What is the synthesis of lysine

Lysine(Lys) is assigned to the 21 L-amino acids that are regularly incorporated into proteins. For this reason, lysine is called proteinogenic and is essential for the biosynthesis of proteins and the maintenance of muscle and connective tissue. A deficit of lysine can impair protein biosynthesis (new formation of proteins) [1].

According to its chemical structure and composition, lysine is one of the basic amino acids, to whom too Histidine and Arginine counting. Since all three amino acids consist of six carbon atoms and one basic group, they are called Hexon bases designated.
In the case of lysine, the free amino group (NH2) in the side chain as a base, especially if the pH value is too low or acidic. If that is the case, take it free NH2-Group of lysine protons (H +) from the environment and will to NH3+. By binding protons, lysine increases the pH value of the environment and at the same time receives a positive charge.
In this way, the basic amino acids maintain the pH value in the extra- and intracellular space of the organism [10].

Lysine is not produced by the human body itself and is therefore essential (vital). In addition to lysine, eight other amino acids are considered essential, all of which must be taken in with food and cannot be replaced by other amino acids [10].
While seven of the essential amino acids in the intermediate metabolism can be formed from their corresponding alpha-keto acids through a transamination reaction, this is the case with lysine and Threonine not the case. These are irreversibly transaminated and consequently referred to as the actual essential amino acids [4].

Digestion and Intestinal Absorption

The partial hydrolysis of food proteins begins in the stomach. Important substances for protein digestion are secreted from various cells of the gastric mucosa. The main cells produce pepsinogen, the precursor of the protein-splitting enzyme pepsin. The parietal or parietal cells produce gastric acid (HCl), which promotes the conversion of pepsinogen to pepsin. In addition, HCl lowers the pH value in the stomach, which increases pepsin activity.

Pepsin breaks down lysine-rich protein into low-molecular-weight breakdown products such as poly- and oligopeptides. Good natural sources of lysine include whey, egg, meat, soy, wheat germ, lentil and amaranth protein, as well as casein. In addition, the cooking water from potatoes has high levels of lysine, as the amino acid is detached from the potato protein by the action of heat [1].

The soluble poly- and oligopeptides then reach the small intestine, the location of the main ones Proteolysis (Protein digestion). Be in the acinar cells of the pancreas (pancreas) Proteases (protein-splitting enzymes). The proteases are first synthesized and secreted as zymogens - inactive precursors. The zymogens are only activated in the small intestine Enteropeptidases, Calcium and the digestive enzyme Trypsin.
Enteropeptidases are enzymes that are formed by the enterocytes (cells of the intestinal mucosa) and released when food protein arrives.
Together with calcium, they lead to the conversion of trypsinogen to trypsin in the intestinal lumen, which in turn is responsible for the activation of other zymogens derived from the pancreatic secretion [3, 5, 8, 10, 23, 24].

The most important proteases are the endo- and exopeptidases. Endopeptidasessuch as trypsin, chymotrypsin, elastase, collagenase and enteropeptidase, split proteins and polypeptides inside the molecules, which increases the terminal vulnerability of the proteins. Exopeptidasessuch as carboxypeptidase A and B as well as amino and dipeptidases attack the peptide bonds of the chain end and can specifically split off certain amino acids from the carboxy or amino end of the protein molecules. They are correspondingly referred to as carboxy or aminopeptidases. Endo- and exopeptidases complement each other due to their different substrate specificity when cleaving proteins and polypeptides.

Through the Endopeptidase trypsin the basic amino acids lysine, arginine, histidine, ornithine and cystine are specifically released at the C-terminal end of the peptide chain. Lysine is then located at the end of the protein and is therefore accessible for cleavage Carboxypeptidase B. This exopeptidase only cleaves basic amino acids from oligopeptides [3, 5, 23].

At the end of protein digestion, lysine is either available as a free amino acid or bound to other amino acids in the form of di- and tripeptides [10].
In its free, unbound form, lysine is mainly actively and electrogenically absorbed in sodium cotransport into the enterocytes (mucosal cells) of the small intestine. The driving force of this process is a cell level directed sodium gradient, which is maintained with the help of the sodium / potassium ATPase.
If lysine is still part of di- or tripeptides, these are transported into the enterocytes against a concentration gradient in the H + cotransport. Intracellularly, the peptides are broken down by amino- and dipeptidases into free amino acids, including lysine.
Lysine leaves the enterocytes via various transport systems along the concentration gradient and is transported to the liver via the portal blood.

The intestinal absorption of lysine is almost complete at almost 100%. However, there are differences in the rate of absorption. Essential amino acids such as lysine, isoleucine, valine, phenylalanine, tryptophan and methionine are absorbed much faster than non-essential amino acids. Compared to the neutral amino acids, the amino acids with a basic side group are taken up into the enterocytes at a significantly slower rate [3, 5, 8, 10, 23, 24].

The splitting of food proteins and endogenous proteins into smaller breakdown products is not only important for the uptake of peptides and amino acids in the enterocytes, but also serves to resolve the alien character of the protein molecule and to exclude immunological reactions [10].


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