Glycine (Gly), is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon).  Amino acids are organic compounds that contain amino (‚ÄìNH2) and carboxyl (‚ÄìCOOH) functional groups, along with a side chain (R group) specific to each amino acid. Glycine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Glycine is found in all organisms ranging from bacteria to plants to animals.  It is classified as an aliphatic, non-polar amino acid and is the simplest of all amino acids. In humans, glycine is a nonessential amino acid, although experimental animals show reduced growth on low-glycine diets. The average adult human ingests 3 to 5 grams of glycine daily. Glycine is a colorless, sweet-tasting crystalline solid. It is the only achiral proteinogenic amino acid. Glycine was discovered in 1820 by the French chemist Henri Braconnot when he hydrolyzed gelatin by boiling it with sulfuric acid. The name comes from the Greek word glucus or "sweet tasting". Glycine is biosynthesized in the body from the amino acid serine, which is in turn derived from 3-phosphoglycerate. In the liver of vertebrates, glycine synthesis is catalyzed by glycine synthase (also called glycine cleavage enzyme). In addition to being synthesized from serine, glycine can also be derived from threonine, choline or hydroxyproline via inter-organ metabolism of the liver and kidneys. Glycine is degraded via three pathways. The predominant pathway in animals and plants is the reverse of the glycine synthase pathway. In this context, the enzyme system involved glycine metabolism is called the glycine cleavage system. The glycine cleavage system catalyzes the oxidative conversion of glycine into carbon dioxide and ammonia, with the remaining one-carbon unit transferred to folate as methylenetetrahydrofolate. It is the main catabolic pathway for glycine and it also contributes to one-carbon metabolism. Patients with a deficiency of this enzyme system have increased glycine in plasma, urine, and cerebrospinal fluid (CSF) with an increased CSF:plasma glycine ratio (PMID: 16151895). Glycine levels are effectively measured in plasma in both normal patients and those with inborn errors of glycine metabolism (http://www.dcnutrition.com/AminoAcids/). Nonketotic hyperglycinaemia (OMIM: 606899) is an autosomal recessive condition caused by deficient enzyme activity of the glycine cleavage enzyme system (EC 2.1.1.10). The glycine cleavage enzyme system comprises four proteins: P-, T-, H- and L-proteins (EC 1.4.4.2, EC 2.1.2.10, and EC 1.8.1.4 for P-, T-, and L-proteins). Mutations have been described in the GLDC (OMIM: 238300), AMT (OMIM: 238310), and GCSH (OMIM: 238330) genes encoding the P-, T-, and H-proteins respectively. Glycine is involved in the body's production of DNA, hemoglobin, and collagen, and in the release of energy. The principal function of glycine is as a precursor to proteins. Most proteins incorporate only small quantities of glycine, a notable exception being collagen, which contains about 35% glycine. In higher eukaryotes, delta-aminolevulinic acid, the key precursor to porphyrins (needed for hemoglobin and cytochromes), is biosynthesized from glycine and succinyl-CoA by the enzyme ALA synthase. Glycine provides the central C2N subunit of all purines, which are key constituents of DNA and RNA. Glycine is an inhibitory neurotransmitter in the central nervous system, especially in the spinal cord, brainstem, and retina. When glycine receptors are activated, chloride enters the neuron via ionotropic receptors, causing an inhibitory postsynaptic potential (IPSP).