Investigation 13: Enzyme Activity
Enzyme activity is measured by tracking how fast product forms or substrate disappears, using the filter-paper disk assay for catalase and absorbance readings for peroxidase across different pH values.
The catalase disk assay turns an invisible reaction into a race you can time. Catalase breaks down hydrogen peroxide into water and oxygen gas. Soak a small filter-paper disk in catalase solution, push it to the bottom of a peroxide-filled tube, and oxygen bubbles collect under it until it floats to the surface. A faster enzyme produces gas faster, so the disk rises sooner. Flipping that time into a rate (1 divided by time, roughly) gives a number that goes up when the enzyme works faster, which is easier to compare across conditions than raw seconds.
The peroxidase version swaps the floating disk for a spectrophotometer. Peroxidase converts a colorless substrate into a colored product, and a colorimeter tracks that color's absorbance over time at a fixed wavelength. Running the same reaction at five pH values (say 3, 5, 7, 9, 11) and plotting absorbance versus time for each gives five curves. The initial slope of each curve is that condition's reaction rate; plotting those five rates against pH reveals the enzyme's optimum, usually a hump centered near neutral pH for many enzymes, though not all.
Either way, the number you want is the initial rate: the slope of the curve while it is still a straight line, right after the reaction starts. This connects directly to the broader science practice of extracting a rate from raw data, the same skill used for population growth curves, cellular respiration, and photosynthesis investigations elsewhere in this unit.
Key terms
- Initial rate
- The reaction rate measured from the slope of the earliest, linear part of a product-versus-time curve.
- Catalase
- An enzyme that breaks down hydrogen peroxide into water and oxygen gas.
- Peroxidase
- An enzyme that converts a substrate into a colored product, letting rate be tracked by absorbance.
- Substrate depletion
- The progressive consumption of reactant molecules during a reaction, which slows the rate over time.
- Absorbance
- A measure of how much light a solution absorbs, used to track colored product formation over time.
Watch it explained
- How it worksHow does the time a disk takes to rise convert into a reaction rate?
- How it worksWalk me through testing peroxidase across five pH values and plotting the result.
- Worked exampleFind the initial rate from a curve of product formed against time at 30 seconds.
- Common mistakeWhy doesn't the rate stay constant for the whole length of the run?
Check yourself
End of topic.