How do competitive and non-competitive inhibitors reduce enzyme activity, and how is inhibition used to control metabolism?
Distinguish competitive and non-competitive enzyme inhibition and explain end-product inhibition in metabolic control
A focused answer to the H2 Biology Energy and Equilibrium outcome on enzyme inhibition. Competitive inhibitors binding the active site, non-competitive inhibitors binding elsewhere, the effect of substrate concentration on each, and end-product inhibition as feedback control.
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What this dot point is asking
SEAB wants you to distinguish competitive from non-competitive inhibition (by where the inhibitor binds, the mechanism, and the effect of increasing substrate), and to explain end-product inhibition as a means of controlling metabolic pathways by negative feedback. This builds on enzyme structure and kinetics.
The answer
Competitive inhibition
A competitive inhibitor has a shape similar to the substrate and binds the active site, competing with the substrate for it. While the inhibitor is bound, the substrate cannot enter, so the rate falls. Because the two compete, increasing the substrate concentration raises the chance that substrate wins the active site, so the inhibition can be overcome and the maximum rate is eventually reached.
Non-competitive inhibition
A non-competitive inhibitor binds a site other than the active site (an allosteric site). This changes the shape of the active site so the substrate can no longer bind effectively. Because the inhibitor does not compete for the active site, increasing substrate cannot overcome it, and the maximum rate is reduced.
End-product inhibition (feedback control)
In a metabolic pathway, the final product often inhibits an enzyme catalysing an early (committed) step, usually by binding an allosteric site. As product accumulates it slows the pathway; as product is used up, inhibition is relieved and the pathway speeds up. This negative feedback prevents waste and keeps product levels balanced.
Examples in context
Example 1. Competitive inhibition in medicine. Some drugs are designed as competitive inhibitors that resemble the natural substrate of a target enzyme, blocking it. Because the effect depends on the relative amounts of drug and substrate, dosing must keep the inhibitor concentration high enough to compete effectively.
Example 2. Respiration and feedback. In glycolysis, the build-up of ATP inhibits an early enzyme of the pathway, slowing glucose breakdown when the cell already has plenty of ATP. This end-product inhibition links enzyme control directly to the regulation of cellular respiration.
Try this
Q1. State where a competitive inhibitor binds on an enzyme. [1 mark]
- Cue. At the active site, competing with the substrate.
Q2. Explain why increasing substrate concentration does not overcome non-competitive inhibition. [2 marks]
- Cue. The inhibitor binds an allosteric site and changes the active site shape, so substrate cannot bind effectively no matter how much is present; the maximum rate stays reduced.
Q3. Explain why end-product inhibition is described as negative feedback. [2 marks]
- Cue. The product of the pathway inhibits an earlier enzyme, so a rise in product reduces its own further production, returning levels toward a set point.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDistinguish between competitive and non-competitive inhibition of an enzyme, and explain how the effect of each can be overcome or not by increasing substrate concentration.Show worked answer →
The answer should contrast binding site, mechanism and the effect of more substrate.
A competitive inhibitor has a shape similar to the substrate and binds the active site itself, competing with the substrate. While it occupies the active site, the substrate cannot bind, so the rate falls. Increasing the substrate concentration raises the chance that substrate, rather than inhibitor, occupies the active site, so the effect can be overcome and the maximum rate is eventually restored.
A non-competitive inhibitor binds a site other than the active site (an allosteric site). This changes the shape of the active site so that the substrate can no longer bind effectively, regardless of how much substrate is present. Increasing the substrate concentration therefore cannot overcome non-competitive inhibition, and the maximum rate is reduced.
Markers reward the binding location for each, the mechanism, and the key contrast that more substrate reverses competitive but not non-competitive inhibition.
Original4 marksExplain how end-product inhibition controls the rate of a metabolic pathway, and state why this is advantageous to the cell.Show worked answer →
The answer should describe the feedback mechanism and its benefit.
In a metabolic pathway, the final product of the pathway acts as an inhibitor of an enzyme that catalyses an earlier step, usually the first committed step. The product binds to that enzyme (often at an allosteric site) and reduces its activity.
As the product accumulates, it slows the enzyme, so the pathway slows and less product is made. As the product is used up, inhibition is relieved and the pathway speeds up again. This is negative feedback.
It is advantageous because it prevents the cell from wasting resources and energy making more product than it needs, and it keeps the amount of product within useful limits.
Markers reward the end product inhibiting an early enzyme, the negative feedback loop, and the benefit of avoiding waste and maintaining balance.
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