General characteristics and stages of digestion
The biochemical essence of protein digestion lies in their sequential hydrolysis into free amino acids. This process is carried out by enzymes of the peptide hydrolase (peptidase) class, which are produced by the cells of the stomach, pancreas, and intestines.
The process starts in the gastric lumen and involves two key events:
- Protein denaturation. The acidic environment disrupts the complex spatial structure of dietary proteins, making peptide bonds accessible to enzymatic attack.
- Primary hydrolysis. Large protein molecules are cleaved into shorter fragments — oligopeptides.
Subsequently, these oligopeptides enter the small intestine. There, during luminal digestion, pancreatic enzymes break them down into di- and tripeptides. Final hydrolysis into free amino acids and their absorption occur at the stage of membrane (juxtaluminal) digestion, including inside enterocytes themselves.
Role of hydrochloric acid and acidity assessment
Gastric juice normally has a markedly acidic reaction (pH 1.5–2.0). This environment is due to the secretion of hydrochloric acid (HCl), which performs critical physiological functions. Firstly, it is responsible for the denaturation of dietary proteins and exhibits a potent bactericidal effect. Secondly, HCl triggers the activation of the zymogen pepsinogen and creates an optimal pH for the resulting active pepsin.
In clinical practice, gastric juice acidity is measured in titration units (TU). The analysis is performed by titrating 100 ml of gastric juice with a 0.1 M sodium hydroxide (NaOH) solution. The following parameters are distinguished:
- Total acidity (norm 40–60 TU) — the sum of all acid-reacting compounds in the secretion collected over one hour.
- Free hydrochloric acid (norm 20–40 TU) — HCl molecules that have not entered into chemical bonding with other substances.
- Bound hydrochloric acid (norm 20–30 TU) — HCl molecules bound to proteins and their hydrolysis products.
Pepsin specificity and intestinal peptidases
The gastric enzyme pepsin has strict substrate specificity. It hydrolyzes predominantly those peptide bonds in which the amino or carboxyl group of aromatic amino acids participates. In particular, it cleaves protein regions adjacent to Phenylalanine (Phe) and Tyrosine (Tyr). Another target of pepsin is the bond between Leucine and Glutamate (Leu–Glu).
Beyond the stomach, in the small intestine, the process continues with pancreatic enzymes, among which exopeptidases stand out — carboxypeptidases A and B.
- They are produced as inactive proenzymes (procarboxypeptidases A, B) and are activated by trypsin.
- Their task is to cleave amino acids from the C-terminus of the peptide chain according to the mechanism: —X—NH—CH(R)—COOH.
- Carboxypeptidase A is specific for amino acids with a hydrophobic radical.
- Carboxypeptidase B cleaves Lysine (Lys) or Arginine (Arg) residues.
Mucosal defense factors
Despite the presence of aggressive hydrochloric acid and the active proteolytic enzyme pepsin, gastric cells normally do not undergo autodigestion. The mucosa is reliably protected by three barrier mechanisms:
- Mucus. The epithelium is covered by a thick secretion containing heteropolysaccharides. They have high chemical resistance and are completely resistant to cleavage by peptide hydrolases.
- Bicarbonate secretion (HCO₃⁻). Epithelial cells continuously secrete bicarbonates, forming a buffer zone with a pH of 5.0–6.0 in the juxtaluminal layer. In such a less acidic environment, pepsin is inactive, which prevents membrane damage.
- Regeneration. Epithelial cells have a high capacity for rapid division and repair of damaged areas.
Briefly: answers to topical questions
What is the exact biochemical mechanism of pepsinogen activation?
The mechanism of activation of the zymogen pepsinogen, which is secreted into the gastric lumen upon food intake, involves two successive stages.
- Slow stage — activation is carried out by hydrochloric acid (HCl), which additionally creates the optimum pH for the enzyme's activity.
- Fast stage — proceeds autocatalytically under the action of the already formed active pepsin.
Thus, the hydrochloric acid of gastric juice initiates the activation process, after which pepsin independently accelerates its own formation.
Which ion pumps do parietal cells use to secrete hydrochloric acid?
Parietal (oxyntic) cells secrete hydrogen ions to form hydrochloric acid using the H⁺, K⁺-ATPase enzyme, which functions as a proton pump.
- Proton pump (H⁺, K⁺-ATPase) — transports hydrogen ions from the cell into the gastric lumen in exchange for potassium ions.
The formation of hydrochloric acid (HCl) itself occurs directly in the gastric lumen via the interaction of secreted hydrogen ions (H⁺) and chloride ions (Cl⁻).
To remember the specificity of pepsin, you can use the mnemonic: "Fenya-Tiran Leyku Glushit". It encodes the amino acids whose peptide bonds are cleaved in the stomach: Fenylalanine, Tyrosine, and the pair Leucine—Glutamate.
Class and exam questions
Will protein digestion be impaired if a major part of the stomach is resected?
No significant impairments in protein digestion will occur. This process is successfully compensated in the small intestine due to the action of pancreatic peptidases and enterocyte enzymes.
Why determine gastric juice acidity parameters?
The assessment of free, bound, and total acidity is used as a biochemical method for the differential diagnosis of various gastric diseases, along with endoscopy and biopsy.
What is the difference between carboxypeptidase A and carboxypeptidase B?
Both enzymes cleave C-terminal amino acids, but carboxypeptidase A cleaves amino acids with a hydrophobic radical, whereas carboxypeptidase B is specific for Arginine and Lysine.
What else to study on this topic
- Mechanism of pepsinogen activation by hydrochloric acid
- Luminal digestion and the role of pancreatic peptidases
- Membrane digestion inside enterocytes
- Intestinal compensatory mechanisms in peptic ulcer disease
- Instrumental diagnostics: X-ray, endoscopy, and biopsy
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