Glutamine
In human blood, glutamine is the most abundant free amino acid.
Serine
-serine was thought to exist only in bacteria until relatively recently; it was the second amino acid discovered to naturally exist in humans, present as a signaling molecule in the brain, soon after the discovery of -aspartate. Had amino acids been discovered in humans sooner, the glycine site on the NMDA receptor might instead be named the -serine site. Apart from central nervous system, -serine plays a signaling role in peripheral tissues and organs such as cartilage, kidney and corpus cavernosum.