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Part A: Biological Catalysts
The biochemical reactions that occur in living things must occur
at certain speeds, or rates, in order for them to be useful. The rate
of a chemical reaction depends on several factors, such as
temperature, concentration of the chemicals, and surface area. If
all these factors are held constant, adding a catalyst can speed up
a chemical reaction. A catalyst is a substance that speeds up the
rate of a chemical reaction without itself being changed by the
reaction.
Most catalysts for reactions are enzymes, a kind of protein. A
enzyme is considered a biological catalyst because they speed up
the chemical reactions in living things. Without enzymes, our
guts would take weeks and weeks to digest our food, our muscles,
nerves and bones would not work properly and so on - we would
not be living!
For a chemical reaction to occur, reactants must come together
with enough energy to break existing bonds and form new ones.
Enzymes speed up reactions by reducing the amount of energy
substrates need to react. In other words, they reduce the
activation energy of the reaction. The activation energy of a
reaction is the minimum amount of energy needed to start a
reaction.
Part B: Lock and Key Model of Enzymes
Enzymes give reactants a site where they can come together to
react. The reactants that are affected by enzymes are known as
the substrates. The surface of an enzyme has a distinct shape
that allows it to interact only with certain substrates. A single
organism may have thousands of different enzymes and each is
specific to one chemical reaction.
Substrates bind to a part of the enzyme called the active site.
The shapes of the substrates and the shape of the active site are
complimentary or opposite (see diagram below). As a result, the
enzyme and substrates fit so precisely together they are
sometimes compared to a lock and key.
The enzyme and the substrates stay bound together until the
reaction is complete. When the reaction finishes, the products are
released. The enzyme, which is unchanged, can then repeat the
process. It is not used up during the process.
Enzyme – substrate complex
Part C: Factors Affecting Enzymes
Enzyme activity can be affected by any factors that influence
chemical reactions. One important factor is temperature.
Enzymes from different organisms tend to work best at different
temperatures. The optimum temperature in humans, for
example, is about 37 ⃘ C, which is normal body temperature.
Enzymes are generally destroyed at temperatures above 50 ⃘C.
Another factor that affects reaction rates in pH. The pH of a
solution describes how acidic or basic the solution is. Most
enzymes function in narrow pH ranges, which vary depending on
the enzyme and its job. For most reactions, the optimum pH is
close to 7, which it neutral. Low (acidic) or high (basic) pH values
tend to inhibit enzyme activity.
The concentration of enzyme and substrate molecules also affects
enzyme activity. Consider a reaction for which the number of
enzyme molecules is fixed. If there are more substrate molecules
than enzyme molecules, the reaction rate will rise until all the
enzyme molecules are attached to substrates. The reaction rate
will then level off.
Part D: Denaturation of Enzymes
Enzymes must have the correct shape to do their job. They are
made of proteins, and proteins are very easily affected by heat, pH
and heavy metal ions. Some people say that enzymes work like a
key in a lock. If the key has been twisted by heat, or dissolved in
acid or stuck up with chewing gum it will not work. Enzymes
change their shape if the temperature or pH changes, so they have
to have the right conditions. Copper ions are poisonous: if you get
copper ions in your blood they will block up some of the
important enzymes in red and white blood cells.
Changes in temperature and pH can change the shape of the
enzyme. At slightly higher temperatures, the hydrogen bonds in
an enzyme may begin to break apart. The enzyme’s structure
changes and it loses its ability to function. This process is called
denaturation. This is one reason why a very high fever is so
dangerous to a person.
Part E; Important Enzyme Reactions
Enzymes are critical to proper cell function. The cells in your
body contain thousands of enzymes that catalyze different
chemical reactions. For example, your nerve cells have enzymes
that produce the chemicals that carry impulses from nerves to
muscles. An inability to make some enzymes can lead to disease
or death. Tay-Sachs is a genetic disorder that affects the human
brain. In this disease, the cells do not produce an enzyme that
helps break down lipids. Over time, the buildup of lipids in and
around the brain cells causes blindness, seizures and usually
death.
Human saliva contains an enzyme called amylase. This enzyme
helps to turn starch into a sugar called maltose. When you
swallow a mouthful of food, the amylase stops working because it
is much too acid in the stomach pH 2. Amyalse works best in
neutral or slightly alkaline conditions, i.e. at about pH 7. When
your food gets into the small intestine, more amylase is made by
the pancreas and this turns the remaining starch into maltose.
Another enzyme (maltase) turns all this maltose into glucose.
Glucose is then absorbed into the blood.
Carbon dioxide in your blood combines with water to form
carbonic acid. This reaction is so slow that if it were not catalyzed,
carbon dioxide might build up in the bloodstream, which could be
fatal. However, the blood contains carbonic anhydrase. This
enzyme makes the reaction about one million times faster than it
would be without the enzyme. As a result, carbon dioxide, which
is toxic in high concentrations, is quickly removed from the blood.