Gamma-glutamyl transpeptidases/transferases are self-processing proteins whose physiological function is to catalyze the hydrolysis of gamma-glutamyl bonds. They are localized to the cell surface and only cleave extracellular substrates, with glutathione and oxidized glutathione being the most abundant substrates. These enzymes can also catalyze a transpeptidation reaction in vitro, transferring the gamma-glutamyl group from the substrate to an acceptor (thus accounting for their common name), but this is not a physiological reaction. (Adapted from PMID:29667297 and PMID:24974180.)
