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3.6 Enzymes
3.6.1 Define enzyme and active site
ENZYMES speed up reaction rates by lowering activation energy.
Control of metabolic pathways is very important
Not all reactions can be or should be done at once.
Activation energy- energy required to initially break the bonds prior to making new
bonds. see p97 graphs.
Active site- area on the enzyme where the reactants connect to the enzyme.
Cofactors/ Coenzymes- also bind to or near the active site. They contribute to the
enzymes function. Many vitamins are coenzymes, or coenzyme precursors.
Option C
C 2.1 State that metabolic pathways consist of chains and cycles of enzyme catalysed
reactions.
3.6.2 Explain enzyme substrate specificity
[The lock and key model can be used as a basis for the explanation. Refer to the
three dimensional structure. The induced fit model is not expected at SL
Every chemical reaction has a specific enzyme that facilitates it. The active sites
on the enzymes are very specific, an sensitive to changes in the shape of the enzyme. If
you change the shape of the enzyme, the reaction may not work.
Option C
C 2.2 Describe the induced- fit model
This is an extension of the lock and key model. Its importance in accounting for
the ability of some enxymes to bind to several substrates should be mentioned.
Use the term: conformational change to describe the change of shape of a protein.
3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme
activity.
[Enzyme activity could be measured using data loggers such as pressure sensors,
pH sensors, or coloromiters.
The effects of environmental acid rain could be discussed.]
Temperature and pH can change the shape of the enzyme and de-activate it.
The graph of temperature increase and enzyme activity increases until the enzyme is
denatured, then the enzyme activity stops.
The graph of pH changes and enzyme activity is a bell curve. All enzymes have an
optimum value of pH that they perform best at. As you decrease or increase the pH away
from the optimal pH, the enzyme activity drops off.
The graph of increasing substrate concentration looks much like the increasing
temperature graph until the enzyme is fully saturated and working at full capacity. At that
point, enzyme activity levels off and adding more substrate will not increase activity.
Option C
C2.3 Explain that enxymes lower the activation energy of the chemical reactions that they
catalyse.
Only exothermic reactions should be considered. Specific energy values do not
need to be recalled.
3.6.4 Define denaturation
Denaturation refers to the changing of the shape of the enzyme. This can be done
by temperataure changes, pH changes, and various control molecule schemes.
( see non-competitive inhibitors, co-factors, co-enzymes and allosteric regulation above)
3.6.5 Explain the use of lactase in the production of lactose-free milk
Some people lack the ability to digest milk because their bodies stop making
lactase after they are weaned. Certain peoples, like Asians, are more likely to be lactose
intolerant than Europeans. Industrially, lactose is added to milk to convert the
disaccharide into galactose and glucose. The lactose is made via genetic engineering and
then added to milk.
C 2.4 Explain the difference between competitive and non competitive inhibition, with
reference to one example of each.
Competitive inhibition is the situation where an inhibiting molecule that is
structurally similar to the substrate molecule binds to the active site, preventing further
substrate binding.
Limit non-competitive inhibition to an inhibitor binding to an enzyme (
not to its active site) that causes a conformational change in its active site, resulting in
decreased activity.
Competitive inhibitors-mimic the substrate shape and block the active site. Can be
overcome by saturating with the substrate. Antibiotics do this to bacteria enzymes
Noncompetitive inhibitors-cause the enzyme to change shape by binding somewhere
else other than the active site.
C 2.5 Explain the control of metabolic pathways by end-product inhibition, including the
role of allosteric sites.
allosteric regulation- regulatory molecules bind on the enzyme and change its shape
either activating it, or inactivating it.
An allosteric enzyme is made of more than one polypeptide chain. There are
two non-competitive sites. One non-competitive site activates the enzyme, the other
non-competitive site inactivates the enzyme. The function of the enzyme then
controlled by the relative amounts of each of the regulatory molecules. Which ever
molecule is in the greatest amount controls the enzyme.
Many pathways have the products of the pathway act as regulatory
molecules for enzymes at the beginning of a pathway. Feedback inhibition- the end
product of a metabolic pathway acts as an allosteric inhibitor preventing more from
being made.