In enzymology, a gamma-glutamylcyclotransferase (EC 2.3.2.4) is an enzyme that catalyzes the chemical reaction

(5-L-glutamyl)-L-amino acid ${\displaystyle \rightleftharpoons }$ 5-oxoproline + L-amino acid

Hence, this enzyme has one substrate, (5-L-glutamyl)-L-amino acid, and two products, 5-oxoproline and L-amino acid.

This enzyme belongs to the family of transferases, specifically the aminoacyltransferases. The systematic name of this enzyme class is (5-L-glutamyl)-L-amino-acid 5-glutamyltransferase (cyclizing). Other names in common use include gamma-glutamyl-amino acid cyclotransferase, gamma-L-glutamylcyclotransferase, and L-glutamic cyclase. This enzyme participates in glutathione metabolism.

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes 2PN7 and 2RBH.