Glutathione is a tripeptide containing the amino acids glutamate, cysteine, and glycine, linked together in that order (see here). The glutamate is linked to the cysteine via the -carboxyl group.

Glutathione's free thiol group provides protection for cells against oxidative stress. For example, it can help to keep cysteine thiol groups in proteins in the reduced state. If two thiol groups become oxidized, they can be reduced nonenzymatically by glutathione.