Glycine

Glycine (abbreviated as Gly or G)^[1] is the organic compound with the formula HO₂CCH₂NH₂. It is one of the 20 amino acids commonly found in proteins, coded by codons GGU, GGC, GGA and GGG. Because of its structural simplicity, this compact amino acid tends to be evolutionarily conserved in, for example, cytochrome c, myoglobin, and hemoglobin.^{[citation needed]} Glycine is the unique amino acid that is not optically active. Most proteins contain only small quantities of glycine. A notable exception is collagen, which contains about 35% glycine.^[2] In its solid, i.e., crystallized, form, Glycine is a free-flowing crystalline material. ^[3]

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Synthesis

Glycine is manufactured industrially:

(1)treatment of chloroacetic acid with ammonia leads to the product in one step.

ClCH₂COOH + NH₃ → H₂NCH₂COOH + HCl

or via

(2)The Strecker Synthesis via hydrolysis of a nitrile.

There are two producers of Glycine in the United States. Chattem Chemicals, Inc. and GEO Specialty Chemicals, Inc., who purchased the Glycine production facilities of Hampshire Chemical Corp.^{[4] [5]} According to information provided to the U.S. Department of Commerce, each uses a different manufacturing process and different raw materials. Chattem's manufacturing process (the "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.^[4]

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 by removing one carbon atom; pyridoxal phosphate is also necessary:^[6]

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:^[6]

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:^[7]

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.^[7]

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.^[7]

Physiological function

As a biosynthetic intermediate

Glycine is a building block to numerous natural products. 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.^[8]

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 an antagonist at ionotropic glycine receptors. 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. The LD₅₀ of glycine is 7930 mg/kg in rats (oral),^[9] and it usually causes death by hyperexcitability.

Industrial Uses

Glycine is used as a sweetener/taste enhancer, buffering agent, reabsorbable amino acid, chemical intermediate, metal complexing agent, dietary supplement as well as in certain pharmaceuticals. ^[3]

Antidumping Tarrifs

Glycine imported from China to the United States has been subject to antidumping duties since March, 1995. ^[10]

In 2007, a United States manufacturer of Glycine, GEO Specialty Chemicals, Inc. 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

In 1994 a team of astronomers at the University of Illinois, led by Lewis Snyder, claimed that they had found the glycine molecule in space. It turned out that, with further analysis, this claim could not be confirmed. Nine years later, in 2003, Yi-Jehng Kuan from National Taiwan Normal University and Steve Charnley claimed that they detected interstellar glycine toward three sources in the interstellar medium.^[11] They claimed to have identified 27 spectral lines of glycine utilizing a radio telescope. According to computer simulations and lab-based experiments, glycine was probably formed when ices containing simple organic molecules were exposed to ultraviolet light.^[12]

In October 2004, Snyder and collaborators reinvestigated the glycine claim in Kuan et al. (2003). In a rigorous attempt to confirm the detection, Snyder showed that glycine was not detected in any of the three claimed sources.^[13]

Should the glycine claim be substantiated, the finding would not prove that life exists outside the Earth, but certainly makes that possibility more plausible by showing that amino acids can be formed in the interstellar medium.

References

• Dawson, R.M.C., Elliott, D.C., Elliott, W.H., and Jones, K.M., Data for Biochemical Research (3rd edition), pp. 1-31 (1986) ISBN 01-985-535-87