In enzymology, a glutaminyl-peptide cyclotransferase (EC 2.3.2.5) is an enzyme that catalyzes the chemical reaction

L-glutaminyl-peptide ${\displaystyle \rightleftharpoons }$ 5-oxoprolyl-peptide + NH₃ or

L-glutamyl-peptide ${\displaystyle \rightleftharpoons }$ 5-oxoprolyl-peptide + H₂O

Hence, this enzyme has one substrate, L-glutaminyl-peptide or L-glutamyl-peptide, and two products, 5-oxoprolyl-peptide and NH₃ or H₂O. The N-terminal 5-oxoproline residue on the peptide is also commonly known as pyroglutamic acid.

This enzyme belongs to the family of transferases, specifically the aminoacyltransferases. The systematic name of this enzyme class is L-glutaminyl-peptide gamma-glutamyltransferase (cyclizing). Other names in common use include glutaminyl-tRNA cyclotransferase, glutaminyl cyclase, and glutaminyl-transfer ribonucleate cyclotransferase.

Structural studies

As of late 2007, 8 structures have been solved for this class of enzymes, with PDB accession codes 2AFM, 2AFO, 2AFS, 2AFU, 2AFW, 2AFX, 2AFZ, and 2IWA.

Human gene

QPCT - note that Q is one-letter abbreviation for glutamine, and glutaminyl is the name of the acyl group.