About protein digestion

[Chemical structure of the peptide bond (bottom) and the three-dimensional structure of a peptide bond between an alanine and an adjacent amino acid (top/inset). The bond itself is made of the CHON elements.](By Chemistry-grad-student - Own work, CC BY-SA 3.0, Link)
It is interesting to look at digestion from the standpoint of the physical laws of nature. ON THE EXAMPLE of digesting proteins, we consider what the challenges facing the digestion.
Although the process of protein digestion is not fully understood and there are many white spots in it, the overall picture can still be presented. Protein digestion is a complex process, as, indeed, any process taking place in a living organism. And as far as I know, until now, man has not been able to repeat any of them in artificial performance.
Protein digestion takes place in several stages, continuously following one after the other. The main purpose of digestion is the disassembly of protein into its constituent parts. As you know, a protein consists of amino acids that are interconnected in a chain by a peptide bond. In total, the human body has, or rather, has in its structure 20 different amino acids. Moreover, some of them our body can synthesize itself, and some must come from outside. They are also called essential amino acids. In total, the human body contains about 15kg of protein. Every day in our body about 400 grams of protein breaks down into amino acids and the same amount is synthesized. Daily intake of protein 100gramm.

[Pepsin in complex with pepstatin. From PDB file 1PSO. Created with PyMol.](By No machine-readable author provided. DrKjaergaard assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims)., Public Domain, Link)
If we talk about the spatial form and structure of the protein, then there are four levels of protein organization. Primary is a chain of connected monomers, a peptide bond. The secondary structure is formed as a result of the interaction of local hydrogen bonds. The protein is already acquiring a different form than a simple chain. The ternary structure is indicated by the spatial form of the protein. Here covalent, ionic and hydrogen bonds, as well as hydrophobic interactions take part. The fourfold structure is formed by the mutual arrangement of several chains in one protein complex. As a result, we have a complex assembly (synthesis) of a protein from “simple” amino acids. To assemble such a complex set, you need to do a lot of work and naturally as much work is needed to break these bonds and disassemble the complex protein to amino acids, which later get through the thin intestinal walls into the bloodstream and rush to their destination.

The digestion process — hydrolysis of food proteins — or simply disassembly of a previously assembled mechanism for parts, occurs through enzymes secreted by the human body.
First, the protein must be disassembled to a simple chain consisting of amino acids. This is necessary to ensure full access of the enzymes to the chain by a tightly bound peptide bond. This process is called protein denaturation or more protein melting. Denaturation of the protein can occur under the action of temperature, pressure, and even shaking (for some proteins), chemical solvents and alkalis, exposure to radioactive radiation, which is known to cause radiation sickness and death of a living organism, ultrasound. In everyday life, we usually produce protein denaturation by heating. As a result of denaturation, the protein breaks down to the primary structure. In our body, denaturation occurs in the stomach. Where as it is known hydrochloric acid is emitted. Which, in addition to denaturation, still has a bactericidal effect on food, forms the PH of the medium 1.5-2, which is necessary for the existence of the enzyme pepsin, while the tech also activates this enzyme. Enzymes are produced in a watery place, and the work is done elsewhere. So that they do not destroy the organs that produce them, and everything that occurs along the way, as a rule, enzymes are produced in an inactive form. And only at the place of use are activated. If we talk about pepsin, which is involved in the digestion process in the first stage in the stomach, then there is protection from its effects on the stomach walls, in the form of secreted mucus from high PH 6-7 and the presence of protective substances in the membranes of stomach cells that are not. object of destruction (substrate) for pepsin.

[A tetrapeptide (example Val-Gly-Ser-Ala) with green marked amino end (L-Valine) and blue marked carboxyl end (L-Alanine).](By Jü - Own work, Public Domain, Link)
Pepsin de-aggregates the protein, destroying peptide bonds. As a result, the protein in the stomach was first unwound up the chain, and then the chain was disassembled into parts - peptides consisting of several residual amino acids. Our goal is individual amino acids.
Next, the food moves into the small intestine where, under the action of other enzymes already, it is broken down into amino acids. In the small intestine proteases of the pancreas trypsin, chymotrypsin, carboxypeptidases. Enzymes of the intestinal mucous membrane, aminopeptidase and dipeptidase, are also involved in the digestion of peptides. It is worth considering that the action of enzymes does not occur simultaneously. They act sequentially, like a conveyor. Gradually analyzing from difficult to simple. Due to the consistent effect on the protein molecule of all enzymes of the gastrointestinal tract, the protein is broken down into amino acids, which are subsequently absorbed into the blood.

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