Serine

Serine is a polar, aliphatic amino acid with a hydroxyl group in its side chain. In proteins it is almost always found in the L- form, which is incorporated during ribosomal protein synthesis. The free amino acid exists as a zwitterion at physiological pH.

Humans synthesize serine via the phosphoserine pathway from 3-phosphoglycerate, making it a nonessential amino acid under normal conditions. It can be converted to glycine and tetrahydrofolate derivatives by serine hydroxymethyltransferase, contributing to one-carbon metabolism. Serine can also be deaminated to pyruvate by serine dehydratase or participate in transsulfuration to support cysteine production, linking serine metabolism to energy production and sulfur amino acid pathways.

In cellular biology, serine residues in proteins are common sites for phosphorylation, a key mechanism for regulating signaling pathways. The hydroxyl side chain enables hydrogen bonding and serves as a site for glycosylation in certain proteins. Serine also plays a role in enzyme catalysis in serine proteases, where the serine residue acts as a nucleophile in peptide bond cleavage within active sites.

The D- form of serine is present in small amounts in the brain and functions as a neuromodulator by acting as a co-agonist at NMDA-type glutamate receptors, influencing neurotransmission.

Dietarily, serine is found in many protein-rich foods such as meat, eggs, dairy products, soy, and grains. Because the body can synthesize serine, dietary requirements are typically modest, and serine is considered nonessential for healthy adults.