Glycine is also co-generated as an impurity in the synthesis of EDTA, arising from reactions of the ammonia co-product.[28]
Chemical reactions
Its acid–base properties are most important. In aqueous solution, glycine is amphoteric: below pH = 2.4, it converts to the ammonium cation called glycinium. Above about pH 9.6, it converts to glycinate.
Glycine functions as a bidentate ligand for many metal ions, forming amino acid complexes.[29] A typical complex is Cu(glycinate)2, i.e. Cu(H2NCH2CO2)2, which exists both in cis and trans isomers.[30][31]
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.[38]
Degradation
Glycine is degraded via three pathways. The predominant pathway in animals and plants is the reverse of the glycine synthase pathway mentioned above. In this context, the enzyme system involved is usually called the glycine cleavage system:[36]
In the second pathway, glycine is degraded in two steps. The first step is the reverse of glycine biosynthesis from serine with serine hydroxymethyl transferase. Serine is then converted to pyruvate by serine dehydratase.[36]
The half-life of glycine and its elimination from the body varies significantly based on dose.[39] In one study, the half-life varied between 0.5 and 4.0 hours.[39]
Physiological function
The principal function of glycine is it acts as a precursor to proteins. Most proteins incorporate only small quantities of glycine, a notable exception being collagen, which contains about 35% glycine due to its periodically repeated role in the formation of collagen's helix structure in conjunction with hydroxyproline.[36][40] In the genetic code, glycine is coded by all codons starting with GG, namely GGU, GGC, GGA and GGG.[8]
Glycine conjugation pathway has not been fully investigated.[43] Glycine is thought to be a hepatic detoxifier of a number endogenous and xenobiotic organic acids.[44]Bile acids are normally conjugated to glycine in order to increase their solubility in water.[45]
In the US, glycine is typically sold in two grades: United States Pharmacopeia ("USP"), and technical grade. USP grade sales account for approximately 80 to 85 percent of the U.S. market for glycine. If purity greater than the USP standard is needed, for example for intravenous injections, a more expensive pharmaceutical grade glycine can be used. Technical grade glycine, which may or may not meet USP grade standards, is sold at a lower price for use in industrial applications, e.g., as an agent in metal complexing and finishing.[49]
Glycine is not widely used in foods for its nutritional value, except in infusions. Instead, glycine's role in food chemistry is as a flavorant. It is mildly sweet, and it counters the aftertaste of saccharine. It also has preservative properties, perhaps owing to its complexation to metal ions. Metal glycinate complexes, e.g. copper(II) glycinate are used as supplements for animal feeds.[27]
Glycine has been researched for its potential to extend life.[53][54] The proposed mechanisms of this effect are its ability to clear methionine from the body, and activating autophagy.[53]
Glycine is a significant component of some solutions used in the SDS-PAGE method of protein analysis. It serves as a buffering agent, maintaining pH and preventing sample damage during electrophoresis.[56] Glycine is also used to remove protein-labeling antibodies from Western blot membranes to enable the probing of numerous proteins of interest from SDS-PAGE gel. This allows more data to be drawn from the same specimen, increasing the reliability of the data, reducing the amount of sample processing, and number of samples required.[57] This process is known as stripping.
Presence in space
The presence of glycine outside the Earth was confirmed in 2009, based on the analysis of samples that had been taken in 2004 by the NASA spacecraft Stardust from comet Wild 2 and subsequently returned to Earth. Glycine had previously been identified in the Murchison meteorite in 1970.[58] The discovery of glycine in outer space bolstered the hypothesis of so-called soft-panspermia, which claims that the "building blocks" of life are widespread throughout the universe.[59] In 2016, detection of glycine within Comet 67P/Churyumov–Gerasimenko by the Rosetta spacecraft was announced.[60]
Glycine is proposed to be defined by early genetic codes.[62][63][64][65] For example, low complexity regions (in proteins), that may resemble the proto-peptides of the early genetic code are highly enriched in glycine.[65]
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^Berzelius J (1848). Jahres-Bericht über die Fortschritte der Chemie und Mineralogie (Annual Report on the Progress of Chemistry and Mineralogy). Vol. 47. Tübigen, (Germany): Laupp. p. 654. From p. 654: "Er hat dem Leimzucker als Basis den Namen Glycocoll gegeben. ... Glycin genannt werden, und diesen Namen werde ich anwenden." (He [i.e., the American scientist Eben Norton Horsford, then a student of the German chemist Justus von Liebig] gave the name "glycocoll" to Leimzucker [sugar of gelatine], a base. This name is not euphonious and has besides the flaw that it clashes with the names of the rest of the bases. It is compounded from γλυχυς (sweet) and χολλα (animal glue). Since this organic base is the only [one] which tastes sweet, then it can much more briefly be named "glycine", and I will use this name.)
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^Leng L, Yang L, Zu H, Yang J, Ai Z, Zhang W, et al. (November 2023). "Insights into glycine pyrolysis mechanisms: Integrated experimental and molecular dynamics/DFT simulation studies". Fuel. 351: 128949. Bibcode:2023Fuel..35128949L. doi:10.1016/j.fuel.2023.128949.
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