Squirrels – one of the most important groups of macromolecules in the human body, presented in a whole variety of forms: cell receptors, signal molecules, structural elements, enzymes, oxygen and carbon dioxide carriers (hemoglobin) – and this is not a complete list. Protein is an integral part of bones, muscles, ligaments, serves for the growth and restoration of body tissues.
In addition to these functions, proteins can also be used as an energy source. An important feature of protein metabolism is the inability of the body to store them in reserve, therefore it is very important to constantly consume proteins with food.
Description of protein metabolism in the human body
Protein metabolism begins in the stomach. When protein-rich food enters the stomach, it is "greeted" by the enzyme pepsin and hydrochloric acid (HCl, 05%), which provides a pH level of 1.5 – 3.5, in which the proteins are denatured. Under the influence of pepsin, proteins break down into polypeptides and their constituent amino acids.
When chyme (food gruel) enters the small intestine, the pancreas secretes juice containing sodium bicarbonate (soda), which neutralizes hydrochloric acid. This helps protect the intestinal lining.
The body synthesizes the proteins it needs from the amino acids that we get from food, and the unnecessary proteins are converted into glucose or triglycerides and used to maintain energy or increase the body's energy reserve.
The small intestine also releases digestive hormones, including secretin and cholecystokinin, which stimulate further protein breakdown. Secretin also stimulates the secretion of pancreatic juice, which also produces most of the digestive enzymes, incl. protease, trypsin, chymotrypsin and elastase, which aid in the digestion of proteins.
Together, these enzymes "break down" complex proteins into individual amino acids that are transported through the intestinal mucosa and used to synthesize new proteins or convert to fats or acetyl coenzyme A and are used in the Krebs cycle.
Role of digestive enzymes and hormones in protein metabolism
Enzymes in the stomach and small intestine break down proteins into amino acids. HCl in the stomach promotes proteolysis, and hormones secreted by intestinal cells regulate the digestion process.
To prevent the breakdown of pancreatic and small intestinal proteins, the pancreas also produces inactive proenzymes that are only activated in the small intestine. In the pancreas, vesicles contain trypsin, chymitrypsin in the form of trypsinogen and chymotrypsinogen.
After entering the small intestine, an enzyme found in the walls of the small intestine (enterokinase) binds to trypsinogen and converts it into its active form – trypsin. After that, trypsin binds to chymotrypsinogen and converts it into the active form – chymotrypsin.
Trypsin and chymotrypsin break down large proteins into smaller peptides during proteolysis. These small peptides are broken down into their constituent amino acids, which are transported across the apical surface of the intestinal mucosa by amino acid transporters.
These transporters bind sodium and an amino acid and then carry it across the membrane. On the basal surface of the mucosal cells, sodium and amino acid are released. Sodium can be reused as a transporter and amino acids enter the bloodstream and are transported to the liver and to all body cells for protein synthesis.
Free amino acids are used to synthesize new proteins. In the case of an excess of amino acids, the body, having no mechanism for storing them, converts them into glucose or ketones, or breaks them down. As a result of the breakdown of amino acids, hydrocarbons and nitrogenous slags are formed. However, nitrogen in high concentrations is toxic, so during the ornithine cycle it is processed, which helps to eliminate nitrogen from the body.
Free amino acids are used to synthesize new proteins. In the case of an excess of amino acids, the body, having no mechanism for storing them, converts them into glucose or ketones, or breaks them down.
Ornithine cycle - urea cycle
Ornithine Cycle – it is a complex of biochemical reactions, as a result of which urea is formed from ammonium ions in order to prevent an increase in the concentration of ammonium in the body to a critical level. The cycle proceeds to a greater extent in the liver, and to a lesser extent – in the kidneys.
Before the start of the ornithine cycle, ammonium ions are formed as a result of the breakdown of amino acids due to the transfer of an amino group from an amino acid to a keto acid.
As a result of this transamination, a molecule necessary for the Krebs cycle is formed, and an ammonium ion, which enters the ornithine cycle and is excreted from the body, combines with CO2, resulting in the formation of urea and water. In turn, urea is excreted by by the kidneys as part of urine.
Amino acids can also be used as a source of energy, especially during fasting. Since the processing of amino acids produces metabolic intermediates, including pyruvic acid, acetyl-coenzyme A, acetoacetyl-CoA, oxaloacetate and alpha-ketoglutarate, amino acids can serve as a source of energy released during the Krebs cycle.
Thus, the amino acids formed as a result of protein metabolism are used either for the synthesis of proteins necessary for the body, or are used for energy, or are excreted as unnecessary, but are not stored in the body. Therefore, a sufficient amount of protein in the diet is very important for growth, tissue repair and maintenance of health.
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