GLYCINE (abbreviated as GLY or G) is the amino acid that has a single
hydrogen atom as its side chain . It is the simplest possible amino
acid . The chemical formula of glycine is NH2 ‐CH2 ‐COOH . Glycine
is one of the proteinogenic amino acids . Its codons are GGU, GGC,
GGA, GGG of the genetic code .
Glycine is a colorless, sweet-tasting crystalline solid. It is unique
among the proteinogenic amino acids in that it is achiral . It can fit
into hydrophilic or hydrophobic environments since it exists as
zwitterion at natural pH, due to its minimal side chain of only one
hydrogen atom. The acyl radical is GLYCYL.
Glycine is a white
* 1 History and etymology
* 2 Production
* 3 Acid-base properties and structures
* 4 Metabolism
* 4.1 Biosynthesis
* 4.2 Degradation
* 5 Physiological function
* 5.1 As a biosynthetic intermediate
* 5.2 As a neurotransmitter
* 6 Uses
* 6.1 Animal and human foods
* 6.2 Cosmetics and miscellaneous applications
* 6.3 Chemical feedstock
* 6.4 Laboratory research
* 7 Presence in space
* 8 See also
* 9 References
* 10 Further reading
* 11 External links
HISTORY AND ETYMOLOGY
Glycine was first isolated from gelatin in 1820. The name comes from
the ancient Greek word γλυκύς "sweet tasting" (which is also
related to the prefixes glyco- and gluco-, as in glycoprotein and
Glycine was discovered in 1820, by
Henri Braconnot who boiled a
gelatinous object with sulfuric acid .
Glycine is manufactured industrially by treating chloroacetic acid
with ammonia : ClCH2COOH + 2 NH3 → H2NCH2COOH + NH4Cl
About 15 million kg are produced annually in this way.
In the United States and Japan, glycine is produced via the Strecker
amino acid synthesis .
There are two producers of glycine in the United States: Chattem
Chemicals, Inc., a subsidiary of Mumbai -based
Sun Pharmaceutical ,
and GEO Specialty Chemicals, Inc., which purchased the glycine and
naphthalene sulfonate production facilities of Hampshire Chemical
Corp, a subsidiary of
Dow Chemical .
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.
Glycine is also cogenerated as an impurity in the synthesis of
arising from reactions of the ammonia coproduct.
ACID-BASE PROPERTIES AND STRUCTURES
In aqueous solution, glycine itself is amphoteric : at low pH the
molecule can be protonated with a pKa of about 2.4 and at high pH it
loses a proton with a pKa of about 9.6 (precise values of pKa depend
on temperature and ionic strength). The nature of glycine in aqueous
solution has been investigated by theoretical methods. In solution
the ratio of concentrations of the two isomers is independent of both
the analytical concentration and of pH. This ratio is simply the
equilibrium constant for isomerization. K = /
Both isomers of glycine have been observed by microwave spectroscopy
in the gas phase. The solid-state structure has been analyzed in
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 the metabolic capacity for glycine
biosynthesis does not satisfy the need for collagen synthesis. In
most organisms, the enzyme serine hydroxymethyltransferase catalyses
this transformation via the cofactor pyridoxal phosphate : 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: CO2 + NH+
4 + N5,N10-Methylene tetrahydrofolate +
NADH + H+ →
tetrahydrofolate + NAD+
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.
Glycine is degraded via three pathways. The predominant pathway in
animals and plants involves the glycine cleavage enzyme :
tetrahydrofolate + NAD+ → CO2 + NH+
4 + 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
Serine is then converted to pyruvate by
serine dehydratase .
In the third pathway of glycine degradation, glycine is converted to
D-amino acid oxidase
D-amino acid oxidase .
Glyoxylate is then oxidized by
hepatic lactate dehydrogenase to oxalate in an NAD+-dependent
The half-life of glycine and its elimination from the body varies
significantly based on dose. In one study, the half-life was between
0.5 and 4.0 hours.
The principal function of glycine is as a precursor to proteins, such
as its periodically repeated role in the formation of the collagen
helix in conjunction with hydroxyproline . It is also a building block
for numerous natural products.
AS A BIOSYNTHETIC INTERMEDIATE
In higher eukaryotes , δ-aminolevulinic acid , the key precursor to
porphyrins , is biosynthesized from glycine and succinyl-CoA by the
ALA synthase .
Glycine provides the central C2N subunit of all
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
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 ) glutamatergic receptors which are
excitatory. The LD50 of glycine is 7930 mg/kg in rats (oral), and it
usually causes death by hyperexcitability.
A 2014 review on sleep aids noted that glycine can improve sleep
quality, citing a study in which 3 grams of glycine before bedtime
improved sleep quality in humans.
Glycine has also been positively
tested as an add-on treatment for schizophrenia .
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
* 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
* 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.
ANIMAL AND HUMAN FOODS
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. Certain 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, and metal
Glycine is an intermediate in the synthesis of a variety of chemical
products. It is used in the manufacture of the herbicide glyphosate .
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
In 2009, glycine sampled in 2004 from comet
Wild 2 by the NASA
spacecraft Stardust was confirmed – the first discovery of glycine
outside the Earth, although glycine had been identified in the
Murchison meteorite in 1970. The discovery of cometary glycine
bolstered the theory of panspermia , which claims that the
"building-blocks" of life are widespread throughout the Universe. In
2016, detection of glycine within Comet 67P/Churyumov-Gerasimenko by
the Rosetta spacecraft was announced.
The detection of glycine outside the solar system in the interstellar
medium has been debated. In 2008, the Max Planck Institute for Radio
Astronomy discovered the glycine-like molecule aminoacetonitrile in
Large Molecule Heimat , a giant gas cloud near the galactic center
in the constellation Sagittarius .
Amino acid neurotransmitter
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