Pyroglutamyl peptidase (PGP) type I, also known as pyrrolidone carboxyl peptidase (pcp) type I: Enzymes responsible for cleaving pyroglutamate (pGlu) from the N-terminal end of specialized proteins. The N-terminal pGlu protects these proteins from proteolysis by other proteases until the pGlu is removed by a PGP. PGPs are cysteine proteases with a Cys-His-Glu/Asp catalytic triad. Type I PGPs are found in a wide variety of prokaryotes and eukaryotes. It is not clear whether the functional form is a monomer, a homodimer, or a homotetramer.