View source for Glycine

{{for|the plant|Glycine (plant)}}
 
{{chembox
| Watchedfields = changed
| verifiedrevid = 306544029
| Reference=<ref>{{Merck11th|4386}}.</ref>
| ImageFileL1 = Glycine-zwitterion-2D-skeletal.png
|   ImageSizeL1 = 120px
| ImageFileR1 = Glycin - Glycine.svg
|   ImageSizeR1 = 73px
| ImageFileL2 = Glycine-from-xtal-2008-3D-balls.png
|   ImageSizeL2 = 120px
| ImageFileR2 = Glycine-3D-balls.png
|   ImageSizeR2 = 100px
| IUPACName = Glycine
| OtherNames = Aminoethanoic acid <br /> Aminoacetic acid
| Section1 = {{Chembox Identifiers
|   Abbreviations = '''Gly''', '''G'''
|   CASNo = 56-40-6
|    CASNo_Ref = {{cascite}}
|   EC-number = 200-272-2
|   ChemSpiderID = 730 
|   PubChem = 750
| IUPHAR_ligand = 727
|   SMILES = NCC(O)=O
  }}
| Section2 = {{Chembox Properties
|   C=2 | H=5 | N=1 | O=2
|   MolarMass = 75.07
|   Appearance = white solid
|   Density = 1.1607 g/cm<sup>3</sup>
|   MeltingPt = 233 °C (decomposition)
|   Solubility = 25 g/100 mL
|   SolubleOther = soluble in [[ethanol]], [[pyridine]] <br> insoluble in [[ether]]
|   pKa = 2.35 and 9.78
  }}
| Section3 = {{Chembox Hazards
|   EUIndex = 
|   FlashPt = 
|   Autoignition = 
|   LD50 = 2600 mg/kg (mouse, oral)
  }}
}}

'''Glycine''' (abbreviated as '''Gly''' or '''G''')<ref>{{IUPAC-IUB amino acids 1983}}.</ref> is an [[organic compound]] with the [[chemical formula|formula]] NH<sub>2</sub>CH<sub>2</sub>COOH. With only two [[hydrogen]] atoms as its 'side chain', glycine is the smallest of the 20 [[amino acid]]s commonly found in [[protein]]s. Its codons are GGU, GGC, GGA, GGG.

Glycine is a colourless, sweet-tasting crystalline solid. It is unique among the [[proteinogenic amino acid]]s in that it is not [[chirality (chemistry)|chiral]]. It can fit into [[Hydrophile|hydrophilic]] or [[Hydrophobe|hydrophobic]] environments, due to its two hydrogen atom side chain.

==Production and key properties==
Glycine was discovered in 1820, by [[Henri Braconnot]] who boiled gelatin with sulfuric acid.<ref>{{cite book |author= R.H.A. Plimmer |editor= R.H.A. Plimmer & F.G. Hopkins |title= The chemical composition of the proteins |url= http://books.google.com/?id=7JM8AAAAIAAJ&pg=PA112 |accessdate= January 18, 2010 |edition= 2nd |series= Monographs on biochemistry |volume= Part I. Analysis |origyear= 1908 |year= 1912 |publisher= Longmans, Green and Co. |location= London|page= 82}}</ref>

Glycine is manufactured industrially by treating [[chloroacetic acid]] with [[ammonia]]:<ref>{{OrgSynth | first1 = A. W. | last1 = Ingersoll | first2 = S. H. | last2 = Babcock | title = Hippuric acid | prep=cv2p0328 | volume = 12 | pages = 40 | year = 1932 | collvol = 2 | collvolpages = 328}}.</ref>
:ClCH<sub>2</sub>COOH + 2 NH<sub>3</sub> → H<sub>2</sub>NCH<sub>2</sub>COOH + NH<sub>4</sub>Cl
About 15M kg are produced annually in this way.<ref>Karlheinz Drauz, Ian Grayson, Axel Kleemann, Hans-Peter Krimmer, Wolfgang Leuchtenberger, Christoph Weckbecker “Amino Acids” in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a02_057.pub2}}</ref>

In the USA (GEO Specialty Chemicals, Inc.) and in Japan by Shoadenko, glycine is produced via the [[Strecker amino acid synthesis]].<ref name="usitc.gov">http://www.usitc.gov/trade_remedy/731_ad_701_cvd/investigations/2007/glycine_from_india_japan_korea/preliminary/DOC/Glycine%20Conference%20(prelim).wpd</ref>

There are two producers of glycine in the United States. Chattem Chemicals, Inc., purchased by Sun Pharmaceutical, who is an international pharmaceutical company based in Mumbai, India and GEO Specialty Chemicals, Inc., who purchased the glycine and naphthalene sulfonate production facilities of Dow/Hampshire Chemical Corp.<ref name="usitc.gov"/><ref>U.S. International Trade Commission, "Glycine From China." Investigation No. 731-TA-718 (Second Review), Publication No. 3810, October 2005</ref>

Chattem's manufacturing process ("MCA" process) occurs in batches and results in a finished product with some residual chloride but no sulfate, while GEO’s manufacturing process is considered a semi-batch process and results in a finished product with some residual sulfate but no chloride.

Its pK values are 2.35 and 9.78, so above pH 9.78, most of the glycine exists as the anionic amine, H<sub>2</sub>NCH<sub>2</sub>CO<sub>2</sub><sup>-</sup>.  Below pH 2.35, its solutions contain mostly the cationic carboxylic acid H<sub>3</sub>N<sup>+</sup>CH<sub>2</sub>CO<sub>2</sub>H.  Its [[isoelectric point]] (pI) is 6.06.

==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]]. In most organisms, the enzyme [[Serine hydroxymethyltransferase]] catalyses this transformation via the cofactor [[pyridoxal phosphate]]:<ref name="Lehninger"/>
: serine + [[tetrahydrofolate]] → glycine + ''N<sup>5</sup>'',''N<sup>10</sup>''-Methylene tetrahydrofolate + H<sub>2</sub>O

In the liver of [[vertebrates]], glycine synthesis is catalyzed by [[glycine synthase]] (also called glycine cleavage enzyme).  This conversion is readily reversible:<ref name="Lehninger"/>
: CO<sub>2</sub> + NH<sub>4</sub><sup>+</sup> + ''N<sup>5</sup>'',''N<sup>10</sup>''-Methylene tetrahydrofolate + NADH + H<sup>+</sup> → Glycine + tetrahydrofolate + NAD<sup>+</sup>

Glycine is coded by [[codons]] GGU, GGC, GGA and GGG. Most proteins incorporate only small quantities of glycine. A notable exception is [[collagen]], which contains about 35% glycine.<ref name="Lehninger">{{Lehninger4th|pages=127, 675–77, 844, 854}}.</ref>

==Degradation==
Glycine is degraded via three pathways. The predominant pathway in animals involves the catalysis of [[glycine cleavage enzyme]], the same enzyme also involved in the biosynthesis of glycine. The degradation pathway is the reverse of this synthetic pathway:<ref name="Lehninger"/>
: Glycine + tetrahydrofolate + NAD<sup>+</sup> → CO<sub>2</sub> + NH<sub>4</sub><sup>+</sup> + ''N<sup>5</sup>'',''N<sup>10</sup>''-Methylene tetrahydrofolate + [[NADH]] + H<sup>+</sup>

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]].<ref name="Lehninger"/>

In the third pathway of glycine degradation, glycine is converted to [[glyoxylate]] by [[D-amino acid oxidase]]. Glycoxylate is then oxidized by hepatic [[lactate dehydrogenase]] to [[oxalate]] in an NAD<sup>+</sup>-dependent reaction.<ref name="Lehninger"/>

==Physiological function==
The principal function of glycine is as a precursor to proteins.  It is also a building block to numerous natural products.
===As a biosynthetic intermediate===
In higher [[eukaryotes]], [[D-Aminolevulinic acid]], the key precursor to [[porphyrins]], is biosynthesized from glycine and [[succinyl-CoA]]. Glycine provides the central C<sub>2</sub>N subunit of all [[purine]]s.<ref name="Lehninger"/>

===As a neurotransmitter===
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). [[Strychnine]] is a strong antagonist at ionotropic glycine receptors, whereas [[bicuculline]] is a weak one. Glycine is a required [[co-agonist]] along with [[glutamate]] for [[NMDA receptor]]s. In contrast to the inhibitory role of glycine in the spinal cord, this behaviour is facilitated at the ([[NMDA]]) glutaminergic receptors which are excitatory.{{Citation needed|date=June 2009}}  The [[LD50|LD<sub>50</sub>]] of glycine is 7930&nbsp;mg/kg in rats (oral),<ref>{{cite web |url=http://physchem.ox.ac.uk/MSDS/GL/glycine.html |title=Safety (MSDS) data for glycine |year=2005 |publisher= The Physical and Theoretical Chemistry Laboratory Oxford University |accessdate=2006-11-01}}</ref> and it usually causes death by [[hyperexcitability]].

==Commercial uses==
In the US, glycine is typically sold in two grades: [[United States Pharmacopeia]] (“USP”), and technical grade. Most glycine is manufactured as USP grade material for diverse uses. USP grade sales account for approximately 80 to 85 percent of the U.S. market for glycine. 
* Pharmaceutical grade glycine is produced for some pharmaceutical applications, such as intravenous injections, where the customer’s purity requirements often exceed the minimum required under the USP grade designation. Pharmaceutical grade glycine is often produced to proprietary specifications and is typically sold at a premium over USP grade glycine.
* Technical grade glycine, which may or may not meet USP grade standards, is sold for use in industrial applications; e.g., as an agent in metal complexing and finishing. Technical grade glycine is typically sold at a discount to USP grade glycine.<ref>http://hotdocs.usitc.gov/docs/pubs/701_731/pub3921.pdf</ref>

===Animal and human foods===
Other markets for USP grade glycine include its use an additive in [[pet food]] and [[compound feed|animal feed]]. For humans, glycine is sold as a sweetener/taste enhancer.  Food supplements and protein drinks contain glycine. Certain drug formulations include glycine to improve gastric absorption of the drug.

===Cosmetics and miscellaneous applications===
Glycine serves as a [[buffering agent]] in [[antacids]], [[analgesics]], [[antiperspirants]], cosmetics, and toiletries.

Many miscellaneous products use glycine or its derivatives, such as the production of rubber sponge products, fertilizers,  metal complexants.<ref name=72FR62827>"Notice of Preliminary Determination of Sales at Less Than Fair Value: Glycine From India" Federal Register 72 (7 November 2007): 62827.</ref>

[[File:Betain-Glycine.png|thumb|right|300px|Zwitterionic salts of glycine at neutral pH]]

===Chemical feedstock===
Glycine is an intermediate in the synthesis of a variety of chemical products.  It is used in the manufacture of the herbicide [[Glyphosate]]. Glyphosate (N-(phosphonomethyl) glycine) is a non-selective systemic herbicide used to kill weeds, especially perennials and broadcast or used in the cut-stump treatment as a forestry herbicide. Initially, Glyphosate was sold only by Monsanto under the Monsanto tradename [[Roundup]], but is no longer under patent.

==Presence in space==
The detection of glycine in the [[interstellar medium]] has been debated.<ref name=Snyder>{{cite journal |author=Snyder LE, Lovas FJ, Hollis JM, et al. |title=A rigorous attempt to verify interstellar glycine |journal=Astrophys J |volume=619 |issue=2 |pages=914–930 |year=2005 |doi=10.1086/426677}}</ref>  In 2008, the glycine-like molecule [[amino acetonitrile]] was discovered in the [[Large Molecule Heimat]], a giant gas cloud near the galactic center in the constellation [[Sagittarius (constellation)|Sagittarius]] by the [[Max Planck Institute for Radio Astronomy]].<ref>{{cite web |url=http://www.sciencedaily.com/releases/2008/03/080326161658.htm |author=Staff|title=Organic Molecule, Amino Acid-Like, Found In Constellation Sagittarius 27 March 2008 - Science Daily |accessdate=2008-09-16}}</ref> In 2009, glycine sampled in 2004 from comet [[Wild 2]] by the [[NASA]] spacecraft [[Stardust (spacecraft)|Stardust]] was confirmed, the first discovery of extraterrestrial glycine. That mission's results bolstered the theory of [[panspermia]], which claims that the "seeds" of life are widespread throughout the universe.<ref>{{cite web |url=http://www.reuters.com/article/scienceNews/idUSTRE57H02I20090818 |author=Reuters|title=Building block of life found on comet - Thomson Reuters 2009 |accessdate=2009-08-18}}</ref>

== References ==
{{Reflist|2}}

==Further reading==
On attempts to detect glycine in interstellar medium
*{{cite journal |author=Kuan YJ, Charnley SB, Huang HC, et al. |title=Interstellar glycine |journal=Astrophys J |volume=593 |issue=2 |pages=848–867 |year=2003 |doi=10.1086/375637}}
*{{cite web |url=http://www.newscientist.com/news/news.jsp?id=ns99992558 |author=Rachel Nowak |title=Amino acid found in deep space - 18 July 2002 - New Scientist |accessdate=2007-07-01}}

==External links==
* [http://www.pdrhealth.com/drugs/altmed/altmed-mono.aspx?contentFileName=ame0084.xml&contentName=Glycine&contentId=247 Glycine] at PDRHealth.com
* [http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/AminoAcid/GlyCleave.html Glycine cleavage system]
* [http://www.schizophrenia.com/glycinetreat.htm Glycine Therapy - A New Direction for Schizophrenia Treatment?]
* {{cite journal | title = Organic Molecule, Amino Acid-Like, Found In Constellation Sagittarius | journal = ScienceDaily |date=27 March 2008| url = http://www.sciencedaily.com/releases/2008/03/080326161658.htm}}
* {{cite journal |  journal = Psychiatric Times | volume = 25 | issue = 14 | title = A New Class of Antipsychotic Drugs: Enhancing Neurotransmission Mediated by NMDA Receptors | author = Guochuan E. Tsai |date=1 December 2008| url = http://www.psychiatrictimes.com/display/article/10168/1357569}}
* [http://chemsub.online.fr/name/glycine.html ChemSub Online (Glycine)].
* [http://www.jpl.nasa.gov/news/news.cfm?release=2009-126 NASA scientists have discovered glycine, a fundamental building block of life, in samples of comet Wild 2 returned by NASA's Stardust spacecraft.]

{{AminoAcids}}
{{Amino acid metabolism intermediates}}
{{Neurotransmitters}}
{{Glycinergics}}

[[Category:Proteinogenic amino acids]]
[[Category:Neurotransmitters]]
[[Category:Glucogenic amino acids]]
[[Category:Flavour enhancers]]

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[[Category:2Fix]]