Glutathione peroxidases (GPxs) are a family of phylogenetically related oxidoreductases distributed in all living domains. They were named after GPx-1, the enzyme first purified from bovine red blood cells that reduces H2O2 by glutathione. In vertebrate GPxs the redox active residue is mainly a peroxidatic selenocysteine, while invertebrate and plant GPxs contain a peroxidatic cysteine.
These residues are part of a strongly conserved active center including Gln, Trp, and Asn. In selenocysteine-containing GPxs, the oxidized selenocysteine is reduced back in two steps by glutathione, through a mixed selenodisulfide intermediate. In the majority of the Cys homologs, instead, the oxidized cysteine gives rise to an intramolecular disulfide that is reduced by thioredoxin. Since reduction of hydroperoxide prevents the formation of reactive oxygen-centered radicals, an antioxidant function has been classically assigned to these enzymes.