Glycine

Glycine (Gly, G) is a nonpolar amino acid. It is the simplest of the 20 standard (proteinogenic) amino acids: its side chain is a hydrogen atom. Because there is a second hydrogen atom at the &alpha; carbon, glycine is not optically active.

Since glycine has such a small side chain, it can fit into many places where no other amino acid can. For example, only glycine can be the internal amino acid of a collagen helix.

Glycine is very evolutionarily stable at certain positions of some proteins (for example, in cytochrome c, myoglobin, and hemoglobin), because mutations that change it to an amino acid with a larger side chain could break the protein's structure.

Most proteins contain only small quantities of glycine. A notable exception is collagen, which is about one-third glycine.

Physiological function
Glycine is an inhibitory neurotransmitter in the CNS, especially in the spinal cord. When glycine receptors are activated, Cl- enters the neuron via ionotropic receptors, causing an inhibitory post-synaptic potential (IPSP). Strychnine is an antagonist at these ionotropic receptors. Its LD50 is 0.96 mg/kg in rats, and it usually causes death by hyperexcitability. Glycine is a required co-agonist along with Glu in CNS. In contrast to the inhibitory role of glycine in the spinal cord, this behaviour is facilitated at the (NMDA) glutaminergic receptors which are excitatory.

Glycine is a non-essential amino acid, meaning that cells of the body can synthesize sufficient amounts to meet physiological requirements.

Presence in the Interstellar Medium
In 1994 a team of astronomers from 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 made the extraordinary claim that they detected interstellar glycine toward three sources in the interstellar medium. 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.

In October 2004, Snyder and his 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.

Should any glycine claim be substantiated, it does 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. The finding would also indirectly support the idea of panspermia, the theory that life was brought to Earth from space.