Factors that affect enzyme action
What changes an enzyme's speed
- Change the conditions and you change how fast an enzyme works.
- First, you need a fair way to measure the rate.
- Then we'll see how temperature, pH and concentration each affect it.
Measuring the rate
- Follow how fast product forms (e.g. collect the oxygen gas from catalase) or how fast substrate disappears (e.g. the iodine colour fading as amylase digests starch).
- A colorimeter turns a colour change into an exact number you can plot.
- The reaction is fastest at the start (most substrate present), so the initial rate — the slope at time zero — is the fairest value to compare.
Why is the initial rate (slope at time zero) the fairest value to compare?
At the start the most substrate is present, so the rate is highest and not yet limited by substrate running out.
Temperature
- Heating gives molecules more kinetic energy, so they collide more often — the rate rises.
- But past the optimum, the heat breaks the bonds holding the enzyme's shape: it denatures, the active site no longer fits, and the rate crashes.
Above the optimum temperature, the rate falls steeply because the enzyme:
Heat breaks the bonds holding the enzyme's shape; the active site changes and can no longer bind the substrate.
pH
- Each enzyme has an optimum pH. Move too far either side and it denatures, so the rate drops.
- To study pH fairly, hold it steady with a buffer solution.
To investigate the effect of temperature fairly, you keep pH constant using a:
A buffer holds pH steady so that pH is not an uncontrolled variable while you change temperature.
Enzyme and substrate concentration
- More enzyme (with substrate to spare) means more active sites, so the rate rises in proportion.
- More substrate speeds things up at first — but once every active site is busy, adding more makes no difference and the rate levels off.
- More inhibitor lowers the rate.
Adding more substrate eventually stops increasing the rate because:
Once all active sites are occupied, the rate is at its maximum; more substrate cannot be processed any faster.
With plenty of substrate available, doubling the enzyme concentration roughly doubles the rate.
More enzyme means more active sites; with substrate to spare, the rate rises in proportion to enzyme concentration.
You've got it
- measure the initial rate (steepest, at the start) for a fair comparison; a colorimeter makes colour exact
- temperature: rises to the optimum, then denatures and crashes
- pH: bell curve about the optimum pH; use a buffer to keep it steady
- enzyme ↑ → rate ↑ (proportional); substrate ↑ → levels off when all sites are full