Although glycine can be isolated from hydrolyzed protein, this route is not used for industrial production, as it can be manufactured more conveniently by chemical synthesis.[18] The two main processes are amination of
chloroacetic acid with
ammonia, giving glycine and
ammonium chloride,[19] and the
Strecker amino acid synthesis,[20] which is the main synthetic method in the United States and Japan.[21] About 15 thousand
tonnes are produced annually in this way.[22]
Glycine is also cogenerated as an impurity in the synthesis of
EDTA, arising from reactions of the ammonia coproduct.[23]
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 9.6, it converts to glycinate.
Glycine functions as a
bidentate ligand for many metal ions, forming
amino acid complexes. A typical complex is Cu(glycinate)2, i.e. Cu(H2NCH2CO2)2, which exists both in cis and trans isomers.
As a bifunctional molecule, glycine reacts with many reagents. These can be classified into N-centered and carboxylate-center reactions.
Metabolism
Biosynthesis
Glycine is not
essential to the human diet, as it is biosynthesized in the body from the amino acid
serine, which is in turn derived from
3-phosphoglycerate, but one publication made by supplements sellers seems to show that the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis.[26] In most organisms, the enzyme
serine hydroxymethyltransferase catalyses this transformation via the cofactor
pyridoxal phosphate:[27]
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.[29]
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:[27]
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.[27]
The half-life of glycine and its elimination from the body varies significantly based on dose.[30] In one study, the half-life varied between 0.5 and 4.0 hours.[30]
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.[27][31] In the
genetic code, glycine is coded by all
codons starting with GG, namely GGU, GGC, GGA and GGG.
Glycine
conjugation pathway has not been fully investigated.[34] Glycine is thought to be a hepatic detoxifier of a number endogenous and xenobiotic organic acids.[35]Bile acids are normally conjugated to glycine in order to increase their solubility in water.[36]
The human body rapidly clears sodium benzoate by combining it with glycine to form
hippuric acid which is then excreted.[37] The metabolic pathway for this begins with the conversion of benzoate by
butyrate-CoA ligase into an intermediate product,
benzoyl-CoA,[38] which is then metabolized by
glycine N-acyltransferase into hippuric acid.[39]
Uses
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.[40]
Animal and human foods
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.[22]
The U.S. "Food and Drug Administration no longer regards glycine and its salts as
generally recognized as safe for use in human food".[42]
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. 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. 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.[44] 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.[45] In 2016, detection of glycine within Comet
67P/Churyumov–Gerasimenko by the
Rosetta spacecraft was announced.[46]
Glycine is proposed to be defined by early genetic codes.[48][49][50][51] For example,
low complexity regions (in proteins), that may resemble the proto-peptides of the early
genetic code are highly enriched in glycine.[51]
^Plimmer, R.H.A. (1912) [1908]. Plimmer, R.H.A.; Hopkins, F.G. (eds.).
The chemical composition of the proteins. Monographs on biochemistry. Vol. Part I. Analysis (2nd ed.). London: Longmans, Green and Co. p. 82. Retrieved January 18, 2010.
^Berzelius, Jacob (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.)
^"Glycine Conference (prelim)". USITC. Archived from the original on February 22, 2012. Retrieved June 13, 2014.{{
cite web}}: CS1 maint: bot: original URL status unknown (
link)
^Meléndez-Hevia, E; De Paz-Lugo, P;
Cornish-Bowden, A; Cárdenas, M. L. (December 2009). "A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis". Journal of Biosciences. 34 (6): 853–72.
doi:
10.1007/s12038-009-0100-9.
PMID20093739.
S2CID2786988.
^
abcdefgNelson, David L.; Cox, Michael M. (2005). Principles of Biochemistry (4th ed.). New York: W. H. Freeman. pp. 127, 675–77, 844, 854.
ISBN0-7167-4339-6.
^"Safety (MSDS) data for glycine". The Physical and Theoretical Chemistry Laboratory Oxford University. 2005. Archived from
the original on October 20, 2007. Retrieved November 1, 2006.
^"butyrate-CoA ligase". BRENDA. Technische Universität Braunschweig. Retrieved May 7, 2014. Substrate/Product
^"glycine N-acyltransferase". BRENDA. Technische Universität Braunschweig. Retrieved May 7, 2014. Substrate/Product
^"Glycine From Japan and Korea"(PDF). U.S. International Trade Commission. January 2008.
Archived(PDF) from the original on June 6, 2010. Retrieved June 13, 2014.
^Casari, B. M.; Mahmoudkhani, A. H.; Langer, V. (2004). "A Redetermination of cis-Aquabis(glycinato-κ2N,O)copper(II)". Acta Crystallogr. E. 60 (12): m1949–m1951.
doi:
10.1107/S1600536804030041.