Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Metabolism of Simple Proteins
Protein Digestion
Protein Digestion in the Stomach

The Stomach provides an optimal environment for Protein Digestion. First, gastric juice contains the active enzyme Pepsin. Second, the presence of free Hydrochloric acid in gastric juice creates the ideal pH range (1.5–2.5) for pepsin activity. The critical role of hydrochloric acid in protein digestion deserves special emphasis: it converts inactive pepsinogen into active pepsin and establishes the optimal environment for pepsin function. In the presence of hydrochloric acid, Proteins undergo Swelling, partial Denaturation, and potentially the Hydrolysis of Conjugated Proteins. Furthermore, hydrochloric acid stimulates secretin production in the duodenum, accelerates iron absorption, and exerts a bactericidal effect.

Given the exceptional importance of hydrochloric acid in protein digestion, numerous attempts have been made to elucidate The Mechanism of its secretion in the stomach. While the exact details remain unclear, available data indicate that chloride ions, formed during the dissociation of sodium chloride in the Blood, diffuse across The Cell membrane and combine with hydrogen ions. These hydrogen ions are released via the dissociation of carbonic acid, which is synthesized within parietal Cells from metabolic end products—Н₂О and СО₂. The resulting hydrochloric acid is subsequently secreted by the parietal cells into the gastric lumen. The equilibrium of Сl^- ions between the blood and parietal cells is maintained by the efflux of negatively charged HCO₃^- ions from the cells into the blood in exchange for Сl^- ions moving from the blood into the cells. The involvement of ATP is presumed, as hydrochloric acid synthesis requires Energy Expenditure.

It should be noted that certain gastric disorders (typically inflammatory conditions) can impair hydrochloric acid secretion and, consequently, protein digestion.

Pepsin, which catalyzes the hydrolysis of peptide bonds formed by aromatic amino acid residues, cleaves virtually all natural proteins, with the exception of certain Keratins, protamines, Histones, and mucoproteins. This hydrolysis yields Peptides of varying sizes and, potentially, a small number of free Amino Acids. The gastric juice of infants, as well as the fourth-ventricle secretion of calves and other young ruminants, contains a highly active enzyme distinct from pepsin known as rennin. Rennin catalyzes milk clotting by converting soluble caseinogen into insoluble casein. In adults, this function is performed by pepsin. Despite its apparent simplicity, the exact mechanism of this process is not yet fully understood. It is hypothesized that rennin converts soluble milk caseinogen into paracasein, the calcium salt of which is insoluble and precipitates out of solution. Interestingly, removing Са²⁺ ions from milk prevents precipitation entirely. The presence of active rennin in the gastric juice of infants appears to be of major physiological significance: by coagulating milk—the primary food source at this age—it significantly slows the passage of insoluble casein through the digestive tract, thereby prolonging its exposure to proteinases.