Which of the following enzymes is stable at acidic pH?

Pepsin is an enzyme that breaks down proteins into smaller peptides (that is, a protease). It is produced in the stomach and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pepsin exhibits maximal activity at pH 2.0 and is inactive at pH 6.5 and above, however, pepsin is not fully denatured or irreversibly inactivated. Therefore, pepsin in the solution of up to pH 8.0 can be reactivated upon re-acidification. Lowering pH does not denature many proteins. Unfolding, yes; denaturation, no. Hydrogen bonds play an impoant role in stabilizing pepsin. Lowering pH does not necessarily break the hydrogen bond network. At least one aspayl residue in the catalytic site needs to be protonated for catalytic action, hence the function at low pH. We do not completely understand the stability of aspayl proteases or pepsin