3.1.4—Enzyme reaction investigations

Enzyme activity is the rate of an enzyme-catalysed reaction: how much substrate is used or product is formed per unit time. In general, rate = change in measured amount ÷ time taken, using a consistent signal and time interval.

1. Choose a progress signal: Follow product formation, such as oxygen produced by catalase, or substrate disappearance, such as starch being hydrolysed by amylase.
2. Measure at defined times: Start the reaction consistently and record the signal at set intervals. For amylase, samples can be tested with iodine; starch gives a blue-black result while it remains.
3. Calculate rate: Plot progress against time when appropriate and use the gradient of the early, approximately linear section as the initial rate. A final amount or a plateau is not itself a rate.
4. Compare fairly: Keep temperature, pH, enzyme concentration, substrate concentration, volumes, mixing and timing controlled; repeat measurements and calculate a representative mean when the design requires it.

A faster reaction changes the chosen product or substrate signal more quickly, so it has a steeper initial gradient. The initial section is preferred because later progress can slow as substrate is depleted, products accumulate or another experimental limit is reached.

Do not call a single endpoint amount an enzyme rate, and do not infer a rate from unequal time intervals without accounting for time. This card defines the general progress-over-time method; the separate colorimetry card explains how a colour signal is calibrated into quantitative readings.