Reference
Glycine
**Glycine** is the smallest and structurally simplest of the twenty proteinogenic amino acids, carrying only a single hydrogen atom as its side chain. It is classified as non-essential because the human body can synthesize it, and it is the most abundant amino acid in collagen, the main structural protein of connective tissue.
Structure and metabolic role
Because its side chain is just one hydrogen atom, glycine is the only amino acid that is not chiral, and its small size lets it occupy tight turns in protein folding where larger residues cannot fit. This property is what makes glycine indispensable to the triple-helix architecture of collagen, where it recurs at every third position in the chain.
Beyond its structural role, glycine is a precursor in several biosynthetic pathways. It is one of the three amino acids — alongside glutamate and cysteine — used to make glutathione, the body’s main intracellular antioxidant tripeptide. Glycine also contributes, with arginine and methionine, to the synthesis of creatine, and it functions as an inhibitory neurotransmitter in the spinal cord and brainstem.
Glycine in collagen and dietary proteins
Collagen is glycine-rich but nutritionally incomplete: it contains no tryptophan and is low in the sulfur amino acids. In one analyzed collagen sample, tryptophan content was 0.00 g/100 g, which is why collagen is categorized as an incomplete protein under the PDCAAS method (Paul, Leser & Oesser, Nutrients, 2019, PMID: 31096622). The same analysis reported collagen’s combined cysteine plus methionine at just 0.72 g/100 g, with modest leucine (2.51 g/100 g) and threonine (1.96 g/100 g). A high glycine fraction therefore does little to offset the missing indispensable amino acids, and only a limited share of total daily protein can be replaced by collagen while still meeting amino acid requirements.
Relevance to potato protein
Glycine is present in plant proteins as well, though no single plant source mirrors collagen’s unusually high glycine proportion. Unlike collagen, a complete protein such as a potato protein isolate supplies all of the indispensable amino acids, including the tryptophan and sulfur amino acids that collagen lacks. For readers comparing structural proteins with food proteins, the broader background is covered in our guide to what potato protein is. Glycine itself remains a building block the body can produce, so dietary protein quality is judged primarily on the indispensable amino acids rather than on non-essential residues like glycine.
Related terms