Glutathione (GSH) is a tripeptide of glutamate, cysteine, and glycine, and it is the most abundant non-protein thiol in mammalian cells. Its reactive cysteine sulfhydryl makes it the principal small-molecule redox buffer of the intracellular environment. This note outlines the GSH/GSSG couple, the enzymes that maintain it, and the conjugation reactions that draw on the reduced form.
The tripeptide and its unusual bond
Glutathione is distinguished by a gamma-glutamyl linkage: the glutamate is joined to cysteine through its side-chain carboxyl rather than its alpha-carboxyl. This atypical peptide bond resists cleavage by most general aminopeptidases, contributing to the relative stability of the intracellular pool. The free thiol on the cysteine residue is the chemically active group in nearly all of glutathione's reactions.
The GSH/GSSG redox couple
The cell's main redox buffer rests on the equilibrium between reduced glutathione (GSH) and its oxidized disulfide form (GSSG), in which two glutathione molecules are joined by a disulfide bond. A high ratio of reduced to oxidized form is maintained under normal conditions, and this couple poises the thiol-disulfide status of many proteins. Because the ratio is informative, analytical methods take care to prevent artifactual oxidation of GSH during extraction.
Glutathione peroxidase and reductase turnover
Glutathione peroxidases reduce hydrogen peroxide and organic hydroperoxides, oxidizing GSH to GSSG in the process. Glutathione reductase then regenerates GSH from GSSG using reducing equivalents supplied by NADPH. The continuous turnover between these two enzymes keeps the buffer poised toward the reduced state, and the rate of regeneration depends on NADPH supply from the pentose phosphate pathway.
Conjugation and detoxification reactions
Beyond redox buffering, glutathione participates in conjugation reactions catalyzed by glutathione S-transferases, in which the thiol attacks electrophilic centers on xenobiotics and reactive metabolites. The resulting conjugates are more water-soluble and are routed into downstream processing and export. This detoxification role consumes glutathione and links the pool to the broader handling of reactive species.
Synthesis and feedback regulation
Glutathione is built in two ATP-dependent steps, the first catalyzed by glutamate-cysteine ligase and rate-limiting for the pathway. Cellular demand modulates expression of the biosynthetic enzymes through redox-sensitive transcriptional control, allowing production to rise when oxidative load increases. Cysteine availability frequently constrains the synthesis rate in experimental systems.
These materials are for laboratory research use only and are not for human or veterinary use.