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.
PRR23C
Proline is predicted to be the most abundant amino acid in proline-rich protein 23C "Homo sapiens". In comparison to the prevalence of amino acids in other human proteins, it is predicted that proline-rich protein 23C "Homo sapiens" has a higher abundance of proline along with very low abundances of asparagine, threonine, and lysine. Orthologs for this protein are predicted to also have a high abundance of proline.