Protein Digestion Process Overview

Protein Digestion

• Protein digestion begins in the stomach, where pepsins cleave some of the peptide linkages.
• Pepsins are secreted in the form of inactive precursors (proenzymes) and activated in the gastrointestinal tract.
• The pepsin precursors are called pepsinogens and are activated by gastric acid.

Pepsinogens

• Human gastric mucosa contains a number of related pepsinogens, which can be divided into two immunohistochemically distinct groups: pepsinogen I and pepsinogen II.
• Pepsinogen I is found only in acid-secreting regions, whereas pepsinogen II is also found in the pyloric region.
• Maximal acid secretion correlates with pepsinogen I levels.

Action of Pepsins

• Pepsins hydrolyze the bonds between aromatic amino acids (such as phenylalanine or tyrosine) and a second amino acid.
• The products of peptic digestion are polypeptides of very diverse sizes.
• Pepsins have a pH optimum of 1.6–3.2, and their action is terminated when the gastric contents are mixed with the alkaline pancreatic juice in the duodenum and jejunum.

Pancreatic and Intestinal Digestion

• In the small intestine, the polypeptides formed by digestion in the stomach are further digested by the powerful proteolytic enzymes of the pancreas and intestinal mucosa.
• Trypsin, the chymotrypsins, and elastase act at interior peptide bonds in the peptide molecules and are called endopeptidases.
• The pH of the intestinal contents in the duodenal bulb is 3.0–4.0, but rapidly rises; in the rest of the duodenum it is about 6.5.