Pepsin is an enzyme secreted in our stomach. Explain why pepsin would not function in the small intestine, which has a pH of 8.

It is because the pH in small intestine is too extreme for pepsin.

In stomach, it has a pH of around 2.0. It is the pH value of where the enzyme pepsin works the fastest and most efficient. We call pH 2.0 as the optimum temperature of pepsin.

Since enzymes are made from protein, where the conformation (shape) of them can be easily affected by extreme pH or high temperatures, if the enzyme is exposed to pH that are extreme comparing the optimum pH, the shape of the active site (where substrates are bind into) may be altered. This can affect the rate of enzymatic reaction. If the shape of active site changed so much that it can no longer bind into substrates, we call this enzyme denatured. It can no longer function.

Therefore, since pH 8.0 is very extreme to pH 2.0, pepsin would denature in such environment, causing it to lose its function.