GLYCINE

'Glycine' is the organic compound with the formula HO₂CCH₂NH₂. It is one of the 20 amino acids commonly found in proteins. Its three letter code is gly, its one letter code is G, and its codons are GGU, GGC, GGA and GGG.^[1] Because of its structural simplicity, this compact amino acid tends to be evolutionarily conserved in, for example, cytochrome c, myoglobin, and hemoglobin. 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 one-third glycine.

Contents

Synthesis

Biosynthesis

Physiological function

As a biosynthetic intermediate

As a neurotransmitter

Presence in the interstellar medium

References

External links

Synthesis

Glycine is manufactured industrially: treatment of chloroacetic acid with ammonia leads to the product in one step.
:ClCH₂COOH + NH₃ → H₂NCH₂COOH + HCl

Biosynthesis

Glycine is not essential to the human diet, since it is synthesized in the body. It is biosynthesized from the amino acid serine. The enzyme serine hydroxymethyl transferase catalyses this transformation:Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
: HO₂CCH(NH₂)CH₂OH + H₂folate → HO₂CCH₂NH₂ + CH₂-folate + H₂O

Physiological function

As a biosynthetic intermediate

Glycine is a building block to numerous species. Aminolevulinic acid, the key precursor to porphyrins is biosynthesized from glycine and succinoyl coenzyme A. Glycine provides the central C₂N subunit of all purines.

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),^[2] and it usually causes death by hyperexcitability.

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. Interstellar glycine, Kuan YJ, Charnley SB, Huang HC, et al., , , ASTROPHYS J, 2003 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.^[3]
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. A rigorous attempt to verify interstellar glycine, Snyder LE, Lovas FJ, Hollis JM, et al., , , ASTROPHYS J, 2005
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

1. Nomenclature and Symbolism for Amino Acids and Peptides IUPAC-IUBMB Joint Commission on Biochemical Nomenclature
2. Safety (MSDS) data for glycine
3. Amino acid found in deep space - 18 July 2002 - New Scientist Rachel Nowak


★ 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

External links

★ Glycine

★ Computational Chemistry Wiki

★ Glycine cleavage system