Glutamate (neurotransmitter)
Glutamate is a major constituent of a wide variety of proteins; consequently it is one of the most abundant amino acids in the human body. Under ordinary conditions enough is obtained from the diet that there is no need for any to be synthesized. Nevertheless, glutamate is formally classified as a non-essential amino acid, because it can be synthesized from alpha-Ketoglutaric acid, which is produced as part of the citric acid cycle by a series of reactions whose starting point is citrate. Glutamate cannot cross the blood-brain barrier unassisted, but it is actively transported into the nervous system by a high affinity transport system, which maintains its concentration in brain fluids at a fairly constant level.
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.