Lesson 2 The Catalytic Cycle What do enzymes do Enzymes lower the activation energy E a required for a reaction to occur Catalytic Cycle of Sucrase Enzyme Substrate Complex Lock and Key Model ID: 235070
Download Presentation The PPT/PDF document "Section 3" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Section 3Lesson 2– The Catalytic CycleSlide2
What do enzymes do?
Enzymes lower the
activation energy
E
a
required for a reaction to occur.Slide3
Catalytic Cycle of Sucrase
Enzyme Substrate ComplexSlide4
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.Slide5
Induced Fit - HexokinaseWhat 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.Slide6
Catalytic Cycle of HexokinaseCopy Fig 3.1.4 from the monograph into your notes.
Annotate your diagram using the notes found under the diagram in the monograph.Slide7
Control of Enzyme ActivityCells 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:
Compartmentalisation
– enzymes remain in compartments in the cell such as the mitochondria, chloroplast, Golgi apparatus.
pH
– the pH affects the activity of enzymes so controlling the pH will change which enzymes are active.
Temperature
–
the
temperature affects
the activity of enzymes so
controlling
the
temperature
will change which enzymes are active.Cofactors
– some enzymes require another non-protein molecule to bind to them so that they will work.Altering the shape of the enzyme
– this can happen through inhibitors, allosteric effects, covalent modification and end-product inhibition.Slide8
InhibitorsThere are 2 types of inhibitors – competitive and non-competitive.
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.Slide9Slide10
Competitive Inhibition in ActionCan you think of an example of competitive inhibition in the nitrogen cycle?
O
2
is similar enough to N
2
to act as a competitive inhibitor.
How is this problem overcome?Slide11
Non-competitive InhibitorsThese 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. Slide12
Allosteric EnzymesAn
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.Slide13
In this case the allosteric enzyme is being negatively modulated
.Slide14
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.Slide15
Glycogen
phosphorylase
Glycogen
phosphorylase
is activated by phosphorylationSlide16
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 producedSlide17
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.Slide18Slide19
End-Product InhibitionSome 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.Slide20
Your TasksCreate a mind map of all the ways that enzymes can be modified to affect their activity. Include examples. Use your monograph to help you.
Update your glossary.
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