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Protein Digestion and Absorption

  • The small intestine mucosa can only transport single amino acids or short polymers of two to three amino acids (i.e. dipeptides and tripeptides). Consequently, large macromolecular proteins must be enzymatically digested prior to absorption.
  • Overview
    • Digestion of proteins into single amino acids, dipeptides, and tripeptides is carried out by a variety of peptidases in both the stomach and the small intestine.
  • Stomach
    • Digestion of proteins begins in the stomach with pepsin which is secreted by gastric chief cells of oxyntic glands and is only active in the stomach's low pH environment.
  • Small Intestine Lumen
    • Pancreatic digestive enzymes perform the majority of protein digestion. The major proteolytic enzymes include trypsin, chymotrypsin, elastase, and carboxypeptidase. These enzymes of the exocrine pancreas digest proteins down to short chains of a few amino acids, termed "Oligopeptides".
  • Small Intestine Epithelium
    • The final stage of protein digestion occurs on the brush border of the small intestine epithelium. Here, membrane-bound peptidases complete digestion of oligopeptides to either single amino acids or dipeptides and tripeptides.
  • End Result
    • The end result of protein digestion is the production of single amino acids or dipeptides and tripeptides which are amenable to epithelial absorption.
  • Once fully digested, single amino acids, dipeptides, and tripeptides are transported, by secondary active transport past the enterocytes luminal membrane by a variety of symporters. Once inside the enterocyte any dipeptides and tripeptides are cleaved to form individual amino acids by a variety of cytosolic peptidases. Individual amino acids are then passively transported past the basolateral membrane and into the blood.