glycine

Glycine[1]
IUPAC name	Glycine 2-Aminoacetic acid
Other names	Aminoethanoic acid
Identifiers
Abbreviations	Gly, G
CAS number	[56-40-6]
PubChem	750
SMILES	NCC(O)=O
ChemSpider ID	730
Properties
Molecular formula	C2H5NO2
Molar mass	75.07 g mol−1
Density	1.1607 g/cm3
Melting point	233 °C (decomposition)
Solubility in water	25 g/100 mL
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Glycine (abbreviated as Gly or G)^[2] is the organic compound with the formula NH₂CH₂COOH. It is the smallest of the 20 amino acids commonly found in proteins, coded by codons GGU, GGC, GGA and GGG. Glycine is unique among the proteinogenic amino acids in that it is not chiral. Most proteins incorporate only small quantities of glycine. A notable exception is collagen, which contains about 35% glycine.^[3] Solid glycine is a colourless, sweet-tasting crystalline material.

Production

Glycine is manufactured industrially by treating chloroacetic acid with ammonia:^[4]

ClCH₂COOH + 2 NH₃ → H₂NCH₂COOH + NH₄Cl

It is also produced via the Strecker amino acid synthesis.

Biosynthesis

Glycine is not essential to the human diet, since 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:^[5]

serine + tetrahydrofolate → glycine + N⁵,N¹⁰-Methylene tetrahydrofolate + H₂O

In the liver of vertebrates, glycine synthesis is catalyzed by glycine synthase (also called glycine cleavage enzyme). This conversion is readily reversible:^[5]

CO₂ + NH₄⁺ + N⁵,N¹⁰-Methylene tetrahydrofolate + NADH + H⁺ → Glycine + tetrahydrofolate + NAD⁺

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:^[6]

Glycine + tetrahydrofolate + NAD⁺ → CO₂ + NH₄⁺ + N⁵,N¹⁰-Methylene tetrahydrofolate + NADH + H⁺

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.^[6]

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⁺-dependent reaction.^[6]

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 C₂N subunit of all purines.^[7]

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 receptors. 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]} The LD₅₀ of glycine is 7930 mg/kg in rats (oral),^[8] 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 use in 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.^[9]

Animal and human foodstuffs

Other markets for USP grade glycine include its use an additive in pet food and 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.^[10]

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.^[11]

Antidumping tariffs

Glycine imported from China to the United States has been subject to antidumping duties since March, 1995.^[12]

In 2007, a United States manufacturer of Glycine, GEO Specialty Chemicals, Inc.^[13] filed petitions requesting that antidumping duties also be imposed on Glycine imported from Japan, the Republic of Korea, and India. On September 7, 2007 the Department of Commerce announced its affirmative preliminary determinations in the antidumping duty investigations on imports of glycine from Japan and the Republic of Korea (Korea). On October 29, 2007 the Department of Commerce announced its affirmative preliminary determination in the antidumping duty investigation on imports of glycine from India.

Presence in the interstellar medium

The detection of glycine in the interstellar medium has been debated.^[14] 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 by the Max Planck Institute for Radio Astronomy ^[15]

References

External links

• Glycine at PDRHealth.com
• Computational Chemistry Wiki
• Glycine cleavage system
• Glycine Therapy - A New Direction for Schizophrenia Treatment?
• "Organic Molecule, Amino Acid-Like, Found In Constellation Sagittarius". ScienceDaily. 27 March 2008. http://www.sciencedaily.com/releases/2008/03/080326161658.htm. 
• Guochuan E. Tsai (1 December 2008). "A New Class of Antipsychotic Drugs: Enhancing Neurotransmission Mediated by NMDA Receptors". Psychiatric Times 25 (14). http://www.psychiatrictimes.com/display/article/10168/1357569. 
• ChemSub Online (Glycine).

Further specialized reading

On attempts to detect glycine in interstellar medium

• Kuan YJ, Charnley SB, Huang HC, et al. (2003). "Interstellar glycine". Astrophys J 593 (2): 848–867. doi:10.1086/375637. 
• Rachel Nowak. "Amino acid found in deep space - 18 July 2002 - New Scientist". http://www.newscientist.com/news/news.jsp?id=ns99992558. Retrieved on 2007-07-01.