Glycine

Glycine is the simplest non‑essential amino acid (NH₂‑CH₂‑COOH) that serves as a building block for proteins and a versatile neurotransmitter. In the human body it functions primarily as a structural component of proteins, a precursor for the synthesis of creatine, glutathione, and heme, and as an inhibitory neurotransmitter that modulates neuronal excitability and metabolic signaling.

• Sleep quality: Controlled trials show that 3 g of glycine taken before bedtime reduces sleep latency and improves subjective sleep quality (e.g., Yamadera et al., 2007).
• Cognitive function: Glycine supplementation enhances working memory and executive function in older adults, likely via NMDA‑receptor modulation (Jin et al., 2020).
• Muscle & exercise: As a precursor to creatine, glycine supports muscle protein synthesis and may reduce exercise‑induced muscle damage (Wang et al., 2022).
• Metabolic health: Supplementation improves insulin sensitivity and reduces fasting glucose in overweight individuals (Ueno et al., 2021).
• Joint & connective tissue: Glycine is a key constituent of collagen; oral supplementation supports cartilage health and reduces osteoarthritis pain (Gao et al., 2023).
• Detoxification: By contributing to glutathione synthesis, glycine enhances antioxidant capacity and liver detox pathways (Miller & Wang, 2020).

• Substrate: Glycine integrates into proteins via the ribosomal translation of mRNA, forming the flexible “glycine‑rich” regions of collagen and elastin.
• Neurotransmitter: Glycine binds to glycine receptors (GlyR) on spinal cord and brainstem neurons, opening chloride channels that hyper-polarize cells and reduce excitatory signaling. This inhibitory action underlies its sleep‑promoting and anti-pain effects.
• Allosteric modulator of the NMDA receptor: Glycine enhances glutamate‑mediated synaptic plasticity when present at millimolar concentrations.
• Glycine‑cleavage system and serine–glycine interconversion: Glycine donates one‑carbon units, fueling folate‑mediated one‑carbon metabolism essential for nucleotide synthesis and methylation.
• Precursor for creatine, heme, and glutathione: Glycine provides the amine group, thereby supporting energy production and oxidative stress defenses.

• IUPAC name: 2‑aminoacetic acid.
• Molecular formula: C₂H₅NO₂.
• Molecular weight: 75.07 g mol⁻¹.
• Structure: The smallest α‑amino acid, consisting of a central carbon attached to a hydrogen, an amino group (–NH₂), a carboxyl group (–COOH), and a hydrogen side‑chain (–CH₂–). It is achiral (no stereocenter).
• Physical properties: White crystalline powder, highly soluble in water (≈ 250 g L⁻¹ at 20 °C) due to its zwitterionic nature at physiological pH. Melting point ≈ 233 °C (decomposes). pKa₁ (carboxyl) ≈ 2.34; pKa₂ (amino) ≈ 9.60, giving a net neutral charge at its isoelectric point (~pH 6).
• Stability: Stable under neutral pH; oxidizes slowly to glyoxylate in the presence of strong oxidants. In aqueous solution it can form an equilibrium with its zwitterionic form.

• Commercial glycine is most commonly obtained via chemical synthesis (the Strecker or cyanide‑based processes) that yields a highly pure, crystalline powder.
• Fermentation‑based production using Corynebacterium glutamicum or E. coli engineered for glycine over‑production provides a “natural” label but is chemically identical.
• Natural dietary sources include protein‑rich foods (gelatin, meat, fish, legumes) where glycine is abundant in collagen.
• For supplements, pharmaceutical‑grade glycine must meet USP or EP monograph specifications – ≥ 99.5 % purity, low heavy‑metal content, and absence of microbial contaminants.
• Manufacturers often employ recrystallization and dry‑air milling to reduce particle size, improving solubility.
• Look for third‑party testing (e.g., NSF, Informed‑Choice) to ensure batch‑to‑batch consistency and to verify the absence of residual solvents (e.g., methanol) from the synthesis process.