Glycine, the simplest amino acid, was first called sugar of gelatin
In 1820 a French chemist boiled gelatin with sulfuric acid and got sweet white crystals he named sugar of gelatin. It was glycine, the smallest amino acid, whose side chain is a lone hydrogen atom. That tiny size lets it pack collagen tightly and makes it a vital brake in the spinal cord.
Henri Braconnot made the discovery; in 1838 Jean-Baptiste Boussingault showed the crystals contained nitrogen, so they were no sugar. An American student of Justus von Liebig proposed glycocoll in 1847, but the Swedish chemist Berzelius offered the shorter glycine a year later, from the Greek for sweet, the same root as glucose. Because its side chain is just hydrogen, glycine alone among the standard protein builders lacks handedness: it is identical to its own mirror image.
Its small size is structurally crucial. Collagen, the protein of skin, tendon and bone, is about 35 percent glycine, which allows three chains to wind into a tight triple helix. Glycine also supplies building blocks for heme in blood and for purines, part of DNA and RNA. The body makes some from serine, but not always enough, so it is called semi-essential.
In the spinal cord, brainstem and retina, glycine acts as an inhibitory signal: when it opens its receptors, chloride flows into neurons and quiets them. Strychnine blocks those receptors, and tetanus toxin causes spastic paralysis by stopping glycine release. Oddly, glycine also works in the opposite direction, since excitatory NMDA receptors will open only when both glycine and glutamate are bound.
Industry skips protein altogether and makes about 15 thousand tonnes a year by chemical synthesis, mainly from chloroacetic acid and ammonia or by the Strecker method. It is used to make the herbicide glyphosate, to finish metals and as a mildly sweet flavouring that masks saccharin's aftertaste. The liver also uses it to clear substances: it binds sodium benzoate, a preservative, into hippuric acid for excretion, and it makes bile acids more water-soluble. Since 1971 the US FDA has not classed glycine as generally recognised as safe for food, permitting it only under certain conditions.
Source: Glycine
