Lysine

Lysine (abbreviated Lys or K) is an amino acid component of proteins synthesized by living beings. It has a hydrophilic character, it is one of the 9 essential amino acids for humans, and consequently it must be provided by the diet. Given the uneven distribution of lysine among different foods, some diets, especially those based on grains, may be deficient in it. This deficiency is usually of no consequence, since most diets contain an excess of protein. In this case, those for which there is enough lysine will simply be synthesized, and the rest of the excess amino acids will be metabolized to produce energy and urea.

Chemical Structure

It acts chemically as a base, like arginine and histidine, since its side chain contains a protonatable amino group that is often involved in hydrogen bonding or ionic bonding in protein chains, stabilizing the tertiary and quaternary structures. Since their degree of ionization depends on pH, these bonds are sensitive to changes in pH.

This amino group, in addition to providing a positive charge to proteins when it is protonated, is acetylated by specific enzymes, known as acetyltransferases. This acetylation is considered to be a post-translational modification, since it occurs after translation of the protein from mRNA. However, its most common post-translational modifications include methylation of the ε-amino group, resulting in methyl-, dimethyl-, and trimethyllysine. The latter occurs in calmodulin.

Collagen contains hydroxylysine, which is derived from lysine by the action of lysyl hydroxylase. O-glycosylation of lysine residues in the endoplasmic reticulum or Golgi apparatus is used to mark certain proteins for secretion from the cell.

Biosynthesis

As an essential amino acid, lysine is not synthesized in the body of animals and therefore must be ingested by animals as lysine or lysine-containing proteins. There are two known routes for the biosynthesis of this amino acid:

• The first is done in bacteria and higher plants, through diaminopimelic acid
• The second in most upper fungi, using α-aminoadipic acid.

In plants and microorganisms, lysine is synthesized from aspartic acid, which is first converted to β-aspartyl-semialdehyde. Cyclization generates dihydropicolinate, which is reduced to Δ1-piperidine-2,6-dicarboxylate. Ring opening of this heterocycle generates a series of derivatives of pimelic acid, which will ultimately generate lysine. Some of the enzymes involved in this biosynthesis are the following:

1. Aspartoquinasa
2. β-aspartate semialdehyde dehydrogenase
3. dihydropiclinate syntase
4. Δ1-piperidine-2,6-dicarboxylate dehydrogenase
5. N-succinil-2-amino-6-cetopimelate synthase
6. Succinil diaminopimelate aminotransferase
7. Succinil diaminopimelato desuccinilasa
8. Epimerase Diaminopimellate
9. Diaminopimelato descarboxilasa

Metabolism

Lysine is metabolized in mammals to acetyl-CoA, through initial transamination with α-ketoglutaramate. Bacterial degradation of lysine results in cadaverine, through a decarboxylation process.

Summary

Synthetic, racemic lysine has been known for a long time. A practical synthesis begins from caprolactam.

Dietary sources

Nutritional requirements for lysine are 1.5 g per day. Animal proteins are good sources of lysine, both meat and dairy products (milk and cheese), fish and eggs. Cereal proteins are deficient in this amino acid, which, on the other hand, is very abundant in legumes and nuts.

Lysine is easily altered when the proteins that contain it are heated in the presence of reducing sugars (monosaccharides and some oligosaccharides) in a glycosylation reaction known as the Maillard reaction. This reaction is responsible for toasted colors and odors in food (bread crust, for example). The lysine that has been bound to the sugar is biologically unavailable, although according to most analytical techniques it is still present. To know the amount of lysine available, specific analyzes must be carried out. This alteration is especially important in milk intended for infant feeding. It can also be altered by treatments in an alkaline medium at high temperature, forming lysine-alanine bridges, which reduce the digestibility and biological value of proteins. This alteration can occur when handling proteins for industrial use, such as soy protein or caseinates.

Role in animal nutrition

Lysine is an essential amino acid, limiting for many animal species of zootechnical importance when diets based on cereals such as corn are used. As such, the concept of 'limiting amino acid' is used when balancing livestock feed formulations. to incorporate the correct amount into the diet based on the lysine content of the various protein foods available and the possible use of synthetic lysine. The latter is common and very economical to do when feeding pigs and other species of zootechnical interest. In the case of dairy cattle nutrition, lysine is also limiting along with methionine, but synthetic lysine cannot be used directly because microbial fermentation in the rumen destroys it extensively; however, there are already options for commercial products with a chemical protection that prevents said degradation, although the economic use of said alternative is still disputed. The best option is the use of protein sources rich in lysine, such as soybeans.

In popular culture

This amino acid is mentioned in the science fiction novel "Jurassic Park" by Michael Crichton and in its film adaptation developed by American filmmaker Steven Spielberg. In this work of fiction, the genetic engineering company InGen uses this amino acid as a population control system for cloned dinosaurs, manufacturing the animals so that they are incapable of synthesizing it, with the intention that they will not survive more than seven days if They are not provided in the diet. However, since the dinosaurs were able to obtain lysine through their food plants, InGen's strategy failed and the animals survived and reproduced uncontrollably, being one of the reasons that led to chaos in the park.