Confounded Catalase Kinetics
During a 90-minute laboratory block, students measure catalase activity in potato tissue by collecting oxygen gas in an inverted graduated cylinder. Each group receives five potato discs cut with a 5 mm cork borer and 3.0% hydrogen peroxide. To test pH, one group uses a single disc per trial but reuses the same disc across all five trials, moving from pH 3 to 5, 7, 9, and 11 and rinsing with distilled water between trials. They add 2 mL of 0.1 M buffer to 8 mL of peroxide for each trial. Room temperature rises from 20°C to 27°C during the block. Oxygen volumes collected in 60 s are: pH 3, 1 mL; pH 5, 14 mL; pH 7, 31 mL; pH 9, 18 mL; pH 11, 2 mL. The group concludes that the optimum pH is exactly 7.0 and that pH 9 is only slightly less favorable than pH 7.
1. Identify the two most serious design flaws in this investigation and explain, for each, how it could bias the conclusion about the pH optimum.
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The most serious flaw is reusing one potato disc across all five pH trials. Catalase is a protein, and exposure to pH 3 and then pH 11 can irreversibly denature some enzyme. Even with rinsing, the disc entering the pH 7 trial has already lost activity, and the disc entering the pH 9 trial has been exposed to both extremes. The result is that later measurements are not independent and are likely depressed by cumulative inactivation. The pH 9 value of 18 mL may therefore underestimate the true activity at pH 9, while the earlier pH 7 value may also be lower than it would be with fresh tissue.
The second flaw is uncontrolled temperature. The room warms from 20°C to 27°C, so later trials run warmer. A 7°C rise can increase enzyme activity substantially, but warming also lowers oxygen solubility, so less gas is collected. These two effects push the measured volume in opposite directions, which means the temperature change cannot be corrected by a simple factor. Because trial order is fixed, pH and temperature are confounded: any apparent pH effect is mixed with time and temperature effects. A 60-second collection also measures cumulative gas rather than an initial rate, so substrate depletion and enzyme inactivation further distort the comparison.
2. Describe a revised procedure that would produce a defensible estimate of the pH optimum, including at least three controls or standardizations.
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A defensible design would use a fresh, uniform potato disc for every trial, cut to the same diameter and thickness and blotted to the same dryness. All buffers and peroxide solutions would be equilibrated in a water bath at one temperature, such as 25°C, and the reaction vessel would be held at that temperature. The buffer concentration and ionic strength would be identical across pH values; simply adding 2 mL of 0.1 M buffer to 8 mL of peroxide gives a final buffer concentration of only 0.02 M, which may not hold pH during the reaction. A stronger buffer at the same final concentration would be better.
Activity would be measured as an initial rate over short intervals, for example 10–30 seconds, using a gas syringe or pressure sensor rather than a single 60-second endpoint. Each pH would be replicated at least three times, and trial order would be randomized so that time or temperature drift does not align with pH. Controls should include a boiled-enzyme disc to show that gas production is enzymatic, and a no-peroxide blank. The substrate concentration should be high enough to saturate the enzyme so that small differences in peroxide volume do not change the rate.
3. The group notes that pH 9 produced more oxygen than pH 5. Does this asymmetry support the claim that the enzyme prefers alkaline conditions? Explain.
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No. The asymmetry is interesting, but this experiment cannot use it to claim an alkaline preference. Because the same disc was used in a fixed order, the pH 9 trial occurred later than the pH 5 trial and at a higher room temperature. The pH 9 disc had also been exposed to pH 3, pH 5, and pH 7 before reaching pH 9, whereas the pH 5 disc had only been exposed to pH 3. Those differences mean the two measurements do not differ only in pH.
A real pH profile can be asymmetric if the catalytic groups have different pKa values, so an alkaline-side plateau is biologically possible. To test that, the group would need fresh discs, randomized order, constant temperature, and replicate measurements with error estimates. If pH 9 still exceeded pH 5 under those conditions, the asymmetry would be evidence about the enzyme's ionizable groups. As reported, the 18 mL versus 14 mL difference is too small and too confounded to support any claim about alkaline preference.