Building the tripeptide in two ATP-dependent steps
Glutathione (GSH) is a tripeptide of glutamate, cysteine, and glycine. Synthesis happens in two steps that each consume ATP. First, glutamate-cysteine ligase joins glutamate and cysteine, the rate-limiting step that is feedback-inhibited by GSH itself. Second, glutathione synthetase adds glycine to complete the molecule.
Cysteine availability usually limits how much GSH a cell can make, which is why cysteine supply tends to be the constraining factor rather than glutamate or glycine.
Recycling the oxidized form
When GSH neutralizes oxidants, two molecules are joined into oxidized glutathione (GSSG). Glutathione reductase regenerates GSH from GSSG using electrons from NADPH, much of which comes from the pentose phosphate pathway. The ratio of GSH to GSSG is often used as an indicator of a cell's redox state.
Supplements such as N-acetylcysteine are sometimes discussed as ways to support cysteine supply, but oral glutathione absorption and clinical benefit are not firmly established for healthy people. This is general educational information about a metabolic pathway, not supplement or medical advice; consult a qualified professional before starting any regimen.