Glycine (Gly, G(

The amino acid glycine is incorporated into the nascent polypeptide chain during protein biosynthesis in response to four codons—GGU, GGC, GGA, and GGG—and represents approximately 7.2% of the residues of the proteins that have been characterized. The glycyl residue incorporated has a mass of 57.05 Da, a van der Waals volume of 48 ų, and an accessible surface area of 85 Ų. Gly residues are changed during divergent evolution less frequently than average; when they are, they are interchanged in homologous proteins most frequently with alanine, serine, aspartic acid, and asparagine residues.

Gly is the simplest amino acid residue, with only a hydrogen atom for a side chain. Note that the a-carbon atom of Gly is not asymmetric, in contrast to the other amino acids incorporated into proteins, because it is bonded to two H atoms. Consequently, this amino acid does not occur as D or L isomers.

 The absence of a larger side chain gives the polypeptide backbone at Gly residues much greater conformational flexibility than at other residues. For example, about 61% of the possible values of the torsion angles f and y of a Ramachandran Plot are permitted with a Gly residue, in contrast to the 30% with other residues; moreover, the Ramachandran plot of allowed torsion angles is symmetric. This extra flexibility tends to decrease the overall average dimensions of unfolded polypeptide chains, as the Gly residues permit the chain to reverse directions more readily. This is also used in native protein structures, where Gly residues occur most frequently in reverse turns. The extra flexibility of the Gly residue in an unfolded polypeptide chain is believed to destabilize all fixed conformations, and Gly residues decrease the tendency to adopt an alpha-helical conformation.

The precursors of some peptide hormones, such as melanocyte stimulating hormone, have a Gly residue at the C-terminus that becomes converted to an amide group enzymatically).

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