The digestion of the protein part of the food begins in the stomach.
The semi-digested food from the mouth enters the stomach where it gets mixed with gastric juice that contains enzymes such as pepsin, mucous and HCl. Pepsin is produced by the chief cells of the gastric gland.
It is secreted in an inactive form of pepsinogen but in the presence of HCl, it is converted into active pepsin.
Pepsin breaks down proteins into proteoses or peptones and large polypeptides
Pepsin+ Proteins ⇢ Peptone, proteoses and polypeptides.
Final answer: The enzyme pepsin converts proteins into proteoses.