Glutamine
In human blood, glutamine is the most abundant free amino acid.
Threonine
As an essential amino acid, threonine is not synthesized in humans, and needs to be present in proteins in the diet. Adult humans require about 20 mg/kg body weight/day. In plants and microorganisms, threonine is synthesized from aspartic acid via α-aspartyl-semialdehyde and homoserine. Homoserine undergoes "O"-phosphorylation; this phosphate ester undergoes hydrolysis concomitant with relocation of the OH group. Enzymes involved in a typical biosynthesis of threonine include: