Lysine is a base, as are arginine and histidine. The ε-amino group often participates in hydrogen bonding and as a general base in catalysis. (The ε-amino group, which is attached to the NH₃⁺ group, is the fifth carbon down from the α-carbon, which is attached to the carboxyl (C=OOH) group.)

Common posttranslational modifications include methylation of the ε-amino group, giving methyl-, dimethyl-, and trimethyllysine. The latter occurs in calmodulin. Other posttranslational modifications at lysine residues include acetylation and ubiquitination. Collagen contains hydroxylysine, which is derived from lysine by lysyl hydroxylase. O-Glycosylation of hydroxylysine residues in the endoplasmic reticulum or Golgi apparatus is used to mark certain proteins for secretion from the cell.

It has been suggested that lysine may be beneficial for those with herpes simplex infections.^[31] However, more research is needed to fully substantiate this claim. For more information, refer to Herpes simplex - Lysine.

Lysine has a known anxiolytic action through its effects on serotonin receptors in the intestinal tract. One study on rats^[32] showed that overstimulation of the 5-HT4 receptors in the gut are associated with anxiety-induced intestinal pathology. Lysine, acting as a serotonin antagonist and therefore reducing the overactivity of these receptors, reduced signs of anxiety and anxiety-induced diarrhea in the sample population. Another study showed that lysine deficiency leads to a pathological increase in serotonin in the amygdala, a brain structure that is involved in emotional regulation and the stress response.^[33]

Human studies have also shown correlations between reduced lysine intake and anxiety. A population-based study in Syria included 93 families whose diet is primarily grain-based and therefore likely to be deficient in lysine. Fortification of grains with lysine was shown to reduce markers of anxiety, including cortisol levels, and also led to potentiation of benzodiazepine receptors (common targets of anxiolytic drugs such as Xanax and Ativan).^[34]

There are lysine conjugates that show promise in the treatment of cancer, by causing cancerous cells to destroy themselves when the drug is combined with the use of phototherapy, while leaving non-cancerous cells unharmed.^[35]

While chemically insignificant to lysine itself, it is worth noting that lysine is attached to dextroamphetamine to form the prodrug lisdexamfetamine (Vyvanse). In the gastrointestinal tract, the lysine molecule is cleaved from the dextroamphetamine, thereby making oral administration necessary.

According to animal studies, lysine deficiency causes immunodeficiency.^[36] One cause of relative lysine deficiency is cystinuria, where there is impaired hepatic resorption of basic, or positively charged amino acids, including lysine. The accompanying urinary cysteine results because the same deficient amino acid transporter is normally present in the kidney as well.

Limited studies suggest that a high-lysine diet or L-lysine monochloride supplements may have a moderating effect on blood pressure and the incidence of stroke.^[37]