Protein digestion and absorption

Gastric phase. HCl denatures proteins, and pepsin, an endopeptidase working best at about pH 2, cleaves them into large polypeptides. Pepsin accounts for about 10–20% of protein digestion and is important for digesting collagen in meat. Protein digestion can still be completed without it.

Pancreatic phase. In the duodenum, brush-border enteropeptidase (enterokinase) converts trypsinogen to trypsin. Trypsin then activates more trypsinogen and the other zymogens. The endopeptidases cleave interior bonds: trypsin after basic residues (Arg, Lys), chymotrypsin after aromatic residues (Phe, Tyr, Trp), and elastase after small neutral residues. The exopeptidases remove C-terminal residues: carboxypeptidase A takes neutral and aromatic residues, and carboxypeptidase B takes basic ones. The result is a mix of oligopeptides and free amino acids.

Brush-border and cellular phase. Aminopeptidases, dipeptidyl peptidases and dipeptidases reduce oligopeptides to di- and tripeptides and free amino acids. Free amino acids enter enterocytes on several carriers, mostly Na⁺-coupled, with separate systems for neutral, basic, acidic and imino amino acids. Di- and tripeptides enter faster, on the H⁺-coupled transporter PepT1; NHE3 maintains the H⁺ gradient. Cytosolic peptidases hydrolyse them, and amino acids leave the basolateral membrane into the portal blood.

Digestion is extremely efficient. Endogenous protein from secreted enzymes and shed cells is digested and recycled along with dietary protein, and over 95% is absorbed, mainly in the duodenum and jejunum.