lysines

Lysines refer to the amino acid lysine, commonly in its L- form (L-lysine), as well as to lysine residues within proteins and, rarely in nature, the D-lysine enantiomer. In biology, L-lysine is one of the 20 standard amino acids used to build proteins, while lysine residues are frequent sites for a variety of post-translational modifications.

Chemical properties and role in proteins: L-lysine has a side chain ending in an amino group, which gives the molecule a positive charge at physiological pH (the pKa of the side chain is about 10.5). It is classified as an essential amino acid in humans, meaning it must be obtained from the diet. It is abundant in animal-derived proteins and in some plant proteins, and is commonly limiting in cereal-based diets.

Biosynthesis and metabolism: Humans cannot synthesize lysine due to the absence of the necessary biosynthetic pathway; it must be obtained from the diet. Plants, fungi, and certain microorganisms can synthesize lysine via the diaminopimelate pathway. In human metabolism, lysine is considered strictly ketogenic; its degradation contributes acetyl-CoA and acetoacetyl-CoA rather than glucose.

Biological roles and modifications: Lysine residues in proteins carry a positive charge that facilitates interactions with negatively charged molecules such as DNA and phospholipids. Lysines are major sites of post-translational modification, including acetylation, methylation, ubiquitination, and sumoylation, which regulate gene expression, protein stability, and cellular localization. Lysine-rich regions can influence protein folding and binding.

Dietary considerations: A diverse diet provides lysine through animal products, dairy, eggs, legumes, and some grains. Cereals are relatively lysine-poor, so combining cereals with legume sources helps ensure adequate intake. The adult daily requirement is commonly estimated around 30 mg per kilogram of body weight per day.