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

2. Learning Intentions
State the difference between an anabolic and catabolic metabolic pathway
Discuss reversible, irreversible and alternative pathways in the metabolism
Describe the role of enzymes in the control of pathways

3. Cell Metabolism
The collective term for all the biochemical reactions in a living cell.
Controlled by enzymes

4. Two types of metabolic pathways
Anabolic
- biosynthesis (build up)
- requires energy
Catabolic
breakdown
releases energy

5. Anabolic Pathway
glucose-1-phosphate phosphorylase starch This reaction requires the input of energy to proceed.

6. Catabolic Pathway
starch amylase maltose This reaction releases energy.
NOTE
ATP plays an important role in the transfer of energy between the 2 types of reaction

7. Steps in the Pathway Glucose
A pathway may contain reversible and irreversible steps.
e.g.
glycolysis in respiration
The reversible step is useful if the cell has too much of intermediate 2. It can convert it back to 1 and use 1 in another pathway.
Enzyme A
Intermediate 1
Glycogen or starch
Enzyme B
Intermediate 2
Enzyme C
Intermediate 3
Many enzymes
Pyruvate

8. Steps in the Pathway Glucose Intermediate 1 Intermediate 2
The irreversible step here keeps the glucose concentration inside the cell LOW so glucose keeps diffusing in.
The irreversible step from 2 to 3 is a key regulatory point in the pathway.
Enzyme A
Intermediate 1
Glycogen or starch
Enzyme B
Intermediate 2
Enzyme C
Intermediate 3
Many enzymes
Pyruvate

9. Alternative Pathway Glucose
These allow steps in the pathway to be bypassed.
This is used when the cell has plenty of sugar.
Enzyme A
Alternative route
Intermediate 1
Enzyme B
Sorbitol
Intermediate 2
Enzyme C
Many enzymes
Intermediate 3
Many enzymes
Pyruvate

10. Cell Metabolism Summary
Metabolism describes all biochemical reactions which occur within a cell.
Metabolic pathways involve synthesis (building up of molecules) reactions, termed anabolism, and breakdown reactions, termed catabolism.
Synthetic pathways require the input of energy whereas break down pathways usually release energy.
Some pathways can be reversible, others irreversible.
Pathways may also have more than one route.

11. Enzymes

12. What is an Enzyme? BIOLOGICAL CATALYST
Remain unchanged at the end of the reaction (can be re-used)
Increase the rate of a reaction by lowering activation energy
Activation energy is the energy needed to break bonds and start the reaction.
Without enzymes reactions would be too slow for life!

13. Enzyme Action Globular proteins.
Have an active site determined by the protein structure.
Only work on one specific substrate which has a complimentary shape to the active site.

15. Enzyme Action INDUCED FIT
Active site is flexible and can change slightly to fit the substrate very closely.
Increases the chance of a reaction due to the close contact.
induced fit short film

16. Induced Fit

17. Enzyme Action ORIENTATION
When a reaction involves 2+ substrates the shape of the active site determines the orientation of the reactants.

18. Enzyme Action THE REACTION
Substrate is held in an induced fit while reaction occurs – this reduces activation energy.
After the reaction the affinity is lower so the products are released.

19. Factors Affecting Enzyme Activity
For enzymes to function efficiently they need :
Suitable pH
Suitable temperature
Enough substrate

20. pH B A C As pH increases to the optimum pH the reaction rate increase.
Too acidic or alkaline conditions denature the enzyme. B A C

21. Temperature
Increasing temp (until the optimum) increases reaction rate.
High temp breaks bonds and changes the shape of the active site.
The enzyme is permanently damaged ……..
….DENATURED
The substrate can’t bind.

22. Substrate Concentration
Low
Too few substrates to use all active sites
Low reaction rate
Higher
More substrates fill more active sites
Increased reaction rate
Reaction rate
Substrate concentration
NOTE : the reaction rate remains constant when all active sites are full. There are more substrate molecules than active sites

24. Enzyme concentration is the limiting factor

25. Direct Enzyme Action Complexes Metabolite W Metabolite Z Metabolite Y
Metabolic pathways usually include groups of enzymes.
The product for one reaction is the substrate for the next.
Metabolite Z
Enzyme2
Metabolite Y
Complexes
Enzymes often work in groups – multi enzyme complex e.g. DNA polymerase

26. Reversibility Metabolite W Metabolite X Too high Metabolite Y
An enzyme can catalyse a reaction forward and back.
This depends on concentrations of reactants and products.
It restores equilibrium.
Enzyme 1
Metabolite X
Enzyme2
Too high
Metabolite Y
Enzyme 3
Metabolite Z

27. Enzymes Summary
Globular proteins that speed up a reaction but remain unchanged.
Lower activation energy needed.
They are specific – active site is complimentary to ONE substrate.
Induced fit increases the chance of reaction.
Reaction rate is affected by pH, temperature and substrate concentration.

28. Learning Intentions
State the difference between an anabolic and catabolic metabolic pathway
Discuss reversible, irreversible and alternative pathways in the metabolism
Describe the role of enzymes in the control of pathways