2. 7.6.1 State that metabolic pathways consist of chains and
cycles of enzyme-catalysed reactions.
7.6.2 Describe the induced-fit model.
(This is an extension of the lock-and-key model. Its
importance in accounting for the ability of some enzymes to
bind to several substrates should be mentioned)
7.6.3 Explain that enzymes lower the activation energy of the
chemical reactions that they catalyse.
(Only exothermic reactions should be considered)
3. 7.6.4 Explain the difference between competitive and non-
competitive inhibition, with reference to one example of
each.
Competitive inhibition is the situation when an inhibiting
molecule that is structurally similar to the substrate molecule
binds to the active site, preventing 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 a decrease in activity)
Reversible inhibition, as compared to irreversible inhibition,
is not required.
7.6.5 Explain the control of metabolic pathways by end-product
inhibition, including the role of allosteric sites.
4. Metabolic Pathways
Metabolic pathways have these features:
They consist of many chemical reactions that are carried out in
a particular sequence.
An enzyme catalyses each reaction.
All the reactions occur inside cells.
Some pathways build up organic compounds (Anabolic).
Some pathways break down organic compounds (Catabolic).
Some metabolic pathways consist of chains of reactions
eg: glycolysis
a chain of 10 enzyme controlled reactions
Some metabolic pathways consist of cycles of reactions:
eg: the Krebs Cycle
6. Induced Fit Model
Scientists have discovered the lock and key model of
enzyme action does not fully explain bind of substrates
and active sites.
A modification of the lock and key model is the Induced
Fit Model.
This model proposes that the active site does not fit the
substrate precisely until the substrate binds.
As the substrate binds to the active site, the active site
changes shape to better fit the substrate.
This weakens the bonds in the substrate, thus reducing the
activation energy required for the reaction.
8. Induced Fit Model
Some enzymes have quite a broad specificity:
eg: some proteases
The induced fit model explains this better than the lock
and key.
If the shape of the active site changes when the substrate
binds, several different, but similar substrates could bind
successfully to it and be catalysed.
9. Activation Energy
For reactions to occur, the reactants must meet with
sufficient energy.
This energy is called the Activation Energy.
Enzymes work by lowering the activation energy.
When the enzyme-substrate complex is formed the bonds
of the substrate are stressed/weakened.
This means that less energy is required to break them.
10. Exergonic & Endergonic Reactions
Chemical reactions can be either:
Exergonic:
This is where energy is released.
The products have less energy than the reactants.
Endergonic:
This is where energy is taken in from the surroundings.
The products have more energy than the reactants.
13. Inhibition of Enzymes
Some substances reduce the activity of enzymes or even
prevent it completely.
These substances are called Enzyme Inhibitors.
There are two main classes of inhibitors:
Competitive:
Non-competitive:
14. Competitive Inhibitors
Competitive inhibitors are where:
The inhibitor and the substrate are structurally very similar.
The inhibitor binds to the active site preventing the substrate
from binding.
An example is the inhibition of folic acid synthesis in
bacteria by sulfonamide Prontosil™ (an antibiotic)
16. Non-competitive Inhibitors
Non-competitive inhibitors are where:
The inhibitor and substrate are not structurally similar.
The inhibitor binds to the enzyme at another site, other than the
active site.
This changes the shape of the active site and prevents the
substrate from binding.
An example is cyanide inhibition of cytochrome oxidase
(an important enzyme in cellular respiration).
The cyanide binds to the –SH groups in an enzyme and
breaks the disulphide bridges, changing the shape and
function of the enzyme.
19. Allostery & Metabolic Pathways
A special kind of non-competitive inhibition is Allostery.
In many metabolic pathways, the product of the last
reaction inhibits the enzyme that catalyses the first
reaction.
This is called end-product inhibition.
This is also an example of negative feedback.
The site where the end-product inhibitor binds is called
the allosteric site.
21. Allostery & Metabolic Pathways
The advantage of this method is that if there is an excess
of the end-product, the whole pathway can be switched
off and intermediates do not build up.
If the levels of end-product are low, more and more of the
enzymes that catalyse the first reaction will start to work
and the whole pathway will become activated.
22. 7.6.1 State that metabolic pathways consist of chains and
cycles of enzyme-catalysed reactions.
7.6.2 Describe the induced-fit model.
(This is an extension of the lock-and-key model. Its
importance in accounting for the ability of some enzymes to
bind to several substrates should be mentioned)
7.6.3 Explain that enzymes lower the activation energy of the
chemical reactions that they catalyse.
(Only exothermic reactions should be considered)
23. 7.6.4 Explain the difference between competitive and non-
competitive inhibition, with reference to one example of
each.
Competitive inhibition is the situation when an inhibiting
molecule that is structurally similar to the substrate molecule
binds to the active site, preventing 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 a decrease in activity)
Reversible inhibition, as compared to irreversible inhibition,
is not required.
7.6.5 Explain the control of metabolic pathways by end-product
inhibition, including the role of allosteric sites.
