1. Metabolic Pathways (b)
State 2 ways metabolic pathways are controlled
State the role of intra- and extra-cellular signal molecules
State how a sequence of reactions can be driven in a particular direction

2. Control of Metabolic Pathways
Metabolic pathways consist of several stages
Metabolic pathways are controlled by:
the absence or presence of particular enzymes
the regulation of the rate of reaction of key enzymes within the pathway
Regulation can be controlled by intra and extracellular signal molecules
Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction.
Alternative pathway X
Causes the reaction to reverse
Lots of Substrate 3
Increases the rate of reaction here

3. Example of metabolic pathways in the body
Making Melanin:
What characteristics of metabolic pathways can you see?
It consists of several stages
Each stage is controlled by enzymes

4. Control of Metabolic Pathways
Metabolic pathways consist of several stages
Metabolic pathways are controlled by the absence or presence of particular enzymes
They are also controlled through the regulation of the rate of reaction of key enzymes within the pathway
Regulation can be controlled by intra and extracellular signal molecules
Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction.

5. Example of metabolic pathways in the body
Phenylketonuria (PKU): X X
Toxins produced
Phenylketonuria is a genetic disorder cause by a missense substitution mutation to a gene on chromosome 12 that normally codes for enzyme 1 in the pathway.
The person is unable to make the essential enzyme that converts phenylalanine to tyrosine.
Instead an alternative metabolic pathway occurs that produces toxins that limit metal development and cause epilepsy.

6. Example of metabolic pathways in the body
Phenylketonuria (PKU):
In the UK, PKU occurs in about 1:10000 people.
Babies are screened at birth for the condition. Their blood is tested for excess phenylalanine, if the condition is present it can be controlled with a special low-protein diet.
PKU sufferers have lighter skin than normal as they still get some tyrosine from their diet so are still able to produce some melanin (skin pigment).

7. Example of metabolic pathways in the body
Albinism: X X
Albinos are unable to make the enzyme that allows the synthesis of melanin. Provided they avoid any exposure to UV radiation they have no medical problems.

8. Control of Metabolic Pathways
Metabolic pathways consist of several stages
Metabolic pathways are controlled by the absence or presence of particular enzymes
They are also controlled through the regulation of the rate of reaction of key enzymes within the pathway
Regulation can be controlled by intra and extracellular signal molecules
Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction.

9. Control of Metabolic Pathways
Genes for some enzymes are continuously expressed.
These enzymes are always present in the cell and their control involves regulation of their rate of reaction. This can be through:
intra and extracellular signal molecules
switching genes on or off

10. Epinephrine is released by adrenal glands
It binds to receptors on liver cell membranes
This triggers the a series of events that activate an enzyme to convert glycogen into glucose and release it into the blood stream.
Epinephrine acts a signal molecule

11. Control of Metabolic Pathways
1. Intracellular signal molecules:
Signals that originate within the cell and affect metabolism
2. Extracellular signal molecules:
Signals that come from the cells environment (other cells) and affect metabolism
An example of this would be epinephrine (AKA adrenaline)

12. Control of Metabolic Pathways
3. Switching genes on or off
Some metabolic pathways are only required to operate under certain circumstances.
Therefore in order to regulate the metabolic pathway, the genes which code for the enzymes can either be switched on or off.
An example of this is found in the bacteria E coli.

13. Escherichia coli produces the enzyme -galactosidase to digest the substrate lactose, to release the glucose that it needs for respiration.
The enzyme is only made if lactose is present. When no lactose is present the gene that codes for the synthesis of -galactosidase is switched off.
This ensures that resources are not wasted in the production of an enzyme when it is not needed.

14. + Lactose = Lactose (a sugar found in milk) Glucose Galactose Glucose
Enzyme that catalyses the breakdown of lactose
Glucose
β - Galactosidase
Galactose +
Lactose

15. Tasks:
Complete the “Investigating the control of metabolic pathways in E coli“ sheet

16. Investigating the lac operon in E coli ANSWERS
ONPG is a colourless synthetic chemical that can be broken down by the enzyme B-galactosidase as follows:
ONPG galactose + yellow compound
The presence of yellow indicates activity by B-galactosidase.

17. Investigating the lac operon in E coli ANSWERS
Method:

18. Investigating the lac operon in E coli ANSWERS
Results:

19. Investigating the lac operon in E coli ANSWERS
Results & conclusion:
Test tube 1
Test tube 2
Test tube 3
Test tube 4
Colour of solution
Reason the solution turned this colour
Yellow
Yellow
Colourless
Colourless
Presence of lactose induced E. coli to produce B- galactosidase.
The substrate ONPG was not present, so the yellow colour could not be produced
The enzyme B- galactosidase acted on substrate ONPG forming the yellow compound
The substrate ONPG was not present, so the yellow colour could not be produced