Lesson 2 of 4 / Enzymes and investigations
Explain an optimum, not just a peak
Why does warming first help and then harm?
In this lesson: Explain temperature and pH effects on enzyme-catalysed rates using molecular interactions.
About 5 min
The key ideaTemperature affects molecular motion and enzyme stability; pH affects charges and interactions. Rate reflects productive binding under the particular conditions.
Explore the idea
Read the shape, then explain it
Relative rate: 6 in this original data set. Increasing temperature can increase productive molecular encounters while the enzyme remains stable.
These are invented practice measurements. Joining points guides the eye; it does not establish the exact optimum between tested conditions.
Explanation
At low to moderate temperatures, increasing temperature increases molecular motion and can increase collision frequency and the proportion of collisions with sufficient energy. Productive enzyme-substrate complexes therefore form more frequently, provided the enzyme remains suitably folded.
At sufficiently high temperature, disruption of stabilising interactions alters the active site. Loss of functional enzyme can outweigh the collision benefit and rate falls. The measured optimum depends on the enzyme, assay conditions and duration; a sparse set of tested temperatures does not locate it exactly.
pH affects ionisation of groups in the enzyme and sometimes the substrate. Changed charge can disrupt ionic interactions, hydrogen bonding or catalytic groups, reducing productive binding and activity. Extreme pH may denature the protein. Different enzymes have different suitable pH ranges.
Compare results at a fixed temperature when testing pH and at fixed pH when testing temperature. A low rate alone does not identify the cause: too little enzyme, depleted substrate or an unsuitable assay could also produce it.
Step by step
- 1
Describe the trend
Use data values before suggesting the mechanism.
- 2
Explain the rising and falling regions separately
Increased molecular motion and structural disruption are different effects.
- 3
Limit the conclusion
Report the fastest tested condition, not an exact unmeasured optimum.
Worked example
The fastest tested temperature
Relative rates at 20, 30, 40 and 50 degrees C are 3, 6, 10 and 4. What can be concluded about the optimum?
One way to explain it
40 degrees C gives the highest rate among the temperatures tested. The exact optimum may lie between tested values, so more closely spaced measurements are needed to locate it.
Why this answer works
- Distinguish a measured maximum from the underlying curve maximum.
- Do not infer a universal optimum for all enzymes.
Is this true? "Low temperature normally denatures the enzyme in the same way as high temperature."
Low temperature usually slows motion and reaction reversibly. High-temperature loss of folding is a different mechanism.