1. ENZYMES
AHL Topic 7.6
IB Biology
Miss Werba

2. AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS
7.1 DNA STRUCTURE
7.2
DNA REPLICATION
7.3
TRANSCRIPTION
7.4
TRANSLATION
7.5 PROTEINS
7.6 ENZYMES
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3. THINGS TO COVER Enzymes in metabolic pathways Induced-fit model
Activation energy & catalysis
Competitive & non-competitive inhibition
End-product inhibition
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4. METABOLIC PATHWAYS Command term = STATE
7.6.1
METABOLIC PATHWAYS Command term = STATE
Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions
Metabolic pathways are also known as biochemical pathways
All the reactions occur inside cells.
An enzyme catalyses each reaction.
The product of one reaction is the substrate for the next
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5. METABOLIC PATHWAYS Command term = STATE
7.6.1
METABOLIC PATHWAYS Command term = STATE
Some pathways consist of chains of reactions
eg. glycolysis
a chain of 10 enzyme controlled reactions
Some pathways consist of cycles of reactions
eg. the Krebs Cycle (respiration)
eg. the Calvin Cycle (photosynthesis)
Ref: Biology for the IB Diploma, Allott
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6. METABOLIC PATHWAYS Command term = STATE
7.6.1
METABOLIC PATHWAYS Command term = STATE
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7. THE INDUCED-FIT MODEL Command term = DESCRIBE
7.6.2
THE INDUCED-FIT MODEL Command term = DESCRIBE
The lock and key model of enzyme action does not fully explain enzyme activity.
If the lock and key model were true, then one enzyme would only catalyse one reaction. However, there are some enzymes that are capable of catalysing multiple reactions.
The Induced Fit Model proposes that the active site does not fit the substrate precisely until the substrate binds.
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8. THE INDUCED-FIT MODEL Command term = DESCRIBE
7.6.2
THE INDUCED-FIT MODEL Command term = DESCRIBE
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.
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9. THE INDUCED-FIT MODEL Command term = DESCRIBE
7.6.2
THE INDUCED-FIT MODEL Command term = DESCRIBE
eg. some proteases have quite a broad specificity
The induced fit model explains this better than the lock and key.
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10. ACTIVATION ENERGY Command term = EXPLAIN
7.6.3
ACTIVATION ENERGY Command term = EXPLAIN
Activation energy:
the amount of energy needed to start off a reaction
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.
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11. ACTIVATION ENERGY Command term = EXPLAIN
7.6.3
ACTIVATION ENERGY Command term = EXPLAIN
Exothermic / exergonic / catabolic reactions:
These reactions involve the breakdown of complex compounds into simpler ones.
eg. respiration, digestion
This is where energy is released.
The products have less energy than the reactants.
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Ref: Biology for the IB Diploma, Allott

12. ACTIVATION ENERGY Command term = EXPLAIN
7.6.3
ACTIVATION ENERGY Command term = EXPLAIN
Endothermic / endergonic / anabolic reactions:
These reactions involve the building up of complex compounds from simpler ones.
eg. photosynthesis, protein synthesis, production of glycogen
This is where energy is taken in from the surroundings.
The products have more energy than the reactants.
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Ref: Biology for the IB Diploma, Allott

13. 13

14. ENZYME INHIBITION Command term = EXPLAIN
7.6.4
ENZYME INHIBITION Command term = EXPLAIN
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
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15. ENZYME INHIBITION Command term = EXPLAIN
7.6.4
ENZYME INHIBITION Command term = EXPLAIN
Competitive inhibitors:
inhibitor and substrate are structurally similar
inhibitor binds to the active site
substrate cannot bind
Examples:
the inhibition of folic acid synthesis in bacteria by sulfonamide Prontosil™ (an antibiotic)
the inhibition of enzyme-catalysed synthesis of fumerate by oxaloacetate
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16. ENZYME INHIBITION Command term = EXPLAIN
7.6.4
ENZYME INHIBITION Command term = EXPLAIN
Non-competitive inhibitors:
inhibitor and substrate are not structurally similar
inhibitor binds at another site, other than the active site
changes the shape of the active site
substrate cannot bind
Examples:
cyanide inhibition of cytochrome oxidase (an enzyme in cellular respiration).
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17. ENZYME INHIBITION Command term = EXPLAIN
7.6.4
ENZYME INHIBITION Command term = EXPLAIN
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18. ENZYME INHIBITION Command term = EXPLAIN
7.6.4
ENZYME INHIBITION Command term = EXPLAIN
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19. 19

20. END-PRODUCT INHIBITION Command term = EXPLAIN
7.6.5
END-PRODUCT INHIBITION Command term = EXPLAIN
End-product inhibition prevents the build up of products
It is an example of negative feedback
The site where the end-product inhibitor binds is called the allosteric site.
Binding of the product to the allosteric site changes the shape of the active site of the enzyme.
This prevents binding of the substrate and a build up of the end product.
Allostery is a therefore a form of non-competitive inhibition.
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21. END-PRODUCT INHIBITION Command term = EXPLAIN
7.6.5
END-PRODUCT INHIBITION Command term = EXPLAIN
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.
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22. END-PRODUCT INHIBITION Command term = EXPLAIN
7.6.5
END-PRODUCT INHIBITION Command term = EXPLAIN
Ref: Biology for the IB Diploma, Allott
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23. Sample questions
Q1. Which is correct for the non-competitive inhibition of enzymes?
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24. Sample questions
Q2. Consider the metabolic pathway shown below. If there is end-product inhibition, which product would inhibit which enzyme?
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25. Sample questions Q3. Outline how enzymes catalyse reactions. [7]
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26. Sample questions
A1. D
A2. D
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27. Sample questions they increase rate of (chemical) reaction;
remains unused/unchanged at the end of the reaction;
lower activation energy;
activation energy is energy needed to overcome energy barrier that prevents reaction;
annotated graph showing reaction with and without enzyme;
substrate joins with enzyme at active site;
to form enzyme-substrate complex;
active site/enzyme (usually) specific for a particular substrate;
enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer);
induced fit model / change in enzyme conformation (when enzyme- substrate/ES complex forms);
making the substrate more reactive; 7 max
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