BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 6. AMINO ACID METABOLISM AND FUNCTIONS. PROTEIN BIOSYNTHESIS

6.3. Protein Digestion

6.3.2. Protein Digestion in the Intestine

Gastric contents (chyme) pass into the duodenum during Digestion. The low pH of the chyme triggers the release of the protein hormone secretin into the bloodstream. This hormone, in turn, stimulates the Pancreas to secrete pancreatic juice into the Small Intestine,

which contains HCO₃^-, neutralizing the HCl of gastric juice and inhibiting Pepsin. As a result, the pH rapidly rises from 1.5-2.0 to ~7.0.

The entry of Peptides into the small intestine stimulates the secretion of another protein hormone, cholecystokinin, which promotes the release of pancreatic Enzymes with an optimal pH of 7.5-8.0. Protein Digestion is completed through the action of enzymes produced by the pancreas and intestinal Cells.

The pancreas synthesizes protease zymogens: trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidases A and B. In the intestine, these are converted into active enzymes—Trypsin, Chymotrypsin, Elastase, and Carboxypeptidases A and B—via Limited proteolysis.

Trypsinogen activation. This process is mediated by enteropeptidase, an enzyme located in the intestinal epithelium. Enteropeptidase cleaves off the N-terminal hexapeptide Val-(Asp)₄-Lys from the trypsinogen molecule. The resulting conformational change in the shortened polypeptide chain forms the Active Site, yielding active trypsin. Activation of the zymogen by cleaving the Val-(Asp)₄-Lys sequence is characteristic of most known trypsinogens across various organisms, ranging from fish to humans.

The resulting trypsin activates chymotrypsinogen, producing several active enzymes (Fig. 6.3). Chymotrypsinogen consists of a single polypeptide chain containing 245 amino acid residues and five Disulfide Bonds. Trypsin cleaves the peptide bond between the 15th and 16th Amino Acids, generating active π-chymotrypsin. Subsequently, π-chymotrypsin cleaves off the dipeptide Ser(14)-Arg(15), yielding δ-chymotrypsin. Cleavage of the dipeptide Thr(147)-Arg(148) completes The formation of the stable active enzyme form, α-chymotrypsin, which consists of three polypeptide chains linked by disulfide bonds.

Other pancreatic protease zymogens (proelastase and procarboxypeptidases A and B) are also activated by trypsin through limited proteolysis, resulting in the formation of active elastase and carboxypeptidases A and B.

Protease Specificity. Trypsin preferentially hydrolyzes peptide bonds formed by the carboxyl groups of Arginine and Lysine. Chymotrypsins show the highest activity toward the carboxyl groups of aromatic amino acids (Phe, Tyr, Trp).

Carboxypeptidases A and B are zinc-containing enzymes that cleave C-terminal amino acid residues. Specifically, Carboxypeptidase A preferentially releases amino acids containing aromatic or hydrophobic side chains, whereas carboxypeptidase B targets arginine and lysine residues.

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Fig. 6.3. Activation of chymotrypsinogen

The final stage of digestion involves the Hydrolysis of small peptides by aminopeptidases and dipeptidases, which are synthesized in active form by intestinal cells. Aminopeptidases sequentially cleave N-terminal amino acids from the polypeptide chain. The best-known example is leucine aminopeptidase, a Zn²⁺- or Mn²⁺-containing enzyme that, despite its name, exhibits broad specificity toward N-terminal amino acids. Dipeptidases break down dipeptides into free amino acids but do not act on tripeptides.

Through the sequential action of all digestive proteases, the majority of dietary Proteins are broken down into free amino acids.