The process of mechanical and chemical break down of ingested complex food material into simpler soluble and absorbable molecules is called digestion. In man, it starts from mouth and continues uptill the small intestine.
The digestion of protein in man starts in stomach. The gastric juice of stomach contains pepsin enzyme. It converts complex protein molecules into smaller molecules–Peptones.
In duodenum, pancreatic juice enzyme Trypsin also acts upon protein molecules to convert it into Peptides and Peptones.
In ileum (a part of small intestine) intestinal juice enzymes convert simple molecules of proteins into amino acids. Proteins are absorbed as amino acids by the intestinal villi.