3.1.6Inquiry Process
Metabolic Pathways — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 106 of 160
Part 1 of 4Chains and Cycles
Cells convert one molecule into another through a sequence of small, controlled steps.
Metabolic pathway
- A series of enzyme-catalysed reactions in which the product of one step becomes the substrate of the next step.
Linear pathway
- Substrate is converted to product through a straight chain of intermediates, each step catalysed by a different enzyme,
- as in glycolysis.
Cyclic pathway
- The final step regenerates the starting compound, which is why the Krebs cycle can run continuously as acetyl groups are fed in.
Why many steps
- Several small steps release or take in energy gradually and allow control at each stage, instead of one uncontrolled reaction.
Intermediates
- Compounds formed between the first substrate and the final product, each of which is made and used by a specific enzyme.
Part 2 of 4End-Product Inhibition
1
A pathway converts substrate A through intermediates to a final product, with each step catalysed by a different enzyme.
2
As the final product accumulates, some of it binds to an allosteric site on the first enzyme in the pathway.
3
Binding changes the shape of the active site of the first enzyme, so it can no longer bind its substrate effectively.
4
The whole pathway slows down, because no more intermediates are made, and the final product stops accumulating.
5
As the product is used up, it detaches from the first enzyme, which becomes active again and the pathway restarts.
Part 3 of 4Control by Negative Feedback
Control by Negative Feedback
Negative feedback
- The output of a system reduces its own production, which keeps the level of product within narrow limits.
First enzyme
- Inhibiting the first enzyme prevents waste because no intermediates build up that the cell does not need.
Example
- In bacteria, isoleucine inhibits threonine deaminase, the first enzyme of the pathway that makes isoleucine from threonine.
Reversible
- Binding of the final product is reversible, so the pathway can be switched on again when the product concentration falls.
| Product level | Effect on first enzyme | Pathway activity |
|---|---|---|
| High | Inhibited by product | Slows down |
| Low | Active, no inhibitor bound | Speeds up |
Part 4 of 4Exam Checklist & Tips
3.1.6 – Key Points for the Exam
- Metabolic pathway A sequence of enzyme-catalysed reactions.
- Intermediate A compound formed between the first substrate and final product of a pathway.
- End-product inhibition Inhibition of an early enzyme by the final product of the pathway.
- Allosteric site A site on an enzyme, other than the active site, where a regulator binds.
- Negative feedback A process in which the output reduces its own production.
Common Mistakes
- Saying the end product binds to the active site; in end-product inhibition it binds to an allosteric site.
- Writing that the final enzyme in the chain is inhibited, when it is usually the first.
- Confusing negative feedback with a pathway being switched permanently off.
- Describing pathways as a single reaction; each step has its own enzyme.
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