Glycine (Longevity).

Glutathione is built from glycine, cysteine and glutamate, and glycine plus a cysteine donor supply the two residues most often limiting. This is the settled basis for pairing them.

Cysteine provides the thiol that does the redox work in glutathione and glycine closes the tripeptide. Both residues must be present for synthesis to complete.

Glycine is the third amino acid ligated onto gamma-glutamylcysteine to form glutathione. Supplying the finished tripeptide and supplying its precursor are two routes to the same pool.

Glutamine is deamidated to glutamate, the third residue of glutathione alongside glycine and cysteine. It feeds the same tripeptide synthesis from a different entry point.

Roughly a third of collagen's residues are glycine, so collagen peptides are among the densest dietary glycine sources. Glycine supply is part of what collagen synthesis draws on.

Ascorbate is the cofactor for prolyl and lysyl hydroxylases that stabilise the glycine-rich collagen triple helix. Glycine supplies the residues and vitamin C enables the crosslinking chemistry.

Glycine N-methyltransferase uses glycine as the acceptor that absorbs surplus methyl groups from S-adenosylmethionine, and betaine is a methyl donor into that same pool. Glycine acts as the outlet for the methylation load betaine raises.

S-adenosylmethionine is the methyl donor and glycine is one of its main acceptors, forming sarcosine and regulating methylation balance. The two are opposite ends of one transfer reaction.

The glycine cleavage system feeds one-carbon units onto tetrahydrofolate, and serine hydroxymethyltransferase moves carbon between glycine, serine and the folate pool. Glycine and folate are linked at the same carbon-shuttling step.

Serine hydroxymethyltransferase, which interconverts glycine and serine, is a pyridoxal phosphate enzyme. Vitamin B6 status governs how freely glycine moves through that step.

Glycine and serine convert into each other in one reversible reaction that transfers a single carbon to folate. The two amino acids share one pool in practice.

Magnesium glycinate delivers two glycine molecules per magnesium ion, so it contributes to glycine intake as well as magnesium. The glycine doses add together.

Creatine synthesis begins when arginine:glycine amidinotransferase moves an amidino group from arginine onto glycine, producing guanidinoacetate. Glycine supplies the whole backbone of the eventual creatine molecule, and arginine supplies the guanidino head. Neither amino acid can complete the step alone, which is why the pair is treated as a single supply question in creatine biochemistry.

Endogenous creatine synthesis draws on glycine for the carbon and nitrogen skeleton and on S-adenosylmethionine for the final methyl group. When creatine is supplied directly, less flux is needed through that route. The relationship is a shared substrate pool rather than an added effect on any single measure.

Delta-aminolevulinate synthase joins glycine to succinyl-CoA to make the first committed intermediate of the porphyrin pathway. Eight of those units build the protoporphyrin ring, and ferrochelatase then incorporates ferrous iron into it. Glycine supplies the nitrogen and two carbons of every pyrrole unit, so the two nutrients sit at opposite ends of one pathway.

Choline is oxidised to betaine, betaine donates a methyl group to homocysteine and becomes dimethylglycine, and dimethylglycine is demethylated through sarcosine to glycine. Dietary choline therefore feeds the glycine pool while supporting methyl group supply. The connection runs one way, from choline toward glycine.

The two demethylation steps that convert dimethylglycine into sarcosine and sarcosine into glycine both run on flavin adenine dinucleotide, which the body builds from riboflavin. Both steps also hand their one-carbon units to tetrahydrofolate. Riboflavin status therefore sits directly on the route that regenerates glycine from the choline and betaine side of methylation.

Bile acid-CoA:amino acid N-acyltransferase conjugates bile acids with either glycine or taurine, and the ratio between glycine and taurine conjugates shifts with the relative availability of the two. Adding one amino acid moves the conjugate profile toward that partner. This is a redistribution within a shared enzymatic step, not evidence that either conjugate performs better.

Bile salt preparations are largely glycine and taurine conjugates of cholic and chenodeoxycholic acid. Glycine availability supports the hepatic conjugation step that keeps recirculating bile acids in their amidated form. The pairing is a supply relationship at the conjugation enzyme rather than a measured clinical combination.

The collagen sequence repeats as glycine-X-Y, with proline and hydroxyproline most often in the X and Y slots, because only glycine is small enough to sit at the crowded centre of the triple helix. Roughly a third of collagen residues are glycine as a result. One published human report on a collagen amino acid composition covering this amino acid profile is indexed as PMID 41266379.

Two glycine molecules co-ordinate a zinc ion through their amine and carboxylate groups to form a neutral, ring-stabilised chelate. The glycine acts as the ligand rather than as a nutrient in that context. Chelation changes how the mineral behaves in the gut lumen compared with an inorganic salt, without making glycine itself the active part.

Copper glycinate is one of the textbook amino acid chelates, with the metal held by the amine nitrogen and carboxylate oxygen of two glycine molecules. The complex is stable across a useful pH range, which is why glycine appears as a ligand on mineral labels. The relationship is formulation chemistry rather than a physiological pairing.

Threonine dehydrogenase converts threonine to 2-amino-3-ketobutyrate, which is cleaved to glycine and acetyl-CoA. This is one of the main routes by which the body makes glycine when dietary intake is limited, alongside conversion from serine. The flow runs from threonine toward glycine and not the other way.

Glycine receptors and GABA-A receptors are both pentameric chloride channels from the same gene superfamily, and some spinal interneurons release the two transmitters from the same vesicle. Their signals sum at the postsynaptic membrane because they open the same ionic conductance. Supplemental forms do not necessarily reach those synapses, so the shared mechanism is receptor biology rather than a demonstrated effect of taking the two together.

Glycine acts on the preoptic region where core body temperature is regulated, and evening melatonin lowers core temperature through peripheral vasodilation. The two therefore push the same physiological variable from different directions. No trial in the candidate set measured them together, so the additive framing is mechanistic and worth flagging for anyone stacking evening ingredients.