Glycine

Glycine is the simplest of the 20 standard amino acids and is represented by the symbol Gly (G). Its chemical formula is C2H5NO2, with a molecular weight of about 75.07 g/mol. The side chain consists of a single hydrogen atom, making glycine non-chiral and the smallest amino acid. In proteins, glycine is often considered nonpolar, though its small size gives it particular flexibility in protein structures.

Glycine is encoded by the codons GGU, GGC, GGA, and GGG. It is a nonessential amino acid for humans, meaning it can be synthesized endogenously. It is abundant in collagen, where glycine appears at every third residue to enable the formation of the collagen triple helix, contributing to the structural integrity of connective tissues.

In the body, glycine serves multiple roles. It functions as an inhibitory neurotransmitter in the spinal cord and brainstem, acting on glycine receptors to increase chloride ion conductance and dampen neuronal activity. It also acts as a co-agonist at NMDA-type glutamate receptors, participating in excitatory neurotransmission in conjunction with glutamate. Glycine is a precursor in the biosynthesis of purines, porphyrins (and thus heme), and several other biomolecules, and it participates in the synthesis of creatine and glutathione. Metabolically, glycine is produced mainly from serine via serine hydroxymethyltransferase and is primarily degraded by the mitochondrial glycine cleavage system.

Dietarily, glycine is found in protein-rich foods and is also consumed as a dietary supplement. It is used as a buffering agent in some foods and pharmaceuticals and is highly water-soluble, existing predominantly as a zwitterion at physiological pH.