Gly

Gly is the three-letter abbreviation for glycine, the simplest and smallest amino acid, and is represented by the one-letter code G in protein sequences. Glycine is unique among the standard amino acids in being achiral, owing to its single hydrogen as its side chain. At physiological pH, glycine exists as a zwitterion with a deprotonated carboxyl group and an protonated amino group; its carboxyl and amino pKa values are about 2.3 and 9.6, respectively.

In proteins, glycine is encoded by codons in the genetic code and is commonly written as Gly or G. Its lack of a bulky side chain provides high conformational flexibility, allowing glycine to occupy tight turns and regions of the protein backbone that are inaccessible to others. This property makes glycine frequent in loop regions and in collagen, where a glycine residue appears at every third position to sustain the triple helix.

Biologically, glycine serves several roles. It functions as an inhibitory neurotransmitter in the central nervous system by activating glycine receptors, and it can act as a co-agonist at NMDA receptors to modulate synaptic transmission. Glycine is a nonessential amino acid in humans, meaning the body can synthesize it, but it is also a building block in the synthesis of purines, porphyrins, and other biomolecules.

Biosynthesis and metabolism involve conversion from serine via serine hydroxymethyltransferase and participation in the mitochondrial glycine cleavage system. Glycine is a versatile component in protein structure and metabolism, underpinning both molecular architecture and physiological signaling.