Glycine is a nonpolar amino acid. It is the simplest of the 20 natural amino acids; its side chain is a hydrogen atom. Because there is a second hydrogen atom at the α 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.
More than 130 molecules are found in the interstellar medium, including glycolaldehyde (the simplest 2 carbon sugar) and ethanol. 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 3 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 of 2004, Lewis Snyder and his collaborators reinvestigated the glycine claim by Yi-Jehng Kuan , Steve Charnley and their collaborators. In a rigorous attempt to confirm the detection, Snyder and his collaborators were able to show that glycine was NOT detected in ANY of the 3 sources claimed by Kuan and Charnley in 2003. These results have just been accepted by the Astrophysical Journal for future publication. The preprint is available on astro-ph.
Should any glycine claim be substantiated, it does not prove that life exists outside the Earth, but certainly makes that possibility more likely 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.
Glycine is an inhibitory neurotransmitter in the central nervous system, especially in the spinal cord. When glycine receptors are activated, chloride ions enter the neuron and the cell undergoes a hyperpolarization. Thus the cell tends to be in an inhibited state. Strychnine, a drug that cause convulsions, acts by blocking these glycine receptors. Glycine is also involved as a co-agonist of glutamate for the activation of NMDA receptors.