Lesson 2 of 4 / Enzymes and investigations
Temperature and pH change enzyme rate
Why is a temperature-rate curve not symmetrical?
In this lesson: Explain and investigate temperature and pH effects on catalysis.
About 6 min
The key ideaTemperature affects collision frequency and stability; pH affects binding-site charges and protein interactions.
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 kinetic energy generally increases collision frequency and successful enzyme-substrate encounters. Beyond a suitable range, disruption of the enzyme's folded structure reduces the number of functional active sites. The decline can be steep because these effects are not mirror images.
An optimum is the highest measured rate under the stated conditions and assay duration. Prolonged heating can inactivate an enzyme that seemed efficient during a short initial measurement. Do not treat one graph as a universal fixed optimum for all enzymes.
Changing pH alters ionisation of substrate and active-site groups as well as interactions stabilising protein shape. Binding or catalysis can decline even before complete unfolding. Use a suitable buffer so that changes in reaction products do not cause an unintended pH change.
For a temperature investigation, equilibrate separate enzyme and substrate solutions before mixing, keep concentrations and pH fixed, and measure initial rate with repeats. A water bath controls temperature more reliably than assuming room conditions remain constant. Include a no-enzyme or denatured-enzyme control where suitable.
Step by step
- 1
Control other variables
Hold enzyme, substrate, volume and pH constant.
- 2
Measure the early rate
Avoid confounding substrate depletion with temperature.
- 3
Interpret both sides
Explain low-temperature kinetics and high-temperature structural loss separately.
Worked example
Work through the evidence
A catalase sample is slow at 5 C but recovers at 25 C. Another heated to 80 C does not recover. Explain.
One way to explain it
Cooling reduced molecular motion without necessarily denaturing the enzyme; warming restored rate. Strong heating disrupted functional structure, and failed refolding or aggregation prevented recovery.
Why this answer works
- Recovery after treatment helps distinguish kinetic from structural effects.
- Do not describe cold enzyme as dead.
Is this true? "Every fall in rate means the enzyme has denatured."
Rate can fall because of lower temperature, limited substrate or altered ionisation without complete denaturation.