Glycine is the simplest amino acid and is the only amino acid that is not optically active (it has no stereoisomers). This amino acid is essential for the biosynthesis of nucleic acids as well as of bile acids, porphyrins, creatine phosphate, and other amino acids. On a molar basis, glycine is the second most common amino acid found in proteins and enzymes being incorporated at the rate of 7.5 percent compared to the other amino acids. Glycine is also similar to gamma-aminobutyric acid and glutamic acid in the ability to inhibit neurotransmitter signals in the central nervous system.

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.

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 Glycine is a protein amino acid found in the protein of all life forms. It is the simplest amino acid in the body and the only protein amino acid that does not have chirality. Although most glycine is found in proteins, free glycine is found in body fluids as well as in plants.

Glycine is not considered an essential amino acid, i. e., the cells in the body can synthesize sufficient amounts of glycine to meet physiological requirements. However, glycine is of major importance in the synthesis of proteins, peptides, purines, adenosine triphosphate (ATP), nucleic acids, porphyrins, hemoglobin, glutathione, creatine, bile salts, one-carbon fragments, glucose, glycogen, and L-serine and other amino acids. Glycine is also a neurotransmitter in the central nervous system (CNS). Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the CNS. Recently, a glycine-gated chloride channel has been identified in neurophils that can attenuate increases in intracellular calcium ions and diminish oxidant damage mediated by these white blood cells. Thus, glycine may be a novel antioxidant.