The digestion of proteins begins in the stomach and is completed in the small intestine. The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The food that enters the stomach becomes acidic on mixing with this gastric juice.

The main components of gastric juice are hydrochloric acid, pepsinogen, mucus, and
rennin. Hydrochloric acid dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into pepsin. Pepsin is a protein- digesting enzyme. It is secreted in its inactive form called pepsinogen, which then gets activated by hydrochloric acid.

The activated pepsin then converts proteins into proteases and peptides.

$\mathrm{Proteins}\stackrel{\mathrm{Pepsin}}{\to }\mathrm{Proteases}+\mathrm{Peptides}$

Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin
plays an important role in the coagulation of milk.

$\mathrm{Prorennin}\stackrel{\mathrm{HCl}}{\to }\mathrm{Rennin}$
$\downarrow$
$\mathrm{Milk} \mathrm{casein}\to \mathrm{Paracasein}$