A student investigated the effect of pH on the rate of protein breakdown by the digestive enzyme pepsin, which is found in the stomach.
The student mixed equal volumes of a cloudy egg white (protein) suspension with a pepsin solution in test tubes maintained at a constant temperature of 37∘C37^\circ\text{C}37∘C using different buffer solutions. The time taken for each suspension to turn completely clear (indicating complete breakdown of the protein) was recorded.
Our results are shown in the table below:
| pH of solution | Time taken for the suspension to turn clear (minutes) |
|---|---|
| 1.5 | 3 |
| 3.0 | 8 |
| 7.0 | Suspension remained cloudy after 20 minutes |
| 9.0 | Suspension remained cloudy after 20 minutes |
Explain why the protein was not broken down in the test tube at pH 7.0.