Lysine (Lys) is one of 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 (formation of new proteins). According to its chemical structure and composition, lysine belongs to the basic amino acids, which also include histidine and arginine. Since all three amino acids consist of six carbon atoms and a basic group, they are called hexon bases. In lysine, the free amino group (NH2) in the side chain reacts as a base, especially if the pH is too low or acidic. If this is the case, the free NH2 group of lysine takes up protons (H+) from the environment and becomes NH3+. Through the proton bond, lysine increases the pH of the environment and at the same time acquires a positive charge. In this way, basic amino acids maintain the pH in the extracellular and intracellular space of the organism. Lysine is not produced by the human body itself and is therefore essential (necessary for life). In addition to lysine, eight other amino acids are considered essential, all of which must be supplied with food and cannot be replaced by other amino acids. While seven of the essential amino acids can be formed in intermediary metabolism from their corresponding alpha-keto acids by a transamination reaction, this is not the case with lysine and threonine. These are irreversibly transaminated and consequently are referred to as essential amino acids proper.
Digestion and intestinal absorption
Partial hydrolysis of dietary proteins begins in the stomach. Important substances for protein digestion are secreted from various cells in the gastric mucosa. The main cells produce pepsinogen, the precursor of the protein-cleaving enzyme pepsin. The parietal cells produce gastric acid (HCl), which promotes the conversion of pepsinogen to pepsin. In addition, HCl lowers gastric pH, which increases pepsin activity. Pepsin breaks down lysine-rich protein into low-molecular-weight cleavage 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 of potatoes has high amounts of lysine because the amino acid is dissolved from the potato protein by the action of heat. The soluble poly- and oligopeptides subsequently enter the small intestine, the site of the main proteolysis (protein digestion). Proteases (protein-cleaving enzymes) are formed in the acinar cells of the pancreas (pancreas). The proteases are initially synthesized and secreted as zymogens – inactive precursors. It is not until the small intestine that the zymogens are activated by enteropeptidases, calcium and the digestive enzyme trypsin. Enteropeptidases are enzymes produced by enterocytes (cells of the intestinal mucosa) and secreted 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 pancreatic secretion-derived zymogens. The most important proteases include endopeptidases and exopeptidases. Endopeptidases, such as trypsin, chymotrypsin, elastase, collagenase, and enteropeptidase, cleave proteins and polypeptides inside molecules, increasing the terminal attackability of proteins. Exopeptidases, such as carboxypeptidase A and B, and amino and dipeptidases, attack the peptide bonds of the chain end and can specifically cleave certain amino acids from the carboxy or amino end of protein molecules. They are referred to accordingly as carboxy- or aminopeptidases. Endopeptidases and exopeptidases complement each other in the cleavage of proteins and polypeptides due to their different substrate specificity. The endopeptidase trypsin specifically releases the basic amino acids lysine, arginine, histidine, ornithine and cystine at the C-terminal end of the peptide chain. Lysine is subsequently located at the end of the protein and is thus accessible for cleavage by carboxypeptidase B. This exopeptidase cleaves exclusively basic amino acids from oligopeptides. At the end of protein digestion, lysine is present either as a free amino acid or bound to other amino acids, in the form of di- and tripeptides.In free, unbound form, lysine is predominantly actively and electrogenically taken up in sodium cotransport into the enterocytes (mucosa cells) of the small intestine. The driving force of this process is a cell-value directed sodium gradient, which is maintained with the help of sodium/potassium ATPase. If lysine is still part of di- or tripeptides, these are transported into the enterocytes against a concentration gradient in 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. Intestinal absorption of lysine is almost complete at nearly 100%. However, there are differences in the speed of absorption. Essential amino acids, such as lysine, isoleucine, valine, phenylalanine, tryptophan, and methionine, are absorbed much more rapidly than nonessential amino acids. Compared to neutral amino acids, amino acids with a basic side group are absorbed into enterocytes at a much slower rate. The breakdown of dietary and endogenous proteins into smaller cleavage products is not only important for peptide and amino acid uptake into enterocytes, but also serves to resolve the foreign nature of the protein molecule and to preclude immunological reactions.
