Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine, with the sequence Glu-Cys-Gly, the molecular formula C₁₀H₁₇N₃O₆S, a molecular weight of 307.3 g/mol, and CAS number 70-18-8. What makes the structure unusual is the first bond. Glutamate is linked to cysteine through its side-chain gamma-carboxyl group rather than its alpha-carboxyl, producing a gamma-glutamyl bond that no ribosome makes and that most peptidases cannot cleave. The full chemical name, gamma-L-glutamyl-L-cysteinyl-glycine, encodes that feature. Ever Vital lists glutathione for research use; the redox research overview covers its biological roles. This article stays on the structure.
Why is the gamma-glutamyl bond important?
The gamma-glutamyl bond is the reason glutathione survives in cellular and extracellular environments that would degrade an ordinary tripeptide within minutes. Standard peptide bonds join the alpha-carboxyl of one residue to the alpha-amino of the next, and the proteases and peptidases that recycle cellular peptides recognize that geometry. The gamma linkage places the glutamate's alpha-carboxyl and alpha-amino groups free and unlinked, hanging off the chain as a zwitterionic side group, and presents a bond geometry that the general peptidase repertoire does not recognize. Only one enzyme family, the gamma-glutamyl transpeptidases (GGT), cleaves it, and GGT is membrane-bound and localized, which lets cells control where and when glutathione is broken down.
The bond is also the reason glutathione is synthesized enzymatically rather than ribosomally. Two ATP-dependent ligases build it: glutamate-cysteine ligase forms the gamma-glutamylcysteine dipeptide, and glutathione synthetase adds glycine. The first step is rate-limiting and feedback-inhibited by GSH itself, which is the structural basis for the homeostatic control of intracellular glutathione concentration studied in redox models.
What makes the cysteine thiol the reactive center?
The cysteine residue's side chain terminates in a sulfhydryl group (thiol, SH), and that single sulfur is the site of nearly all of glutathione's chemistry. Thiols are nucleophilic, and the thiolate anion (S minus) that forms when the thiol loses its proton is far more nucleophilic than the protonated form. The pKa of glutathione's thiol is approximately 8.7 to 9.2 in aqueous solution, meaning that at physiological pH near 7.4 only a small fraction is deprotonated at any moment. That fraction is what reacts: with reactive oxygen species, with electrophiles in conjugation reactions catalyzed by glutathione S-transferases, and with oxidized cysteine residues on proteins.
The thiol's position in the middle of the tripeptide, flanked by the gamma-glutamyl group on one side and glycine on the other, keeps it sterically accessible while the charged termini keep the whole molecule water-soluble. Structural studies of glutathione in solution describe it as conformationally flexible with no stable secondary structure, which is consistent with its role as a small diffusible reductant rather than a folded signaling peptide.
How does GSH oxidize to GSSG?
Two glutathione molecules oxidize to one glutathione disulfide (GSSG) by forming a disulfide bond between their cysteine sulfurs, releasing two electrons and two protons. The reaction can be written as 2 GSH gives GSSG plus 2 H plus and 2 electrons. GSSG has the molecular formula C₂₀H₃₂N₆O₁₂S₂ and roughly twice the mass of GSH. In the reverse direction, glutathione reductase uses NADPH to reduce GSSG back to two GSH, regenerating the thiol. The ratio of GSH to GSSG in a cell or compartment, typically reported in the range of 100 to 1 or higher in the cytosol under unstressed conditions, is one of the most widely used readouts of redox state in cellular aging research.
Glutathione peroxidases couple GSH oxidation to the reduction of hydrogen peroxide and lipid hydroperoxides, consuming two GSH per peroxide reduced. This is the enzymatic route by which glutathione's thiol chemistry is applied to peroxide detoxification in the models where it is studied. The NAD+ coenzyme overview covers the NADPH supply side that keeps the cycle running.
What is S-glutathionylation?
S-glutathionylation is the formation of a mixed disulfide between glutathione's cysteine and a cysteine residue on a protein. It is reversible, and it changes the protein's activity, localization, or stability while it persists. Under oxidative conditions, protein thiols can be transiently oxidized to sulfenic acid, which then reacts with GSH to form the mixed disulfide; alternatively, GSSG can exchange directly with a protein thiol. Glutaredoxins reverse the modification. Because it protects protein cysteines from irreversible over-oxidation to sulfinic and sulfonic acid, S-glutathionylation is studied as both a redox-signaling mechanism and a protective buffer in cellular stress models. The chemistry depends entirely on the accessibility of glutathione's single thiol.
What is the charge state of glutathione at physiological pH?
Glutathione carries four ionizable groups: two carboxylates (the glutamate alpha-carboxyl and the glycine carboxyl), one amine (the glutamate alpha-amino group), and the cysteine thiol. At pH 7.4 both carboxylates are deprotonated (negative), the amine is protonated (positive), and the thiol is mostly protonated (neutral), giving a net charge of minus one. This zwitterionic profile makes GSH highly water-soluble and unable to cross lipid membranes passively, which is why transport into and out of compartments, and export to the extracellular space, requires specific transporters. Lyophilized research-grade glutathione is typically supplied as the free acid, and its dissolution behavior reflects these charged groups.
How does the structure determine analytical methods for GSH and GSSG?
The thiol is what the classic assays detect. Ellman's reagent (DTNB) reacts with free thiols to release a yellow chromophore absorbing at 412 nm, which quantifies GSH but not GSSG; GSSG is measured by first reducing it with glutathione reductase and NADPH, then measuring the total. Fluorescent thiol-reactive probes such as monochlorobimane and ortho-phthalaldehyde work on the same principle. HPLC methods separate GSH from GSSG directly, usually after derivatization of the thiol to prevent auto-oxidation during sample handling, with UV, fluorescence, or mass spectrometric detection. Because the thiol oxidizes readily in air, sample preparation for any of these methods includes acid quenching or alkylation immediately on collection, and the GSH to GSSG ratio reported in a study is only as reliable as that step.
For research-grade material, purity by reverse-phase HPLC and identity by mass spectrometry (observed mass near 307.3 Da for the protonated molecule) are the standard release tests, and the presence of a GSSG peak at approximately twice the mass is the usual indicator of oxidative degradation during storage.
How does glutathione's structure compare to other short peptides in longevity research?
Glutathione is shorter than most peptides in the longevity research category, and unlike signaling peptides such as epitalon or MOTS-c, it does not act through a receptor. Its function follows directly from its chemistry: a protected, water-soluble carrier for a single reactive thiol. That makes it a reference compound in redox research rather than a signaling molecule, and it explains why its analytical profile (a small, polar, oxidation-sensitive molecule) differs from the lipophilic, receptor-targeted peptides elsewhere in the catalog.
All compounds referenced in this article are supplied for research purposes only and are not for human use. Nothing here describes or endorses any use of these materials outside laboratory research. These products are not intended to diagnose, treat, cure, or prevent any disease.
Join the list.
Restock alerts, subscriber discounts, and first access to new compound drops.
