ionic bonds of the tertiary structure and the
charges on the amino acids within the active site. Therefore more
substrate molecules could meet and fit the active sites and
enzyme-substrate complexes could be formed and the reaction could take
place. However at pH 11 the amount of gas produced began to fall this
was because the amount of ions in the pH 11 solution caused the
breaking of the ionic bonds that hold the tertiary structure of the
enzyme in place and also effected charges on the amino acids within
the active site, meaning fewer reactions could take place. At pH 13
and 14 I predict the rate of reaction would return to zero, as the ion
content of the pH 13/14 solution would cause the enzymes to denature.

Evaluation

To enable this experiment to be completed as accurate as possible, I
repeated the procedure three times and calculated the average of all
the results, converted them into rate of reaction (cm3/s) and plotted
a graph with a line of best fit. I kept all the variables apart from
the one I was testing (pH) the same. This meant the amount of hydrogen
peroxide remained the same as well as the amount of time the celery
was reacting with the substrate and the size of the celery. I tried to
make the measurements as accurate as possible, however the measuring
cylinders went up in intervals of 1mm3 so my measurements were
accurate to the nearest mm3. There was also a delay between putting
hydrogen peroxide in with the peroxidase solution and placing the bung
on top. However I feel the results are reliable enough to make a solid
conclusion, as there are no anomalous results. To make sure the pH was
not effected I used separate equipment for each pH level instead of
rinsing with water as this would effect the pH level.

Improvements

To improve the results to make them even more accurate and reliable I
would next time cut the weigh the amount of celery to be used or
either use another source such as jarred horseradish as it can be
easily weighed out. I would also use more accurate measuring apparatus
such as a large burette.

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"The Effect of pH on Enzyme Activity." 123HelpMe.com. 20 Oct 2014
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