Proteins are broken down during digestion into amino acids used by cells to create proteins or macromolecules such as DNA. Digestion of proteins in the mouth begins with chewing: the mechanical splitting of large pieces into smaller ones, and with the help of saliva.
When food enters the esophagus, the stomach secretes gastric juice and begins mechanical contractions – peristalsis. Gastric juice contains hydrochloric acid and the enzyme pepsin, which is necessary for the chemical digestion of protein. As a result, the digested mixture – chime – moves into the small intestine.
Chemical processes include the denaturation of proteins when their three-dimensional structure unfolds, and the polypeptide network is exposed, which also destroys the protein function. Denatured peptide bonds become more convenient for subsequent enzymatic digestion in the stomach.
The enzyme pepsin secreted by the cells of the stomach is activated by hydrochloric acid and breaks peptide bonds, which are converted into polypeptides. Proteins take longer to digest, so an individual feels full longer.
In the small intestine, thanks to the digestive juices secreted by the pancreas, the enzymes chymotrypsin and trypsin continue to break down polypeptides. Trypsin activates proteases, the enzymes with the function of protein digestion.
Together, these enzymes break down proteins into dipeptides, tripeptides, and individual amino acids. Cells on the surface of the small intestine secrete additional enzymes to speed up the breakdown of polypeptides.
Further, dipeptides, tripeptides, and individual amino acids are absorbed into the enterocytes of the small intestine, for which active transport systems are used. Inside enterocytes, dipeptides and tripeptides are broken down into individual amino acids and absorbed into the bloodstream. Undigested proteins enter the large intestine and are excreted in the feces.