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.
Titin
Titin is the third most abundant protein in muscle (after myosin and actin), and an adult human contains approximately 0.5 kg of titin. With its length of ~27,000 to ~33,000 amino acids (depending on the splice isoform), titin is the largest known protein. Furthermore, the gene for titin contains the largest number of exons (363) discovered in any single gene, as well as the longest single exon (17,106 bp).