Pepsin: The Effect of pH on Enzyme Activity

Pepsin is a gastric enzyme which is secreted by the stomach wall and aids in protein digestion. It is a protease produced when pepsinogen is secreted by the gastric glands and due to the highly acidic nature of the stomach, converted to pepsin. It changes proteins into peptides and amino acides therefore beginning protein digestion. For pepsin to be effective, it requires an acidic enviroment such as the one provided in the stomach by hydrochloric acid.

There are many factors affecting enzyme activity. These include temperature,pH, concentration and enzyme inhibitors. The following experiment will investigate the effect of pH on Pepsin.


Hard-boiled egg white

6 Test tubes


1% Pepsin

1% Pepsin in 0.4 Hcl

0.4 Hcl

1% Pepsin in 0.5% sodium bicarbonate

Sodium Bicarbonate (0.5%)

Distilled water


  1. Label the test tubes appropriately from 1 to 6
  2. Put a third of each solution as listed above in the corresponding test tube.
  3. Cut the egg white into approximately 2 millimeter sized cubes and drop one in each test tube.
  4. Incubate at room temperature for at least 12 hours.
  5. Repeat the experiment but this time incubate at thirty degrees Celsius.
  6. Repeat a second time but this time use, extremely thinner strips of egg white at room temperature conditions.


Once the 12 hours expire, little egg white in tube 2 that contained 1% pepsin in 0.4% HCl remains while the rest remain unchanged. Due to the appropriate acidic conditions, pepsin is able to breakdown the egg white. If the 12 hour period is extended, the egg white in tube 4 is observed to be smaller (Miller, 1992, pp. 153-161). When incubated at thirty degrees Celsius as well as when very thin egg white pieces are used, the egg white disappears very first in tube 2. Concisely, an increase in temperature to optimum leads to an increase in the rate of enzyme activity. Similarly an increase in substrate concentration also increases the rate of enzyme action.


















Miller, S. B. (1992). Tested studies for laboratory teaching: Proceedings of the 6th             Workshop/Conference of the Association for Biology Laboratory Education (ABLE) (Vol.           6). Place of publication not identified: The Association.