GLUTATHIONE

Glutathione (GSH) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by reactive oxygen species such as free radicals, peroxides, lipid peroxides, and heavy metals. It is a tripeptide with a gamma peptide linkage between the carboxyl group of the glutamate side chain and cysteine. The carboxyl group of the cysteine residue is attached by normal peptide linkage to glycine.
Glutathione is the most abundant thiol in animal cells, ranging from 0.5 to 10 mM. It is present both in the cytosol and the organelles.
Glutathione biosynthesis involves two adenosine triphosphate-dependent steps:

First, gamma-glutamylcysteine is synthesized from L-glutamate and cysteine. This conversion requires the enzyme glutamate–cysteine ligase (GCL, glutamate cysteine synthase). This reaction is the rate-limiting step in glutathione synthesis.
Second, glycine is added to the C-terminal of gamma-glutamylcysteine. This condensation is catalyzed by glutathione synthetase.
While all animal cells are capable of synthesizing glutathione, glutathione synthesis in the liver has been shown to be essential. GCLC knockout mice die within a month of birth due to the absence of hepatic GSH synthesis.

The unusual gamma amide linkage in glutathione protects it from hydrolysis by peptidases

Humans synthesize glutathione, but a few eukaryotes do not, including Leguminosae, Entamoeba, and Giardia. The only archaea that make glutathione are halobacteria. Some bacteria, such as cyanobacteria and proteobacteria, can biosynthesize glutathione.