Any living organism feeds on organic food, which is destroyed in the digestive system and is involved in cellular metabolism. And for a substance like protein, digestion means complete breakdown to its constituent monomers. This means that the main task of the digestive system is the destruction of the secondary, tertiary or domain structure of the molecule, and then the elimination of amino acids. Later, protein monomers will be carried by the circulatory system to the cells of the body, where new protein molecules necessary for life will be synthesized.
Enzymatic protein digestion
Protein is a complex macromolecule, an example of a biopolymer consisting of many amino acids. And some protein molecules consist not only of amino acid residues, but also of carbohydrate or lipid structures. Enzymatic or transport proteins may even contain a metal ion. More often than others, protein is present in foodmolecules found in animal meat. They are also complex fibrillar molecules with a long amino acid chain.
For the breakdown of proteins in the digestive system, there is a set of proteolysis enzymes. These are pepsin, trypsin, chemotrypsin, elastase, gastrixin, chymosin. The final digestion of proteins occurs in the small intestine under the action of peptide hydrolases and dipeptidases. This is a group of enzymes that break the peptide bond in strictly specific amino acids. This means that one enzyme is needed to break the peptide bond between the residues of the amino acid serine, and another is needed to cleave the bond formed by threonine.
Enzymes of protein digestion are divided into types depending on the structure of their active center. These are serine, threonine, aspartyl, glutamine and cysteine proteases. In the structure of their active center, they contain a specific amino acid, which gave them their name.
What happens to protein in the stomach?
Many people mistakenly say that the stomach is the main organ of digestion. This is a common misconception, since the digestion of food is partially observed already in the oral cavity, where a small part of the carbohydrates is destroyed. This is where partial absorption takes place. But the main processes of digestion do take place in the small intestine. At the same time, despite the presence of pepsin, chymosin, gastrixin and hydrochloric acid, digestion of proteins in the stomach does not occur. These substances under the action of the proteolytic enzyme pepsin and hydrochloric aciddenature, that is, lose their special spatial structure. Chymosin also curdles milk protein.
If we express the process of protein digestion as a percentage, then approximately 10% of the destruction of each protein molecule occurs in the stomach. This means that in the stomach, not a single amino acid breaks away from the macromolecule and is not absorbed into the blood. The protein only swells and denatures to increase the number of available sites for proteolytic enzymes to work in the duodenum. This means that under the action of pepsin, the protein molecule increases in volume, exposing more peptide bonds, which are then joined by proteolytic enzymes of pancreatic juice.
Protein digestion in the duodenum
After the stomach, processed and carefully ground food, mixed with gastric juice and prepared for further stages of digestion, enters the duodenum. This is the section of the digestive tract located at the very beginning of the small intestine. Here, further splitting of molecules occurs under the action of pancreatic enzymes. These are more aggressive and more active substances that can break up a long polypeptide chain.
Under the action of trypsin, elastase, chymotrypsin, carboxypeptidases A and B, a protein molecule is split into many smaller chains. In fact, after passing through the duodenum, the digestion of proteins in the intestine is just beginning. And ifexpressed as a percentage, then after processing the food bolus with pancreatic juice, the proteins are digested by about 30-35%. Their complete “disassembly” to their constituent monomers will be carried out in the small intestine.
Results of pancreatic protein digestion
Protein digestion in the stomach and duodenum is a preparatory step that is needed to break down macromolecules. If a protein with a chain length of 1000 amino acids enters the stomach, then the output from the duodenum will be, for example, 100 molecules with 10 amino acids each. This is a hypothetical figure, since the endopeptidases mentioned above do not divide the molecule into equal sections. The resulting mass will contain molecules with a chain length of 20 amino acids, and 10, and 5. This means that the crushing process is chaotic. Its goal is to maximally simplify the work of exopeptidases in the small intestine.
Digestion in the small intestine
For any high molecular weight protein, digestion is its complete destruction to the monomers that make up the primary structure. And in the small intestine, under the action of exopeptidases, the decomposition of oligopeptides into individual amino acids is achieved. Oligopeptides are the above-mentioned residues of a large protein molecule, consisting of a small number of amino acids. Their splitting is comparable in terms of energy costs with synthesis. Therefore, the digestion of proteins and carbohydrates is an energy-intensive process, as is the very absorption of the resulting amino acids by epithelial cells.
Walldigestion
Digestion in the small intestine is called parietal, as it takes place on the villi - the folds of the intestinal epithelium, where exopeptidase enzymes are concentrated. They attach to the oligopeptide molecule and hydrolyze the peptide bond. Each type of amino acid has its own enzyme. That is, to break the bond formed by alanine, you need the enzyme alanine-aminopeptidase, glycine - glycine-aminopeptidase, leucine - leucine-aminopetidase.
Because of this, protein digestion takes a long time and requires a large number of different types of digestive enzymes. The pancreas is responsible for their synthesis. Its function is affected in patients who abuse alcohol. But it is almost impossible to normalize the lack of enzymes by taking pharmacological preparations.
