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With the discovery of glutathione peroxidase (GPx1), the role of glutathione in counteracting oxidative challenge became clear. GPx1 was the first selenoprotein discovered in mammals. It contains a selenocysteine residue integrated into the peptide chain. The phospholipid hydroperoxide glutathione peroxidase (GPx4) also proved to be a selenoprotein. In the cytosol, it inhibits lipid peroxidation and ferroptosis; in the nucleus, it supports protamine compaction; its mitochondrial expression form builds the sheath surrounding the mitochondria in spermatozoa and is essential for male fertility. In the meantime, glutathione peroxidases have grown into a large family of enzymes that work with selenium or with sulfur catalysis. With the growing knowledge that hydroperoxides are not just toxic, but normal intermediates of biosynthetic processes and signaling molecules, glutathione peroxidases had to be re-considered as regulatory proteins in signaling cascades, as modulators of enzyme activities, or as hydrogen peroxide sensors. The extraordinary catalytic efficiency of both the selenium- and sulfur-containing enzymes remained enigmatic for a long time. Recent quantum mechanical approaches revealed a concerted dual attack on the peroxide bond, which explains why their speed of peroxide reduction is orders of magnitude higher than that of any low molecular weight thiol or selenol.

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