3.6 Enzymes
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Topic 3: Chemistry of Life
3.6 Enzymes
3.6.1 Define enzyme and active site (pg. 71, 45)
3.6.2 Explain enzyme-substrate specificity (the lock-and-key model can be used
as a basis for the explanation. Refer to the 3-D structure). (pg. 71, 45)
3.6.3 Explain the effects of temperature, pH and substrate concentration on
enzyme activity. (pg. 72-75, 45-46)
3.6.4 Define denaturation (pg. 72-75, 46)
3.6.5 Explain the use of lactase in the production of lactose-free milk (pg. 77,
46)
3.6.1 Active Site
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3.6.1 Define enzyme and active site
Orange book  pg. 71
Green book  pg. 45
To do:
In your green exercise book define the terms “enzyme” and “active site”
Enzymes are proteins. Thus, enzymes are long chains of amino acids that have
taken on a very specific three-dimensional globular shape. Enzyme act as
biological catalysts. Somewhere in the three-dimensional shape of the enzyme is
an area that is designed to match a specific molecule known as that enzyme’s
substrate. This area of the enzyme is called the active site.
3.6.2 Specificity
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3.6.2 Explain enzyme-substrate specificity (the lock-and-key model can be used
as a basis for the explanation. Refer to the 3-D structure).
Orange book  pg. 71
Green book  pg. 45
To do:
In your green exercise books, use a diagram to illustrate the ‘lock-and-key’
model for enzyme-substrate specificity.
The active site of an enzyme matched the substrate in a similar way to the way
a glove fits over a hand. In this analogy, the glove represents the active sire and
the hand represents the substrate.
Another analogy that is very commonly used for enzyme-substrate activity is a
lock and key. In this analogy, the lock represents the enzyme’s active site and
the key represents the substrate. Because the three-dimensional shape of the
internal portion of the lock is complex and specific, only one key will fit. The
same principle is generally true for enzymes and their substrates – they are
specific for each other.
As catalysts, enzymes influence the rate of reactions. As a general rule, a set
of reactants in the presence of an enzyme will form product(s) at a faster rate
than without the enzyme. Enzymes cannot force reactions to occur that would
not otherwise occur. The real role of an enzyme in a reaction is to lower the
energy level needed to start the reaction. This energy is referred to as the
activation energy of the reaction. Thus, enzymes lower the activation energy of
reactions. Enzymes are not considered reactants and are not used up in the
reaction. An enzyme can function as a catalyst many, many times.
3.6.3 Enzyme Activity
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3.6.3 Explain the effects of temperature, pH and substrate concentration on
enzyme activity.
Orange book  pg. 72-75
Green book  pg. 45-46
To do:
In your green exercise books sketch and annotate graphs to explain the effects
of temperature, pH and substrate concentration on enzyme activity.
Reactions rates of enzymes are affected by temperature, pH and substrate
concentration.
Effect of temperature
Imagine an enzyme and its substrate floating freely in a fluid environment. Both
the enzyme and the substrate are in motion and the rate of that motion is
dependent on the temperature of the fluid. Fluids with higher temperatures will
have faster-moving molecules (more kinetic energy). Reactions are dependent on
molecular collisions and, as a general rule, the faster molecules are moving the
more often they collide and with greater energy. Reactions with or without
enzymes will increase their reaction rate as temperature (and thus molecular
motion) increases. The limit is based on the temperature at which the enzyme
(as a protein) begins to lose its three-dimensional shape due to intra-molecular
bonds being stressed and broken. When an enzyme loses it shape, including the
shape of the active site, it is said to be denatured. Denaturation is sometimes
permanent and sometimes only temporary until the molecule re-forms its normal
shape.
Effect of pH
The pH of a solution is dependent on the relative number of hydrogen ions (H+)
compared to hydroxide ions (OH-) in the same solution. Any substance that gives
off hydrogen ions, for example HCl, is an acid and results in a solution with a pH
higher than 7. Pure water has a neutral pH of 7 because when water dissociates
(splits), it results in an equal number of hydrogen ions and hydroxide ions.
The active site of an enzyme typically includes many amino acids of that protein.
Some amino acids have areas that are charged either positively or negatively.
The negative and positive areas of a substrate must match the opposite charge
when the substrate is in the active site of an enzyme in order for the enzyme to
have catalytic action. When a solution has become too acidic, the relatively large
number of hydrogen ions (H+) can bond with the negative charges of the enzyme
of substrate and not allow proper charge matching between the two. A similar
scenario occurs when a solution has become too basic; the relatively large
number of hydroxide ions (OH-) can bond with the positive charges of the
substrate or enzyme and once again not allow proper charge matching between
the two. Either of these scenarios will result in an enzyme becoming less
efficient and sometimes becoming completely inactive in extreme situations.
One further possibility is that the numerous extra positive and negative
charges of acidic and basic solution can result in the enzyme losing its shape and
thus becoming denatured.
There is no one pH that is best for all enzymes. Many of the enzymes active in
the human body are most active when in an environment that is near neutral.
There are exceptions to this; for example, pepsin is an enzyme that is active in
the stomach. The environment of the stomach is highly acidic and pepsin is most
active in an acidic pH.
Effect of substrate concentration
If there is a constant amount of enzyme, as the concentration of a substrate
increases, the rate of reaction will increase as well. This is explained by the
idea of increased molecular collisions. If you have more reactant molecules,
there are more to collide. There is a limit to this however. The limit is due to
the fact that enzymes have a maximum rate at which they can work. If every
enzyme molecule is working as fast as possible, adding more substrate to the
solution will not further increase the reaction rate.
3.6.3 Enzyme Questions (52 Marks)
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1. The graphs below show the relationship between pH and the relative activity
of three different protein digesting enzymes: trypsin, pepsin and papain.
(a) Explain why changes in pH usually affect the activity of the enzymes. (3)
(b) Comment on the effect of changes in pH on the activity of trypsin, pepsin
and papain. (4)
(c) Which of these three enzymes would be most suitable to use as a meat
tenderiser? Give an explanation for your answer. (3)
2. An experiment was carried out to determine the effect of temperature on
the activity of an enzyme digesting the protein gelatin.
Gelatin was incubated with the enzyme at a range of temperatures from 5ºC to
60ºC. The rate of amino acid production was measured over a three-hour
period.
The results are shown in the table below, expressed as rate of amino acid
production in mg dm-3 h-1
(a) Comment on the effect of temperature on the activity of the enzyme. (3)
(b) The experiment was continued at 45ºC for a further 7 hours. At the end of
this time, an additional 292 mg dm-3 of amino acid had accumulated.
Calculate the mean rate of reaction during the 10 hours at 45ºC. (1)
(c) Give two possible reasons for the difference between the rate at the end of
10 hours and the rate after 3 hours incubation. (3)
3. Urease is an enzyme which catalyses the breakdown of urea to ammonia and
carbon dioxide.
An experiment was carried out into the effect of pH on the activity of urease.
10 cm3 of pH 3 buffer solution was mixed with 1 cm3 of urease solution. This
mixture was then added to 10 cm3 of urea solution and the concentration of
ammonia in the mixture was measured after 60 minutes. This procedure was
repeated using buffer solutions of pH 4, 5, 6, 7, 8 and 9.
The results are shown in the graph below.
(a) What do these results suggest is the optimum pH for urease activity? (1)
(b) Explain why no ammonia was produced at pH 3. (3)
(c) Explain why less ammonia is produced at pH 9 than at pH 8. (2)
4. Glucose oxidase is an enzyme which catalyses the oxidation of glucose from
gluconic acid and hydrogen peroxide. An experiment was carried out to
investigate the effect of pH on the activity of glucose oxidase. The activity of
this enzyme was determined at a range of pH values. The results are shown in
the graph below.
(a) State how the different pH values could be obtained in this experiment. (1)
(b) Describe the effect of changes in pH on the activity of this enzyme. (2)
(c) Explain why changes in pH affect the activity of enzymes. (3)
5. Read the following passage.
Starch is an important storage carbohydrate in most plants and is found as
insoluble granules in the cytoplasm. It is a polymer of glucose.
Corn starch is a very cheap substance and can be converted to fructose using
various enzymes obtained from microorganisms. The starch suspension is heated
to a temperature of 105°C and an enzyme called a-amylase obtained from
bacteria is added. It is a thermostable enzyme and it begins to hydrolyse the
starch. The temperature is then reduced to 90°C and hydrolysis continues for 1
- 2 hours. The bonds within the polysaccharide molecule are hydrolysed and
the long chains are broken into smaller chains called dextrins.
The next step involves the conversion of dextrins to glucose and this is carried
out by a fungal enzyme called amyloglucosidase. The substrate is adjusted to
conditions producing the maximum rate of reaction, a temperature of 60 0C and
a pH of 4.5, before the enzyme is added. Amyloglucosidase removes glucose, one
molecule at a time from the end of the
dextrin molecule.
After concentration, the resulting glucose syrup can be converted into fructose.
This process involves glucose isomerase produced by bacteria.
Use information from the passage and your own knowledge to answer the
following questions.
(a) Draw a simple flow chart showing the steps in converting starch to glucose.
Add to your flowchart the names of the enzymes that control the different
steps. (2)
(b) Explain what is meant by a thermostable enzyme. (1)
(c)The diagram shows part of a dextrin molecule.
Draw a similar diagram to show the structure of one of the molecules produced
when amyloglucosidase is added to dextrin. (2)
(d) Explain why a-amylase may be described as a polymer. (2)
(e) Describe and explain why the rate of reaction of amyloglucosidase would
vary with temperature. (3)
6. The turnover number of an enzyme is defined as the number of substrate
molecules converted to product by one molecule of enzyme in one minute. In an
experiment carried out at 20°C, the turnover number for an enzyme was found
to be 2500 at the start of the experiment but dropped to 1000 after 5
minutes.
(a) Suggest why the turnover number decreased after 5 minutes. (2)
(b) How would you expect the turnover number to differ from 2500 at the start
of an identical experiment but carried out at 30°C? Explain your answer. (2)
(c) Explain why it would be important to have a control in the experiment at
20°C and at 30°C. (1)
7. Meat is the muscle tissue of an animal. When animals are slaughtered,
biochemical changes occur in the muscle which result in the meat becoming
tough. There are several ways of making the meat tender again. These involve
partly breaking down the proteins which make up muscle tissue.
One way is to add a protein-digesting enzyme called papain just before freezing
the meat. As the meat is thawed and cooked, this enzyme digests the protein.
Use your knowledge of enzymes to explain why:
(a) the rate at which the protein is digested increases as the meat warms up in
the early stages of cooking; (3)
(b) protein digestion stops once the meat has been heated to 90 °C. (3)
(c) Another way is to leave the meat for 10 days before freezing. Explain how
lysosomes in muscle cells make the meat tender before it is frozen. (2)
3.6.4 Denaturation
3.6.4 Define denaturation
Orange book  pg. 72-75
Green book  pg. 46
To do:
Explain denaturation in your own words.
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3.6.5 Lactose Free Milk
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3.6.5 Explain the use of lactase in the production of lactose-free milk
Orange book  pg. 77
Green book  pg. 46
To do:
In no more than three bullet points summarise the use of lactase in lactose-free
milk in your green exercise books.
Almost all humans on Earth are born with the ability to digest lactose, one of
the most common sugars found in milk. The reason for this is that we are born
with the ability to produce the enzyme lactase in our digestive tract. Lactase is
the enzyme that digests the disaccharide lactose into two monosaccharides.
The monosaccharides are much more readily absorbed into the bloodstream.
Most people lose the ability to produce lactase as they get older, and by
adulthood no longer produce any significant amounts of lactase. Normal milk and
milk products enter their digestive tract and are not digested; instead bacterial
colonies in their intestines feed directly on the lactose. This leads to such
symptoms as cramping, excessive gas and diarrhoea.
There are actually more people on Earth with lactose intolerance than there are
without it!
Milk and milk products can be treated with lactase before consumption. When
this treatment occurs, the nutrients in the milk are not affected, but the
person is able to absorb the sugars as they have been pre-digested. The
technology to treat milk and milk products in this way must be improved in order
for it to be useful on a large scale. Lactose intolerance has been shown to be
extremely high incidence in some ethnic groups and be relatively low in others.
This is a good example of natural variation in a population.
HW: Extended Response (9 marks)
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Title: Discuss factors that affect enzyme activity (100 words).
You can use this space to plan your answer however it is to be hand written and
submitted for marking.