Protein digestion

Classification and enzyme specificity

Enzymes that degrade proteins are divided into two large groups based on the location of the attacked peptide bond within the molecule:

• Endopeptidases hydrolyze internal regions of the protein chain, away from its ends. These include gastric pepsin, as well as pancreatic juice enzymes — trypsin, chymotrypsin, and elastase.
• Exopeptidases split off amino acids strictly from the edges of the chain (N- or C-terminus) or hydrolyze the shortest fragments (di- and tripeptides). This group includes intestinal aminopeptidase and pancreatic carboxypeptidases A and B.

Each peptidase possesses relative substrate specificity. This means that the enzyme preferentially cleaves bonds between specific amino acid residues. For example, trypsin attacks sites formed by carboxyl groups of basic amino acids (Arg, Lys), while elastase targets small aliphatics (Gly, Ala). Pepsin and chymotrypsin specialize in aromatic residues (Tyr, Phe, Trp).

Stages of hydrolysis in the gastrointestinal tract

The digestion process proceeds in stages, with each section of the GI tract providing the appropriate environment and its own set of enzymes.

1. Stomach. Chief cells of the mucosa secrete inactive pepsinogen, and parietal cells secrete hydrochloric acid (HCl). Acid denatures dietary proteins and triggers the slow activation of pepsin. The environment here is strongly acidic (pH 1.5–2.0). Large molecules are broken down into shorter peptides.
2. Lumen of the small intestine. Pancreatic secretion enters here. The acidic chyme is neutralized by bicarbonates (HCO₃⁻). In a slightly alkaline environment (pH 7.0–7.8), pancreatic enzymes — trypsin, chymotrypsin, elastase, and carboxypeptidases — continue to break down peptides.
3. Small intestine epithelium. Aminopeptidases operate on the epithelial surface. Inside the enterocytes themselves, dipeptidases and tripeptidases cleave short fragments into free amino acids.

The final result of hydrolysis is free amino acids, which are absorbed from the epithelium into the blood.

Organ defense mechanisms and cascade activation

The spatial separation of sites of synthesis and enzyme activation is a crucial mechanism preventing self-digestion of secretory cells. Powerful peptidases are synthesized as proenzymes (zymogens).

Their activation occurs at the site of action via limited proteolysis — the cleavage of a small protective peptide, which exposes the active center.

In the intestine, this process proceeds as a cascade:

• Intestinal cells secrete the enzyme enteropeptidase. It cleaves a hexapeptide (composition: Val-(Asp)₄-Lys) from inactive trypsinogen.
• Active trypsin is formed.
• Next, trypsin becomes the central activator: via autocatalysis, it activates new molecules of trypsinogen and also converts chymotrypsinogen, proelastase, and procarboxypeptidase into their active forms.

Interestingly, the enzymes of the intestinal epithelium itself (aminopeptidases and dipeptidases) are synthesized directly in active form and do not require such a cascade. In pathological conditions (peptic ulcer disease, acute pancreatitis), premature intracellular activation of proenzymes can occur, leading to tissue autolysis.

Gastric juice in the diagnosis of pathologies

The biochemistry of gastric juice serves as an important diagnostic tool. Normally, its pH is 1.5–2.0, free HCl is present, and blood and lactic acid are absent.

Key disease markers:

• Lactic acid (lactate): its appearance indicates anaerobic glycolysis. This occurs during the intensive reproduction of lactic acid bacteria against the background of reduced HCl secretion (achlorhydria) or in malignant tumors.
• Blood: a characteristic sign of gastric ulcer or cancer.
• Achylia: a state of complete absence of hydrochloric acid and pepsin, in which the environmental pH shifts toward neutral (7.0).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Limited proteolysis as a universal mechanism of protein activation
• Specificity of proteases acting on peptides with specific examples (Lysine, Tyrosine bonds)
• Pathogenesis of acute pancreatitis upon premature trypsin activation
• The role of pancreatic juice bicarbonates
• Association of lactate in gastric juice with malignant tumors

Other topics in the "Amino acid metabolism" section

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