Serine

Serine (abbreviated as Ser or S) is an amino acid with the formula HO2CCH(NH2)CH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.

Occurrence and biosynthesis
This compound is one of the naturally occurring proteinogenic amino acids. Its codons are UCU, UCC, UCA, UCG, AGU and AGC. Only the L -stereoisomer appears naturally in proteins. It is not essential to the human diet, since it is synthesized in the body from other metabolites, including glycine. Serine was first obtained from silk protein, a particularly rich source, in 1865. Its name is derived from the Latin for silk, sericum. Serine's structure was established in 1902.

The biosynthesis of serine starts with the oxidation of 3-phosphoglycerate to 3-phosphohydroxypyruvate and NADH. Reductive amination of this ketone followed by hydrolysis gives serine. Serine hydroxymethyltransferase catalyzes the reversible, simultaneous conversions of L -serine to glycine (retro-aldol cleavage) and 5,6,7,8-tetrahydrofolate to 5,10-methylenetetrahydrofolate (hydrolysis).

This compound may also be naturally produced when UV light illuminates simple ices such as a combination of water, methanol, hydrogen cyanide, and ammonia, suggesting that it may be easily produced in cold regions of space.

Production
Industrially, L-serine is produced by fermentation, with an estimated 100-1000 tonnes per year produced. In the laboratory, racemic serine can be prepared from methyl acrylate via several steps:
 * Synthesis of dl-serine.png

Metabolic


Serine is important in metabolism in that it participates in the biosynthesis of purines and pyrimidines. It is the precursor to several amino acids including glycine and cysteine, and tryptophan in bacteria. It is also the precursor to numerous other metabolites, including sphingolipids and folate, which is the principal donor of one-carbon fragments in biosynthesis.

Structural role
Serine plays an important role in the catalytic function of many enzymes. It has been shown to occur in the active sites of chymotrypsin, trypsin, and many other enzymes. The so-called nerve gases and many substances used in insecticides have been shown to act by combining with a residue of serine in the active site of acetylcholine esterase, inhibiting the enzyme completely.

As a constituent (residue) of proteins, its side chain can undergo O-linked glycosylation, which may be functionally related to diabetes.

It is one of three amino acid residues that are commonly phosphorylated by kinases during cell signaling in eukaryotes. Phosphorylated serine residues are often referred to as phosphoserine.

Serine proteases are a common type of protease.

Signaling
D -Serine, synthesized in the brain by serine racemase from L -serine (its enantiomer), serves as both a neurotransmitter and a gliotransmitter by activating NMDA receptors, making them able to open if they then also bind glutamate. D -serine is a potent agonist at the glycine site of the NMDA-type glutamate receptor. For the receptor to open, glutamate and either glycine or D -serine must bind to it. In fact, D -serine is a more potent agonist at the glycine site on the NMDAR than glycine itself. D -serine was only thought to exist in bacteria until relatively recently; it was the second D amino acid discovered to naturally exist in humans, present as a signalling molecule in the brain, soon after the discovery of D -aspartate.  Had D amino acids been discovered in humans sooner, the glycine site on the NMDA receptor might instead be named the D -serine site.