Serine

Serine is a small, polar amino acid with a hydroxyl (–OH) group on its side chain. It is encoded by six codons (UCU, UCC, UCA, UCG, AGU and AGC), and it is made by the body from an intermediate of glycolysis, so it is not essential in the diet.

The hydroxyl group can form hydrogen bonds and can serve as a nucleophile or as an attachment point. Cells make serine from 3-phosphoglycerate, which comes from glycolysis, and the amino acid is a source of one-carbon units for building nucleotides and for the synthesis of glycine and cysteine. It also forms the head group of the membrane lipid phosphatidylserine.

In serine proteases such as trypsin and chymotrypsin, a serine side chain is activated by a nearby histidine and aspartate, a set called the catalytic triad, to cut peptide bonds. Nerve agents and some insecticides act by blocking a catalytic serine in the enzyme acetylcholinesterase. The chemistry shows how amino acid side chains do the work in enzymes.

Serine is one of three amino acids, with threonine and tyrosine, that can be modified by phosphorylation, the attachment of a phosphate group by kinases. This adds negative charge and changes the protein's shape and activity, and it is one of the main ways of passing signals within cells. Serine and threonine residues can also carry sugars, as in O-linked glycosylation.

• Threonine — a similar hydroxyl-bearing amino acid
• Phosphorylation — a modification at its hydroxyl
• Protease — enzymes with a catalytic serine