Glycine

The Discovery: A Pot of Boiling Gelatin

It was 1820 in Nancy, France. Henri Braconnot, a chemist and botanist, was doing what curious scientists of his era did: boiling things in sulfuric acid to see what happened. When he turned his attention to gelatin — the protein extracted from animal bones — something crystallized out of the solution that caught his eye. It was sweet.

Braconnot called his new substance sucre de gélatine — "gelatin sugar" — because of its unexpectedly pleasant taste. It would later be renamed glycine, from the Greek word glykys, meaning sweet. And so began the story of the amino acids: with something sweet, and a chemist boiling bones.

Glycine holds a notable distinction: it was the first amino acid ever isolated from a protein by acidic hydrolysis — a method that would go on to unlock the rest of the amino acid family. The discovery opened a new chapter in biochemistry, demonstrating that proteins could be broken down into discrete chemical units and that those units could be studied individually.

What Makes Glycine Unique

Among the 20 standard amino acids, glycine is in a class by itself for one simple reason: its side chain is just a hydrogen atom. Where every other amino acid has some group hanging off the central carbon — a methyl group, a benzene ring, a sulfur atom — glycine has nothing but a lone H.

This seemingly minor detail has enormous consequences. Because there are only two hydrogens and no side chain, the central carbon of glycine has no chirality. It is the only amino acid in the standard set that is not chiral — the only one that has no left-handed (L) or right-handed (D) form. All amino acids found in proteins are L-form, but glycine simply has no handedness at all.

This lack of a side chain also makes glycine extremely flexible. Proteins kink, coil, and fold into complex shapes partly because some amino acids can swivel freely — and glycine, with nothing in the way, is the most flexible of all. Wherever a protein chain needs to make a sharp turn or bend, it often uses glycine to do it.

Glycine in Space: A Cosmic Discovery

In 2009, NASA's Stardust spacecraft returned from its visit to comet Wild 2, carrying a precious cargo of comet dust. When scientists analyzed the material, they found glycine — the first amino acid ever identified in a comet sample. It was a remarkable confirmation of what many astronomers had long suspected: the building blocks of life are scattered throughout the cosmos, forming not just on Earth, but wherever chemistry has time to operate.

This wasn't glycine's first extraterrestrial sighting. It had already been detected in the Murchison meteorite, a carbonaceous chondrite that fell in Victoria, Australia in 1969. Meteorites like Murchison carry amino acids that formed through chemistry in space — the same basic reactions, happening in the cold void between stars.

The discovery raises a striking possibility: some of the amino acids in your body might trace their ancestry to chemistry that happened before the Solar System existed, drifting through interstellar space for billions of years.

Functions of Glycine in the Body

Glycine is a non-essential amino acid — the body synthesizes it primarily from serine, via the enzyme serine hydroxymethyltransferase. Despite its structural simplicity, it is metabolically versatile and participates in a wide range of essential processes.

Inhibitory neurotransmitter

Glycine acts as an inhibitory neurotransmitter in the central nervous system, particularly in the spinal cord, brainstem, and retina. It binds to strychnine-sensitive glycine receptors on neurons, reducing their excitability and helping regulate muscle tone and coordinated movement. This inhibitory function is distinct from its role as a co-agonist at NMDA receptors in the brain, where glycine binding is required for glutamate to activate the receptor — making glycine simultaneously inhibitory in some circuits and facilitatory in others.

Collagen and connective tissue

Nearly one third of all amino acid residues in collagen are glycine. The triple-helix structure of collagen — which gives skin, tendons, bones, and cartilage their mechanical properties — depends critically on glycine's small size: every third position in the helix must be occupied by an amino acid small enough to fit into the interior of the coiled structure, and only glycine qualifies. Without adequate glycine, the body cannot synthesize or repair collagen, affecting wound healing, skin integrity, and joint health.

Bile acid synthesis

Glycine is conjugated with bile acids in the liver — combining with cholic acid and chenodeoxycholic acid to form glycocholate and glycochenodeoxycholate. These glycine-conjugated bile salts are the primary form in which bile acids are secreted into the small intestine, where they emulsify dietary fats and enable their absorption. Glycine's role in bile acid conjugation makes it directly relevant to fat digestion and fat-soluble vitamin absorption.

Creatine synthesis

Glycine is one of three amino acids required for the biosynthesis of creatine — along with arginine and methionine. Creatine is stored in muscle tissue as phosphocreatine and serves as a rapid energy buffer, releasing phosphate groups to regenerate ATP during short, high-intensity muscular effort. The body's capacity to synthesize creatine depends on a continuous supply of glycine.

Nucleic acid synthesis

Glycine is a direct precursor in the biosynthesis of purines — the nitrogen-containing bases adenine and guanine that make up DNA and RNA. Its carbon atoms are incorporated into the purine ring structure, making glycine essential for cell division and the replication of genetic material.

Interesting Facts

Where to Find Glycine in Food

Glycine is abundant in foods rich in connective tissue proteins — collagen in particular. Many nutritionists argue that modern diets contain less glycine than traditional ones, since people eat less skin, bone, and cartilage than their ancestors. Values below are approximate per standard serving:

Glycine in Industry and Chemistry

Beyond its role in biology, glycine has practical applications in chemistry and food science. It acts as a buffer in many laboratory solutions, maintaining stable pH levels during experiments. As a food additive, it is approved in many countries under the code E640, used mainly as a flavor enhancer and mild sweetener.