Explain how the concentration of an enzyme affects the rate of an enzyme-catalysed reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a cell, the concentration of enzymes is regulated by gene expression. Increasing the amount of enzyme produced can dramatically alter the rate at which metabolic reactions proceed.
Model answer (4 marks)
Increasing the concentration of enzyme raises the rate of the reaction when substrate is not limiting, because more enzyme molecules give more active sites for substrate binding.
Each enzyme molecule can form an enzyme–substrate complex; therefore, a higher enzyme concentration allows more complexes to form simultaneously, increasing the overall reaction rate.
If substrate concentration is high enough that all active sites are already occupied, further increases in enzyme concentration still raise the rate, as more complexes can form.
However, when substrate becomes the limiting factor and all enzyme molecules are saturated, the rate reaches a plateau; additional enzyme cannot increase the rate because there are not enough substrate molecules to occupy all active sites.
Each enzyme molecule can form an enzyme–substrate complex; therefore, a higher enzyme concentration allows more complexes to form simultaneously, increasing the overall reaction rate.
If substrate concentration is high enough that all active sites are already occupied, further increases in enzyme concentration still raise the rate, as more complexes can form.
However, when substrate becomes the limiting factor and all enzyme molecules are saturated, the rate reaches a plateau; additional enzyme cannot increase the rate because there are not enough substrate molecules to occupy all active sites.
Examiner tips
- Use the phrase "increases the rate" and mention "active sites" and "enzyme–substrate complexes". Show the two regimes: substrate not limiting and substrate limiting. Explain the plateau when enzyme is saturated with substrate.
Common mistakes
- Confusing enzyme concentration with substrate concentration. Saying the rate always increases with enzyme concentration, even when substrate is limiting. Forgetting to mention the plateau when all active sites are occupied.
Mark scheme (4 marks)
- Increasing enzyme concentration increases the rate of reaction (when substrate is not limiting).
- More enzyme molecules provide more active sites, so more enzyme–substrate complexes can form at any one time.
- When substrate concentration becomes limiting (substrate is in excess of enzyme), further increases in enzyme concentration continue to raise the rate.
- If substrate concentration is the limiting factor, a plateau in rate is reached where additional enzyme molecules cannot increase the rate because substrate molecules are insufficient to occupy all active sites.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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