Protein digestion
Dietary, endogenous, microbial and shed mucosal cells proteins are capable of being hydrolyzed by endopeptidases, the proteolytic enzymes present in gastric , pancreatic and intestinal juices break the proteins at internal peptide points.
In the luminal phase of digestion, gastric proteases (eg. pepsin and rennin) and pancreatic proteases (eg. trypsin, chymotrypsin, carboxy peptidase a & b) yield oligopeptides and some amino acids.
These oligopeptides are further hydrolyzed by oligopeptidases in the glycocalyx to produce amino acids, di- and tri- peptides.
Luminal phase of digestion
The proteolytic enzymes are secreted from gastric glands and pancreas as inactive zymogens. In the stomach, pepsinogenand prorennin (chymosinogen) are activated by HCl.
Pepsin also autocatalytically activates pepsinogen.
Trypsinogen from pancreas is activated as trypsin by the action of enterokinase, an enzyme produced from duodenal mucosa.
Trypsin autocatalytically activates trypsinogen.
The active trypsin inturn activates other proteolytic pancreatic enzymes (Chymotrypsinogen and procarboxypeptidase a & B) .
The HCl of stomach also has hydrolytic properties on protein
Membranous phase of digestion
In the membranous phase of digestion, the exopeptidases present on the enterocyte apical membrane act at the ends of peptide chain to release free amino acids at the surface of mucous membrane.
Some long-chain peptides are incompletely hydrolysed leading to the production of di- and tri-peptides.
These di- and tri-peptides are easily absorbed by the epithelial cells; where they are subsequently hydrolysed by the intracellular peptidases, forming free amino acids.
Thus, the free amino acids are produced at two sites: on the surface of the enterocytes and the second within the cell.
| Enzyme | Source | Precursor | Activator | Action |
| Pepsin | Gastric glands | Pepsinogen | HCl & Pepsin | Endopeptidase |
| Chymosin (Rennin) | Gastric glands | Pro-rennin | HCl | Endopeptidase |
| Trypsin | Pancreas | Trypsinogen | Enteokinase & Trypsin | Endopeptidase |
| Chymotrypsin | Pancreas | Trypsinogen | Trypsin | Endopeptidase |
| Elastase | Pancreas | Pro-elastase | Trypsin | Endopeptidase |
| Carboxypeptidase A | Pancreas | Procarboxypeptidase A | Trypsin | Endopeptidase |
| Carboxypeptidase B | Pancreas | Procarboxypeptidase B | Trypsin | Endopeptidase |