Section 3
Lesson 2– The Catalytic Cycle
What do enzymes do?
Enzymes lower the activation energy Ea required
for a reaction to occur.
Catalytic Cycle of Sucrase
Enzyme
Substrate
Complex
Lock and Key Model
On your desks use an example to explain the
lock and key model of enzyme activity.
Why is this model misleading?
Enzymes are not the rigid structures that the
diagrams make them out to be. We already
know that they can change their configuration
in response to changes in their environmental
conditions. They also change in response to
interactions with other molecules.
Induced Fit - Hexokinase
What do kinases do?
Kinases catalyse the transfer of a phosphate group onto
another molecule. Hexokinase catalyses the transfer of
a phosphate from ATP onto glucose.
Catalytic Cycle of Hexokinase
Copy Fig 3.1.4 from the monograph
into your notes.
Annotate your diagram using the
notes found under the diagram in the
monograph.
Control of Enzyme Activity
Cells have a number of ways to control the activity of their enzymes.
Without control the reaction of the cell would be disordered.
Cells can control enzyme activity by:
1. Compartmentalisation – enzymes remain in compartments in the cell
such as the mitochondria, chloroplast, Golgi apparatus.
2. pH – the pH affects the activity of enzymes so controlling the pH will
change which enzymes are active.
3. Temperature – the temperature affects the activity of enzymes so
controlling the temperature will change which enzymes are active.
4. Cofactors – some enzymes require another non-protein molecule to
bind to them so that they will work.
5. Altering the shape of the enzyme – this can happen through
inhibitors, allosteric effects, covalent modification and end-product
inhibition.
Inhibitors
There are 2 types of inhibitors – competitive and noncompetitive.
Competitive Inhibitors
These are molecules that are similar to the substrate
and so are able to compete with the substrate for the
active site. If they get there first the enzyme will be
unable to catalyse the reaction it is intended for.
Competitive Inhibition in Action
Can you think of an
example of competitive
inhibition in the nitrogen
cycle?
O2 is similar enough to
N2 to act as a
competitive inhibitor.
How is this problem
overcome?
Non-competitive Inhibitors
These are able to bind to the enzyme in a place other
than the active site. By binding they change the active
site which reduces the efficiency of the enzyme to
catalyse the intended reaction.
Allosteric Enzymes
An allosteric enzyme is a type of enzyme that
changes form when a regulating molecule binds
to it. The regulating molecule is often called a
modulator or effector. (Non-competitive
inhibition is an example of an allosteric
interaction)
There are 2 types – positive or negative.
Positive – these stabilise the active form of
the enzyme allowing it to work.
Negative – these stabilise the inactive form
of the enzyme stopping it from working.
In this case the allosteric enzyme is
being negatively modulated.
Covalent Modification of Enzymes
Covalent modification involves the addition or removal
of molecular units (often phosphate groups) which
either activate or inactivate the enzyme.
Protein kinases add phosphate groups and
phosphatases remove them.
Some enzymes require phosphorylation to become active
while others are inactivated by phosphorylation.
Glycogen phosphorylase
Glycogen
phosphorylase is
activated by
phosphorylation
Allosteric Modification of Glycogen
phosphorylase
In addition to the covalent modification of glycogen
phosphorylase, it is also modified by glucose and ATP
which act as negative modulators. AMP (adenosine
monophosphate) acts as a positive modulator.
Having additional allosteric modifiers improves the
control of this enzyme. Use your monograph to
discover:
- Where this enzyme is produces
- Why it is produced
Proteolytic Cleavage
Trypsin is an enzyme which is made in the pancreas and is
responsible for the digestion of proteins. When it is produced it is
inactive – it has extra amino acids and is called trypsinogen.
Trypsin is VERY important as it is powerful enough to begin self
digestion of the pancreas should it be activated!
When trypsinogen is secreted in the small intestine it is
cut by a protease to remove the additional amino acids.
The result is the active trypsin.
End-Product Inhibition
Some metabolic pathways are controlled by end-product
inhibition. This involves the end product of a series of
reactions acting as an inhibitor of the enzyme in one of
the earlier reactions.
Your Tasks
1. Create a mind map of all the ways that enzymes can
be modified to affect their activity. Include
examples. Use your monograph to help you.
2. Update your glossary.
3. Past Paper Questions on catalysis:
2002 MC Q9
2003 MC Q6,10,11
2004 MC Q9,10,11
Q1A (ER)
2005 MC Q7,9
Q8B (ER)
2007 MC Q10,11
Due Monday
December
10th