Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Protein Biochemistry
Protein Digestion and Amino Acid Absorption

Dietary Proteins are not digested in the Oral Cavity because saliva contains no hydrolytic Enzymes.

Chemical Digestion of proteins begins in The Stomach under the action of Proteolytic Enzymes (peptide Hydrolases) that cleave peptide bonds between Amino Acids:

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These enzymes are synthesized in an inactive form by the Cells of the gastric mucosa, Small Intestine, and Pancreas. This inactive form prevents self-digestion of proteins within the cells where they are synthesized, as well as the walls of the gastrointestinal tract.

In the stomach, Protein Digestion takes place with the participation of Pepsin, a gastric juice enzyme formed from inactive pepsinogen under METABOLISM/18.html">The Influence of Hydrochloric acid. Pepsin exhibits maximum enzymatic activity in a strongly acidic environment at pH 1–2. In addition, hydrochloric acid causes protein Swelling and partial Denaturation, which increases the surface area available for enzyme-protein interaction. All of this facilitates Protein Digestion in the stomach. Pepsin cleaves peptide bonds in protein molecules, resulting in The formation of high-molecular-weight Peptides and prosthetic groups (Fig. 95).

In the duodenum, the resulting peptides undergo further breakdown through the action of Pancreatic and Intestinal juice enzymes: Trypsin and Chymotrypsin. The pancreas produces an inactive enzyme, trypsinogen, which is converted into active trypsin by enterokinase, an enzyme of the small intestinal mucosa. Trypsin then acts on another inactive pancreatic juice enzyme, chymotrypsinogen, converting it into active chymotrypsin.

Fig. 95 Protein transformation during digestion and amino acid absorption

Trypsin and chymotrypsin exhibit maximum activity in a mildly alkaline environment at pH 7.8. They break down proteins (peptides and Polypeptides) into simpler compounds, namely low-molecular-weight peptides (oligopeptides) and a certain amount of free amino acids.

The final breakdown of low-molecular-weight peptides into amino acids takes place in the small intestine under the action of highly specific enzymes: aminopeptidases, Carboxypeptidases, and dipeptidases. Protein digestion, much like carbohydrate digestion, occurs not only in the intestinal lumen but also On the surface of mucosal cells (contact or membrane digestion). Relatively large protein molecules are primarily digested in the intestinal lumen, whereas smaller peptides are broken down on The Cell surface (between microvilli). The resulting free Amino Acids and some simple peptides are absorbed into the bloodstream via complex biochemical processes and delivered to The Liver and other Tissues.

Proteins that escape DIGESTION IN THE small intestine are degraded in the Large Intestine by peptidases synthesized by the local microflora. The Enzymes of the large intestinal microflora are capable of breaking down many dietary amino acids to produce various toxic substances, such as phenol, cresol, indole, hydrogen sulfide, mercaptans, etc. This MICROBIAL TRANSFORMATION OF amino acids in the large intestine is known as protein putrefaction. These toxic substances are absorbed into the bloodstream and transported to the liver, where they are detoxified. The entire process of protein digestion in the gastrointestinal tract takes on average 8–12 hours following a meal.

The intestinal Absorption of Amino acids can involve various mechanisms for their Transport Across the intestinal wall and capillaries, including osmosis, diffusion, and Active Transport. A special role in absorption belongs to the intestinal mucosal villi, which mediate ATP-dependent Amino Acid Transport coupled with The transport of sodium (Na⁺) or hydrogen (H⁺) ions.