Glycine is the smallest and simplest of all amino acids — it consists of a single carbon atom attached to an amino group and a carboxyl group, making it the only amino acid that is not chiral and that does not have a side chain. Despite its structural simplicity, glycine is one of the most abundant amino acids in the human body — it constitutes approximately 33% of the amino acid content of collagen (the most abundant protein in the body, making up approximately 25-30% of total body protein), it is a critical neurotransmitter in the central nervous system, and it is a component of the creatine phosphocreatine energy system that powers muscle contraction. Glycine is classified as a non-essential amino acid, meaning that the body can synthesise it from serine and from choline, but under conditions of high collagen turnover (such as recovery from injury, during growth, during intense physical training, and during ageing), the demand for glycine may exceed the body capacity to synthesise it, making glycine a conditionally essential amino acid in these contexts.
Glycine and Collagen Synthesis
Collagen is synthesised by fibroblasts, chondrocytes, osteoblasts, and other cells of the mesenchymal lineage, and the process begins with the transcription of the collagen genes (COL1A1, COL1A2, COL2A1, COL3A1, and others) to produce pre-procollagen mRNA. The pre-procollagen protein is then translated on ribosomes, signal peptide is removed in the ER, and the pro-alpha chains undergo a remarkable series of post-translational modifications — including the hydroxylation of proline and lysine residues (by prolyl hydroxylase and lysyl hydroxylase, which require vitamin C as a cofactor), the glycosylation of hydroxylysine residues, and the assembly of the triple helix. Each amino acid in the collagen triple helix is in a specific position — every third position in the repeating sequence Gly-X-Y (where X is frequently proline and Y is frequently hydroxyproline) is occupied by glycine, because only the glycine residue is small enough to fit in the centre of the triple helix without causing steric hindrance. This glycine requirement is absolute — if any other amino acid is substituted for glycine in the triple helix, the triple helix is unstable and the collagen that is synthesised is defective. The importance of glycine for collagen synthesis is most clearly seen in the conditions that result from glycine deficiency or from mutations in the glycine residues of collagen genes — including the Ehlers-Danlos syndromes (which are characterised by joint hypermobility, skin hyperextensibility, and in some forms, by serious cardiovascular complications), where mutations in the glycine residues of collagen genes produce defective collagen that is unable to form stable triple helices.
Glycine and Neurotransmission
Glycine is one of the two primary inhibitory neurotransmitters in the mammalian central nervous system (the other is GABA), and it acts through the glycine receptor (GlyR), which is a ligand-gated chloride channel that is expressed on neurons in the spinal cord, the brainstem, and the brain. The glycine receptor is a member of the Cys-loop receptor superfamily (which includes the GABA-A receptor, the nicotinic acetylcholine receptor, and the 5-HT3 receptor), and it is composed of four transmembrane helices that form the chloride channel and of a extracellular ligand-binding domain that binds glycine. When glycine binds to the glycine receptor, the channel opens and chloride ions flow into the neuron, hyperpolarising the cell membrane and reducing the probability of action potential firing. The inhibitory effects of glycine are particularly important in the spinal cord, where glycine-mediated inhibition coordinates the timing of reflex arcs and of voluntary movement. The clinical importance of glycine neurotransmission is most clearly seen in the conditions that result from impaired glycine receptor function — including hyperekplexia (startle disease), which is characterised by an exaggerated startle response and by muscle stiffness that results from the loss of glycinergic inhibition in the spinal cord.
Practical Application
For general glycine supplementation, the evidence-based dose is 3-5g of glycine daily, taken before bed for its sleep-promoting effects or in divided doses throughout the day for its connective tissue supporting effects. Glycine is generally well-tolerated with no significant adverse effects at therapeutic doses, though very high doses can produce mild GI upset. For comprehensive connective tissue support, glycine pairs well with vitamin C (which is required for the hydroxylation of proline and lysine residues in the collagen molecule and which is the most important cofactor for collagen synthesis), with the other amino acids that are abundant in collagen (including proline, hydroxyproline, and lysine), with the minerals that are important for connective tissue health (including zinc, copper, and manganese, which are required for the cross-linking of collagen by lysyl oxidase), and with the omega-3 fatty acids (which have anti-inflammatory effects that reduce the collagen-degrading activity of the matrix metalloproteinases that are activated during inflammatory states). For sleep support, glycine (at 3g before bed) pairs well with magnesium (for muscle relaxation and for the reduction of neuronal excitability), with the B-complex vitamins (for the synthesis of the neurotransmitters that regulate sleep), and with the maintenance of good sleep hygiene (darkness, cool temperature, regular sleep-wake times).
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