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QUESTION SHEET
4 Enzymes
In living cells, most chemical reactions require an input of energy before the molecules will
react together. This energy is known as the activation energy. Enzymes are biological
catalysts, which increase the rate of chemical reactions by reducing activation energy.
Enzymes as globular proteins
Enzymes are globular protein molecules with a well-defined tertiary structure. They have a
specific region, known as the active site, which is complementary to the shape of the
substrate molecule and into which the substrate molecule fits. The rest of the enzyme
molecule is involved in maintaining the shape of the active site. The shape of the active
site is specific to one substrate and a temporary enzyme–substrate complex is formed by
a ‘lock-and-key’ mechanism. In some enzymes, the shape of the active site changes when
the substrate molecule becomes attached. This process is referred to as ‘induced fit’.
When the products have been formed, they leave the active site, which is then left free to
combine with a new substrate molecule.
Enzymes are not used up in the reactions they catalyse; they can be used again and
again.
Factors affecting enzyme activity
Changes in their environment can affect enzyme activity. Changes in temperature and pH
affect the shape of the enzyme molecule and thus affect the shape of the active site.
At low temperatures, a rise in temperature will increase the rate of an enzyme-controlled
reaction. There will be an increase in the kinetic energy of the enzyme and substrate
molecules, so that there are more collisions and more enzyme–substrate complexes are
formed. The optimum temperature for a particular enzyme is the temperature at which it
functions most rapidly. This temperature varies: some enzymes can function efficiently at
temperatures up to 95 °C, whereas those in the human body function best at around 37
°C.
Above the optimum temperature, an increase affects the shape of the enzyme molecule.
The active site becomes distorted and enzyme–substrate complexes are unable to form.
The enzyme loses its catalytic properties and is said to be denatured.
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Most enzymes have an optimum pH at which they function most efficiently. Any changes
in pH affect the ionisation of the side groups of amino acid residues, which affect the
overall shape of the molecule. The shape of the active site is disrupted and the formation
of enzyme–substrate complexes is affected. At extremes of pH the enzyme may become
denatured. Intracellular enzymes function best at a pH of 7, but enzymes present in the
stomach have optima of around 1.5 to 3.5.
The rate of an enzyme-controlled reaction is affected by the concentrations of the
reactants. As the concentration of the enzyme increases, more active sites become
available and, provided that there is excess of the substrate, the rate of the reaction will
increase.
If the substrate concentration increases, the rate of reaction will increase up to the point
where all the enzyme molecules are being used. If more substrate is added, the rate will
remain constant as the active sites are all occupied and the reaction is at its maximum
velocity.
Inhibition of enzyme activity
Inhibitors are substances which reduce the activity of enzymes. They can affect the active
site directly (active site-directed) or affect another part of the molecule which will then alter
the shape of the active site (non-active site-directed).
Some inhibitors bind to the active site of the enzyme, preventing the formation of the
normal enzyme–substrate complex. These inhibitors often have similar shapes to the true
substrate molecules and compete for the active site. The presence of such inhibitors
reduces the rate of the reaction, but high substrate concentrations reduce the effect of the
inhibitor.
Some inhibitors become attached to parts of the enzyme molecule other than the active
site, causing the molecule to lose its catalytic properties.
Commercial uses of enzymes
Enzymes are used in industrial processes and the development of enzyme technology has
resulted in the large-scale production of enzymes from microorganisms. These microbial
enzymes are used in the production of paper, textiles, food and biological detergents.
Pectinases are obtained commercially from fungi. They are enzymes which break down
pectins, present in plant cell walls, to galacturonic acid, which can be broken down further
to sugars and other compounds. Pectinases are added to crushed fruit, such as apples
and grapes, to increase the yield of juice which can be extracted. In addition, they improve
the colour derived from the fruit skins. They are also used to clarify wines, vinegar and
fruit juices, by removing suspended pectic material.
Proteases catalyse the hydrolysis of peptide bonds in proteins and peptides. They act by
removing amino acid residues from the ends of the polypeptide chains or hydrolysing
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bonds within the chain. They have a large number of commercial uses, including
tenderisation of meat and modification of the proteins in flour used for breadmaking.
Bacterial proteases are incorporated into biological detergents where they contribute to
the breaking down of protein stains when washing clothing.
Many commercial enzymes are immobilised when used. This involves attaching the
enzyme to inert, insoluble materials, such as agar gels, cellulose or polyacrylamide.
Immobilisation has a number of advantages over using enzymes in free solution. The
advantages include purity of the product, reduction of cost, ability to re-use the enzyme
and stability. Some enzymes are more stable when immobilised and less likely to become
denatured.
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QUESTION SHEET
4 Enzymes
1. Explain what is meant by the term ‘catalyst’.
[1]
2. Describe the structure of an enzyme molecule.
[4]
3. Describe the ‘lock-and-key’ mechanism of enzyme action.
[3]
4. How does the ‘induced fit’ model differ from the ‘lock-and-key’ mechanism?
[2]
5. Explain how enzymes speed up the rate of chemical reactions in living organisms.
[3]
6. Describe the effect of temperature on enzyme action.
[5]
7. Explain why pH affects enzyme activity.
[4]
8. By means of graphs, explain the effect of:
(i) increasing the enzyme concentration and
(ii) increasing the substrate concentration.
[7]
9. Give an example of active site-directed inhibition of enzyme action and explain how the
rate of the reaction is affected.
[6]
10. Explain why non-active site-directed inhibition slows the speed of an enzymecontrolled reaction.
[2]
11. List the advantages of using immobilised enzymes in industrial processes.
[3]
Total marks [40]
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