n. an amino acid that serves as one of the two major inhibitory neurotransmitters in the central nervous system (particularly the spinal cord), the other being gamma-aminobutyric acid (GABA). Glycine is also a cotransmitter with glutamate at excitatory NMDA receptors. Glycine synthesis occurs via two different pathways; in the most important of these, glycine is synthesized from the amino acid serine in a single reaction catalyzed by the enzyme serine hydroxymethyltransferase.