Sérine

Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), a carboxyl group (which is in the deprotonated −COO- form under biological conditions), and a side chain consisting of a hydroxymethyl group, classifying it as a polar amino acid. It can be synthesized in the human body under normal physiological circumstances, making it a nonessential amino acid. It is encoded by the codons UCU, UCC, UCA, UCG, AGU and AGC. This compound is one of the proteinogenic amino acids. 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 by Emil Cramer. 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 (an intermediate from glycolysis) to 3-phosphohydroxypyruvate and NADH by phosphoglycerate dehydrogenase (). Reductive amination (transamination) of this ketone by phosphoserine transaminase () yields 3-phosphoserine (O-phosphoserine) which is hydrolyzed to serine by phosphoserine phosphatase (). In bacteria such as E. coli these enzymes are encoded by the genes serA (EC 1.1.1.95), serC (EC 2.6.1.52), and serB (EC 3.1.3.3). Glycine biosynthesis: Serine hydroxymethyltransferase (SHMT = serine transhydroxymethylase) also catalyzes the reversible conversions of L-serine to glycine (retro-aldol cleavage) and 5,6,7,8-tetrahydrofolate to 5,10-methylenetetrahydrofolate (mTHF) (hydrolysis). SHMT is a pyridoxal phosphate (PLP) dependent enzyme. Glycine can also be formed from CO2, NH4+, and mTHF in a reaction catalyzed by glycine synthase. Industrially, L-serine is produced from glycine and methanol catalyzed by hydroxymethyltransferase.