1. Higher Human Biology Subtopic 6 (b)
Metabolic Pathways
Higher Human Biology
Subtopic 6 (b)

2. Learning Intentions
Describe how gene expression and enzymes are regulated
Discuss the role of extracellular and intracellular signal molecules
Explain the difference between competitive and non-competitive inhibitors and describe their effect on enzyme activity

3. Control of Pathways
Each step in a metabolic pathway is driven by a specific enzyme.
Each enzyme is coded for by at least one gene.
If one enzyme is absent, the pathway stops.

4. Pathways can be regulated through gene expression.
Control of Pathways
Pathways can be regulated through gene expression.
Some pathways are needed at all times so are always turned on.
Some are only needed at certain times.
Stages (genes) are switched on and off to stop resources being wasted.

5. The Lac Operon Lactose is a sugar in milk Glucose Galactose
E. coli (a bacteria) use lactose as an energy source for respiration but they must first break it down.
Glucose Galactose
Lactose molecule

6. The Lac Operon
E. coli has a gene that codes for β galactosidase – an enzyme that breaks down lactose.
E. coli only produce the enzyme when lactose is present
β galactosidase
glucose
Lactose
galactose

7. This process is called ENZYME INDUCTION.
The Lac Operon
E. coli switch the gene for β galactosidase on when lactose is present and off when it’s not.
This process is called ENZYME INDUCTION.

8. The Lac Operon – How it works
An operon is 1 or more structural genes that code for the enzyme
+ a neighboring operator gene that controls them.
operon
operator gene
Controls the switching on and off
structural gene

9. In the absence of lactose
- genes turned off
- no enzyme produced
RNA polymerase
repressor protein binds to operator
cannot bind to promoter
promoter
no transcription of structural genes
repressor gene
operator
no enzymes for lactose breakdown
no translation of structural proteins
produces (via transcription and translation) repressor protein

10. In the presence of lactose
- genes turned on
- enzyme produced
- lactose broken down L
RNA polymerase
binds to promoter
promoter
transcription of structural genes
repressor gene
operator
mRNA for structural proteins
altered repressor cannot bind operator
produces (via transcription and translation) repressor protein
lactose binds to and alters repressor L
enzymes for lactose breakdown produced

11. Investigating The Lac Operon
β galactosidase
ONPG yellow substance + galactose

12. Controlling Pathways - signal molecules
Some pathways are always switched on.
These are controlled in different ways :
- signal molecules
- inhibitors

13. Signal Molecules These molecules can be e.g. hormones.
They trigger the activation of the enzymes.
They are intracellular
- the signal is released within the cell
Or extracellular
- the signal comes from the environment outside the cell

14. Inhibitors
This is a substance that decreases the rate of the reaction.
There are 2 types :
- competitive
- non-competitive

15. Competitive Similar shape to the substrate
Competes for the active site
Blocks the active site (substrate can’t bind)
Reaction rate decreases
Active site ISN’T damaged

16. Competitive
Increasing the substrate concentration will increase the rate of reaction as the substrate outnumbers the inhibitor.
Textbook page 93

17. NON - Competitive Doesn’t combine with the active site
Attaches elsewhere to an allosteric site
Indirectly changes the shape of the active site
Substrate can’t bind

18. Non - Competitive
These inhibitors change the active site so increasing substrate concentration makes no difference to activity.

19. NEGATIVE FEEDBACK CONTROL
End Point Inhibition
As the concentration of the end product increases it can bind to e.g. enzyme 1 in the pathway.
Less B, C and D will be produced (saves waste).
As levels of D fall, less enzyme 1 is affected so the reaction increases.
Animation 2
NEGATIVE FEEDBACK CONTROL

20. Activators Enzymes can exist in inactive or active forms.
A regulatory molecule can bind to an allosteric site to activate the enzyme.
This increases the reaction rate.
Textbook page 93

21. Expt - Phosphotase Textbook page 95 phosphatase
Phenolphthalein phosphate phenolphthalein phosphate

22. Learning Intentions
Describe how gene expression and enzymes are regulated
Discuss the role of extracellular and intracellular signal molecules
Explain the difference between competitive and non-competitive inhibitors and describe their effect on enzyme activity

23. Q1. Enzymes… a) speed up reactions and remain unchanged
d) slow down reactions and are used up in the reaction
c) speed up reactions and are used up in the reaction
b) slow down reactions and remain unchanged

24. Q2. What is an active site?
a) The place on a substrate where the enzyme binds.
d) The place on an enzyme where the substrate binds.
c) The place on the product where the substrate binds.
b) The place on the substrate where the product binds.

25. Q3. Which graph shows the effect of temperature on enzyme activity?d) c) b) a)
activity
activity
temperature
temperature
activity
activity
temperature
temperature

26. Q4. What will bind to an active site?
a) All types of substrate molecule.
d) One type of product molecule.
c) One type of substrate molecule.
b) All types of product molecule.

27. Q5. Enzymes will work at … a) only one pH d) acidic pHs
c) a range of pHs
b) all pHs

28. Q6. Which of the following is correct?
a) Starch catalase maltose
d) Starch catalase glucose
c) Starch amylase glucose
b) Starch amylase maltose

29. Q7. Which of the following is correct?
Amylase is a synthesis enzyme.
Catalase is a breakdown enzyme.
d) Amylase is a synthesis enzyme.
Phosphorylase is a breakdown enzyme.
c) Catalase is a synthesis enzyme.
Amylase is a breakdown enzyme.
b) Phosphorylase is a synthesis enzyme.

30. Q8. Which term best describes a denatured enzyme?
a) Its active site has changed shape.
d) Attached to the substrate.
c) Working at its fastest rate.
b) Dead.