Responsible for the biosynthesis of pyroglutamyl peptides. Has a bias against acidic and tryptophan residues adjacent to the N-terminal glutaminyl residue and a lack of importance of chain length after the second residue. Also catalyzes N-terminal pyroglutamate formation (By similarity). N-terminal L-glutaminyl-[peptide] = N-terminal 5-oxo-L-prolyl-[peptide] + NH4(+) Expressed mainly in brain tissue. Belongs to the glutaminyl-peptide cyclotransferase family. It is unclear whether this protein requires a metal cofactor for catalysis. It was originally proposed to be a Zn(2+)-dependent metalloenzyme based on structural similarities to bacterial aminopeptidases and the observation that it can bind Zn(2+) ions, typically in a 1:1 stoichiometry (By similarity). However, a recent study suggests a Zn(2+)-independent catalytic mechanism (By similarity).