Glutamine
Glutamine is the most abundant naturally occurring, nonessential amino acid in the human body, and one of the few amino acids that can directly cross the blood–brain barrier. Humans obtain glutamine through catabolism of proteins in foods they eat. In states where tissue is being built or repaired, like growth of babies, or healing from wounds or severe illness, glutamine becomes conditionally essential.
Glutaric aciduria type 1
While GCDH deficiency is a rare disease, GLO deficiency is the most common of metabolic diseases affecting children, limiting ascorbic acid biosynthesis to a minute fraction of what other non-primate species synthesize. It was thus called by OMIM (Online Mendeleian Inheritance in Man) a "public" error of metabolism. Ascorbic acid (Vitamin C) is a necessary cofactor for the utilization of lysine in collagen synthesis. Collagen, the most abundant protein in the human body, requires great amounts of lysine, the most abundant amino acids in proteins. Ascorbic acid, the main hydroxyl radical quencher, works as the cofactor providing the hydroxyl radical required to collagen cross-linking; lysine thus becomes hydroxylysine.